Amplifiers My op-amp is oscillating. What should I do? The frequency of the oscillation is a very important clue. Frequencies near or above the op-amp''s GBW generally show instability in the output stage, usually caused by capacitive loading on the output or poor supply bypassing. Also try adding a 0.1uF ceramic bypass capacitor to the power supplies. If the frequency is within the operating range of the circuit (below GBW and near GBWP), and the overall circuit has a relatively high gain, it could be input-to-output coupling causing the feedback. Try shielding the input sections from the output sections and move input and output components away from each other. If the frequency is below 10Hz (commonly referred to as "motorboating"), and the output is driving a relatively heavy load, it is usually an inadequate supply current, poor power supply bypassing, or the lack of a "star-ground" layout. This happens frequently with audio power amplifiers. Add larger supply bypass capacitors and make sure the load''s ground is returned directly to the power supply''s ground. Major causes of oscillations are poor grounding techniques, poor input to output isolation, and/or poor power supply bypassing. What tools does National offer to aid in the design of active filters? In the WEBENCH suite of online design tools is the Active Filter Designer. The Active Filter Designer supports design and simulation of semi-custom lowpass and highpass filters from 2nd to 10th order in a large variety of filter response types, including Chebyshev, Butterworth, Bessel, and others. Begin your filter design here. Is "Operating Temperature Range" based on Junction Temperature (Tj) or Ambient Temperature (Ta)? "Junction Temperature," or Tj, is the temperature within the silicon device, while "Ambient Temperature," or Ta, is the temperature of the ambient environment in which the packaged silicon device is being used. The operating characteristics of silicon are dependent on Tj, and for a given Ta, the end-user''s circuit board (PCB), thermal management, and loading play a big role in what the final Tj will be.  To allow the end-user maximum flexibility, silicon manufacturers normally do not control these factors.. Thus, the Temperature Range for a device generally relates to Junction Temperature, unless otherwise indicated. For more information about power dissipation and its effect on integrated circuits, please see "Understanding Integrated Circuit Package Power Capabilities (also known as "Appendix E" or AN-450). What are the advantages of the BiCMOS process? A BiCMOS process offers the IC designer the ability to use bipolar transistors where speed or voltage-noise levels matter while allowing the use of CMOS transistors where high impedance levels or substantial mixed-signal content is necessary. The VIP50 MOS transistors are optimized to offer analog-grade matching and noise performance. What are the advantages of National''s VIP50 process for low power products? The VIP50 process is a silicon-on-insulator based IC process. SOI processes offer oxide insulation between the circuit elements and the wafer substrate. Oxide insulation results in a strong reduction in parasitic capacitances of the circuit elements. Products implemented in the VIP50 process will have greatly improved bandwidth-to-power ratios compared to conventional technologies. For more information on National''s VIP50 process, technology and products, visit : http://www.national.com/appinfo/amps/vip50.html. How can I create a BOM (Bill of Materials) for a regulator, PLL, or amplifier application? Using the wireless WEBENCH online design tools, you can create a regulator, PLL, or amplifier application. The design process includes developing a bill of materials (BOM) and initial evaluation of circuit performance through simulation. For devices in the LM339 family of comparators, is it OK to have the output pullup voltage different from the supply voltage? Yes. No matter what the supply voltage, the ouput can be pulled up to any value between 2V and 36V (28V for LM2901/3). This is mentioned in the Applications Hints section in the datasheet. Also see Application Note AN-74. Relevant Part: LM139;LM239;LM339;LM193;LM293;LM393;LM2901;LM2903;LM3302 What is Output High Voltage (VOH) for a comparator? Output High Voltage (VOH) is the high DC output voltage of the comparator, with output driven high with specified output current. This specification is typically associated with comparators having totem-pole or push-pull outputs Relevant Part: LM160;LM161;LM360;LM361;LMC6762;LMC7211;LMC7215;LMV7219;LMV7239;LMV7251 What is Output Low Voltage (VOL)? Output Low Voltage (VOL) is the low DC output voltage with the output driven low with specified sinking current. This specification is typically associated with comparators having totem-pole or push-pull outputs Relevant Part: LM160;LM161;LM360;LM361;LMC6762;LMC7211;LMC7215;LMV7219;LMV7239;LMV7251 What is Strobe "OFF" Voltage of a comparator? Strobe "OFF" Voltage is the minimum voltage on the strobe terminal that will guarantee that it does not interfere with the operation of the comparator Relevant Part: LM161;LM361 What can be done for an LM675 power op-amp that is oscillating? Try using an R-C "snubber" circuit. Place a series 1 Ω resistor and 0.22µF capacitor from the output to ground. The LM675 must be used at closed-loop gains of 10 or higher. See the datasheet "Typical Applications" section for low gain application circuits. Relevant Part: LM675 What would be a good replacement for a LM741 or a LM324? The LM324 and LM741 are general purpose devices. The newer generation of op-amps are optimized for high speed and/or low power operation - and each comes with its own set of tradeoffs. Process technology evolves every few years such that a device with similar, or improved, performance to the LM741 can be made cheaper, smaller and with less power consumption. In general, there is nothing wrong with the LM741 in non-power critical applications. If you are designing a battery powered device, then you may want to upgrade to a device with less supply current and lower supply voltage for the same speed. If you wish to get a bit more headroom, you could use a newer device with wider output swing and common mode range. Asking which new device will replace the old LM741 is like asking what car you should buy to replace your old Ford Galaxy. Would you like better fuel economy? (lower power) ? Would you like it to be more comfortable (wider CMR and output swing)? Would you like more acceleration? (higher speed) Would you like a quieter ride? (lower noise) Do you want a smaller car for easier parking in crowded areas? (smaller package) There are many options to choose from now. Part of the "fun" of the design process is choosing the right device that meets your needs. Please refer to our online Catalog selection guide, or the offline selection guide, Selguide, to help you find the best fitting device for your needs. Also check out the WEBENCH Sensor Designer or the WEBENCH Amplifier online design tool, to select op amps according to system requirements. Relevant Part: LM741;LM324 How does the LMH1251 YPbPr to RGBHV Converter and 2:1 Video Switch work? Using the integrated 2:1 MUX, there are two possible signal paths.  The input can take an RGBHV input and simultaneously take a YPbPr input. RGBHV video source (input) is selected: If the RGBHV input is chosen via the 2:1 MUX, the RGB video and H/V syncs are sent through the chip to the output pins, unchanged and unaltered.  The RGBHV has a 400 MHz path for video resolutions up to and beyond the UXGA standard (1600x1200).  YPbPr (Component) video source (input) is selected: If the YPbPr (Component) source is selected, the signal is converted to the RGBHV format and then sent to the output pins.  The product will automatically detect which format of video (480p, 720p, 1080i, 1080p) and apply the proper conversion technique.  The sync processor strips the sync information off of the Y and converts to H and V at the outputs.  Similarly, the Pb and Pr information is sent through the complex color space matrix to convert the video color information to the R, G, and B format at the outputs.  The result is a YPbPr input signal that is converted to RGBHV at the outputs, all done in the analog domain and within 1% accuracy. Relevant Part: LMH1251 What is a Programmable Gain Buffer? A Programmable Gain Buffer is an Op Amp with gain setting resistors integrated on the die allowing possible gains of +1, +2, or -1 using simple external connections. These devices are ideal for minimizing external component count, minimizing signal lead lengths, and simplifying designs Relevant Part: LMH6718;CLC5633;CLC5632;CLC5623 Why should I use dynamic offset reduction techniques with the LMP2011/LMP2012/LMP2014? The LMP2011/12/14 uses a proprietary switching offset reduction technique that reduces the effect of op amp imperfections like offset voltage and 1/f or flicker noise. In addition, the offset reduction technique almost completely eliminates amplifier drift. This results in amplifiers with an extremely low offset voltage that remains constant over both time and temperature. Systems using the LMP2011/12/14 can therefore amplify low-level signals with high accuracy and without a need for periodic calibration. The elimination of 1/f noise allows for high-accuracy measurements at frequencies near DC which can not be achieved with traditional JFET-based precision amplifiers. Relevant Part: LMP2011;LMP2012;LMP2014 What is the minimum operating supply voltage (Vcc) for the LP339? The LP339 quad low-power comparator is specified for 5V to 15V operation only. It may function below 5V, but the input common mode range quickly collapses and we do not guarantee operation below 5V. Refer to the LMV339 datasheet for a comparator with low-voltage (2.7V-5V) operation. Relevant Part: LP339;LMV339 Where can I find a copy of Tom Frederickson''s book, "Intuitive IC Op Amps"? The classic paperback book, "Intuitive IC Op Amps", was originally published in 1984. The book describes how op amps work and how they are used, from a practical, common-sense perspective. It is currently out of print. However, you may be able to find it in university libraries or by browsing the internet. As of March 2005, the book was also available from Rector Press. How does the voltage controlled oscillator of application note AN-74 function? The voltage-controlled oscillator of Figure 17 in application note AN-74 uses a classic approach of an integrator followed by a comparator and automatic reset circuit. The comparator on the left, Comparator 1, is used in an integrator configuration, producing a triangle waveform at a rate proportional to the input voltage. The comparator on the right, Comparator 2, detects whether the integrator output is above or below a mid-supply-voltage reference, converting the triangle wave to a square wave. When the inverting input of Comparator 2 is high and the output of Comparator 3 is off, capacitor C1 is charged through the 100KΩ resistor R1, ramping Comparator 1 output, or Vtri, down. When Vtri reaches about Vcc/2, Comparator 2 output goes low and Comparator 3 output turns on (sinking current). This causes the capacitor to discharge through the 50KΩ resistor R2, ramping Vtri up. When the Vtri reaches Vcc/2 again, Comparator 2 output goes low and turns off Comparator 3 output, causing the process to repeat. With C1=500pF, the approximate frequency of oscillation is: f = (2666) X (Vc) See additional details and application hints in application note AN-74. When considering the ideal AC performance of an op amp, the Bode plot (gain vs. frequency response) is a one pole system. What is the rate at which the gain rolls off, in dB/decade? In a one-pole system, the gain rolls off (or declines) at 20dB per decade. This is also 6 dB per octave. This is true for any one-pole response (ie: a simple RC filter or an ideal op amp). However, because the op amp has additional high-frequency poles, the phase shift will start to increase as the frequency approaches the op-amp unity gain frequency Where can I find the available buffers that National provides? National''s Analog-Buffers product table offers DSL line drivers, programmable buffers, high speed video buffers, and closed-loop buffers. I am using a CMOS op-amp as an output driver. Although the circuit works well, I have noticed that if I use a long (1 Meter) shielded cable, the op-amp oscillates at around 1MHz with no input signal. If I shorten the cable to 10 cm, the oscillation subsides. What is causing this? Some op-amps have trouble driving direct capacitive loads, such as a hunk of shielded cable which is a good capacitive load. 1 Meter of coax cable is about 60-100pF. Try placing a 50 to 500 Ω resistor between the op-amp output and the cable. The datasheets for CMOS op-amps have a section on how to compensate for capacitive loads. What is the difference between an op amp''s "common mode voltage" and "input voltage range"? While similar, these terms do not mean exactly the same thing. "Input voltage range" is the range of acceptable voltages applied to EITHER of the input pins. This is listed in the datasheet as "CMR", "CMVR"; or "input voltage range". See "What is CMR" . "Common Mode voltage" is a voltage applied to BOTH inputs simultaneously. Remember that an op-amp is supposed to REJECT common mode voltages and only amplify the difference BETWEEN the input pins. See "What is CMRR" What''s the difference between a voltage feedback amplifier and a current feedback amplifier? The internal circuitry of the two types of op-amps are different, and these two types of op-amps are not necessarily interchangeable in a given configuration. Voltage feedback op-amps are constrained by their internal design to have very low input bias currents, but the differential input voltage is unconstrained internally and is only constrained when external feedback applied around it. Conversely, for current feedback op-amps, the differential input voltage is constrained internally, but the input bias current is unconstrained by the internal design to be low, and is only constrained when external feedback applied around it. Although most colleges do not yet teach the basics in current feedback amplifiers, there are many advantages to using them, especially in high speed applications. See the following Application Notes: OA-30, Current vs. Voltage Feedback Amplifiers OA-07, Current Feedback Op Amp Applications Circuit Guide OA-13, Current Feedback Loop Gain Analysis and Performance Enhancement OA-15, Frequent Faux Pas in Applying Wideband Current Feedback Amplifiers OA-20, Current-Feedback Myths Debunked Amplifiers WEBENCH supports both current mode and voltage mode types of amplifiers. The amplifier datasheet gives an Input Offset Voltage specification of "1 mV max". Is this +1mV max, or -1mV max, or somewhere between +/- 0.5mV, or somewhere between +/- 1mV? The input offset voltage (Vos) specification for an op amp is given as a magnitude. From the perspective of one of the input pins, the offset voltage may be anywhere between +Vos and -Vos. In a real application, the offset voltage will be multiplied by the non-inverting gain of the circuit to produce an offset at the output of the op amp What''s the difference between "Open Loop Gain" and "Closed Loop Gain"? "Open Loop Gain" is the actual gain "inside" of the op-amp without feedback, usually anywhere from 1,000 to 10 million. See the "Open Loop Gain" plots in the datasheet. "Closed Loop Gain" is the gain of the overall circuit, with feedback, and is selected by the user by choosing the appropriate feedback resistor ratios. This is your "gain of +10" or "gain of -2" What does "Avol" stand for, and what is it? Avol stands for "open loop voltage gain." The "A" is the symbol for gain. The "v" is a subscript that indicates voltage gain, as opposed to current gain. The "ol" is also a subscript and stands for open-loop. The open-loop voltage gain is the voltage gain of the amplifier (Vout/Vin) without feedback, compensating for any errors due to offset voltage What are Differential Phase and Differential Gain? "Differential Gain" and "Differential Phase" (DG/DP) are video measurements, and are a standard of measurements in the broadcast field. These measurements are the change in the amplitude, or phase, of the video signal over a stepped video waveform (think of it as delta gain or delta phase vs. Vout). The standard test signal used is a six-step monochrome video test pattern with a steady chroma subcarrier (NTSC 3.579MHz / PAL 4.2MHz) applied. The resultant video waveform resembles a six stair-step ramp (0 to 100% brightness) with the color subcarrier "fuzz" riding on top. For differential phase, the test instrument establishes a phase lock with the colorburst reference signal, then compares the phase of the subcarrier "riding" along on each "step" of the stairstep, and displays the phase error for each "step". A good video amplifier will cause less than 0.1 degrees error. For differential gain, the test instrument compares the stairstep to a known amplitude reference, and displays the results. A good amplifier will have less than 0.1% of gain shift. For our measurements, we use the industry standard Tektronix VM700A test set. Tektronix defines DG/DP as: Differential Gain: Measured as deviation of the peak-to-peak amplitude of each color bar from the nominal value for that color bar expressed as a percent of the nominal value. Differential Phase: Measured as deviation of the phase of each color bar from the nominal phase for that color bar, relative to burst phase. See OA-24 for more information and a procedure to measure DG/DP with a standard Network Analyzer. What does "CMR" stand for, and what is it? CMR stands for Common Mode Range. The Common Mode Range is also known as input voltage range, and is a measure of the range of input voltages that the op-amp can accept at its input pins. This is usually specified with respect to the supply rails. An LM741 for example, is guaranteed to accept input voltages that are inside the supply rails by at least 3 volts. So when operating from a dual supply of +15V and -15V, it can accept input pin voltages between +12V and -12V; this is the Common Mode Range in this situation What is -3dB Bandwidth (or small signal bandwidth, SSBW)? -3dB Bandwidth (or small signal bandwidth (SSBW)) is the frequency at which the closed loop amplifier small signal magnitude response is 3dB below its nominal value at low frequency. It is sometimes specified for various signal amplitudes What is Closed Loop Gain? Closed Loop Gain is the ratio of an output voltage change to an input voltage change after feedback and input networks are added. Usually, external resistors are used to set this parameter What is Common Mode Input Resistance? Common-Mode Input Resistance is the ratio of the common-mode input voltage change to the inverting or non-inverting input current change What is Common-Mode Input Resistance? Common-Mode Input Resistance: The ratio of the common-mode input voltage change to the inverting or non-inverting input current change What is Common-Mode Rejection Ratio (CMRR)? Common-Mode Rejection Ratio (CMRR): The ratio of differential voltage amplification to common-mode voltage amplification. It is measured by determining the ratio of a change in input common-mode voltage to the resulting change in input offset voltage change. CMRR(dB) = 20log 10 (DVCM /DVOS What is Common-Mode Voltage Range (Vcm)? Typically the range of voltages on the input terminals for which the amplifiers''s performance is specified What is Current Feedback? Current Feedback is a technique used in Current Feedback Amplifiers, which generates an output signal in response to the current flowing into the inverting input node (transimpedance gain function). This topology offers operational advantages in certain areas, compared to the traditional voltage feedback. See application note OA-30, "Current vs. Voltage Feedback Amplifiers. What is Differential Gain and Phase? Differential Gain and Phase: Differential Gain refers to change in gain with output input and Differential Phase refers to change in phase with input level. Both parameters are used in video broadcast applications as a measure of consistency of video signal relative to changes in illumination What is Differential Input Resistance? Differential Input Resistance: The ratio of the differential input voltage change to the input current change What is Gain Bandwidth (GWB)? Gain Bandwidth (GWB): The open loop gain times the frequency at a specified frequency higher than the first pole What is Gain Bandwidth Product? Gain Bandwidth Product is an Arithmetic Product of a given input frequency and the op-amp open loop gain at that frequency (usually specified in MHz, voltage feedback amplifiers only.) For an ideal op amp, this is a constant for all frequency after the dominant pole frequency, but other poles and zeroes in the forward path could make the number vary with frequency What is Gain Flatness? Gain Flatness is specified as "Peaking" and "Rolloff" numbers in dB over a given frequency band. It is a measure of an op amp''s closed loop frequency response gain flatness. Phase margin, Gain margin, and sufficient loop gain are the most important parameters affecting these specifications What is Gain Margin (Cm)? Gain Margin (Cm ): Open loop gain at the frequency where the phase between inverting input and output crosses zero What is Harmonic Distortion? Harmonic Distortion is unwanted spurious signals generated at the output of an amplifier due to non-linearity in the signal flow path. With sinusoidal input, these spurs will occur at integer multiples of the input frequency (e.g. 2nd harmonic, 3rd harmonic) What is Input Current (IB or Iin)? The Input Current specification is the average of the currents drawn by the two input pins. Input current is also often called "bias current" What is Input Current Noise (in)? Input Current Noise (in ): The equivalent current noise applied in parallel with the input of the noiseless amplifier What is Input Impedance (Zin)? Input Impedance (Zin ): The ratio of input AC voltage to input AC current What is Input Offset Current (Ios)? Input Offset Current (Ios): The difference of the currents between the two input terminals What is Input Offset Voltage (Vos)? Input Offset Voltage (Vos): The DC error voltage which exists between the input terminals due to non-ideal balancing of the input stage to the output. It is multiplied by the closed loop gain What is Input Voltage Noise (en) ? Input Voltage Noise (en ): The equivalent voltage noise applied in series with the input of the noiseless amplifier What is Large-Signal Voltage Gain (Av)? Large-Signal Voltage Gain (Av ): The ratio of the output voltage change to the change in input voltage. This parameter is usually specified at a large output voltage, less than maximum output Voltage, and typically under DC condition What is Linear Phase Deviation? Linear Phase Deviation: Specified over a given frequency band, it is a measure of how close an op amp’s closed loop phase response follows a linear relationship with respect to frequency What is Logic Threshold Voltage (VT)? Logic Threshold Voltage (VT) is the voltage that exceeds the input offset voltage causing the comparator output to change state What is Offset Current Temperature Coefficient (TCIOS)? Offset Current Temperature Coefficient (TCIOS ): The average rate of change in offset current for junction temperature variation over a specified temperature range What is Offset Voltage Temperature Coefficient (TCVos)? Offset Voltage Temperature Coefficient (TCVos ): The average rate of change in offset voltage for the junction temperature variation over a specified temperature range What is Open loop Gain ? Open loop Gain: The ratio of the voltage change at the output to the voltage change at the input, usually under AC condition What is Output Current? Output Current is the current available at the output of the op amp to drive a load. It is usually a function of input over-drive, output voltage relative to supplies, and temperature. Sourcing and sinking characteristics could be different What is Output Impedance (Zo)? Output Impedance (Zo ): The apparent output impedance of an op amp, typically illustrated with an ideal op amp with zero output impedance in series with an output impedance, Zout, measured under AC condition What is Output Leakage Current (ILEAKAGE)? Output Leakage Current (ILEAKAGE ) is the current into the comparator''s output terminal with the output driven high. It applies to open collector or open drain outputs What is Output Resistance (Ro)? Output Resistance: The apparent output resistance of an op-amp, typically illustrated with an ideal op amp with zero output resistance in series with an output resistor, Rout, measured under DC condition What is Output Sink Current (ISC-)? Output Sink Current (ISC- ) is the maximum negative output current that can be sunk by the comparator What is Output Source Current (ISC+) ? Output Source Current (ISC+) is the maximum positive output current that can be sourced by the comparator with a push/pull output state What is Output Voltage Swing (Vo)? Output Voltage Swing (Vo ): The maximum peak-to-peak output voltage swing under specified load and supply voltages What is Phase Margin? Phase Margin: The open-loop phase shift between the output and the inverting input at the unity frequency What is Power Supply Rejection Ratio (PSRR)? Power Supply Rejection Ratio (PSRR): The ratio of the change in input offset voltage to the change in power supply voltages producing it. PSRR(dB) = 20log 10 (DVOS /DVS What is Response Time (tr)? Response Time (tr ): The interval between the application of an input step function and the time when the output crosses the logic threshold voltage What is Rise Time (tr) ? Rise Time (tr ): The time required for an output voltage step to change from 10% to 90% of its final value What is Saturation Voltage (VSAT)? Saturation Voltage (VSAT) is the collector-to-emitter voltage of a transistor in a saturated condition. In saturation, both the emitter-base and collector-base junctions are forward biased. resulting in a very low collector-emitter voltage, typically 0.1-0.3V. Saturation voltage is measured at a specified collector current. It is usually of concern with an open-collector output which has a resistive pull-up to a positive supply voltage What is Settling Time? Settling Time: The time between the initiation of the input step function and the time when the output voltage has settled to within a specified error band, which is expressed as the ±percentage of the total voltage change What is Short-Circuit Output Current ? Short-Circuit Output Current: The maximum available current out of the output of an op amp What is Slew Rate (SR)? Slew Rate (SR): The rate that an amplifier output changes from one voltage level to another when a step or square wave input is applied. Typically it is the average rate measured from 10% to 90% of the total output voltage change What is Supply Current (Is) ? Supply Current (Is ): The current required from the power supply to operate the amplifier with no load and the output midway between the supplies What is Total Harmonic Distortion (THD)? When a pure sinusoid given as Vin (w) = Vp sin(wt) is applied to the input of an operational amplifier, the output with harmonic distortion will be:    Vout (w)a1 Vp sin(wt)+a2 Vp sin(wt)+...+an Vp sin(nwt). THD is expressed as:    THD(%) = [sqrt(a2xa2 +a3xa3 +...+anxan)/a1 ] x 100 What is Transient Response ? Transient Response: The closed-loop step-function response of the amplifier under small-signal conditions. Usually small signal is less than 100mV What is Unity Gain Bandwidth? Unity Gain Bandwidth: The frequency where the amplifier open loop gain equals to one. It equals GBW if the op amp has a single pole roll-off in its frequency response What is Unity Gain Frequency? Unity Gain Frequency: The frequency at which a voltage feedback op amp gain is 1 (0dB). For an ideal op amp, this is equal to the Gain Bandwidth product What is Voltage Feedback? Voltage Feedback is a technique used in traditional Op Amps, where a portion of the output voltage is fed back to the amplifier input, contributing to the voltage difference between the two inputs which is then amplified by the op amp What is Voltage Gain (AV)? Voltage Gain (AV) is the ratio of the change in output voltage to the change in voltage between the input terminals producing it What is Voltage Overdrive? Voltage overdrive (or overdrive voltage) is the amount of applied input step voltage beyond the minimum drive required to change the comparator output state from one logic level to the opposite logic level What is a Closed Loop Buffer? A Closed Loop Buffer is an amplifier with high input impedance and low output impedance, with a fixed gain of +1. It is typically used for isolation, increased output drive, capacitive load drive, etc. No gain setting resistors are required What is a Open Loop Gain, Voltage feedback Op Amp? Open Loop Gain, Voltage feedback Op Amp: The open loop gain is specified at DC and is defined as the ratio of an output voltage change to an input voltage change. It is also referred to as differential voltage gain (no feedback or input networks added). When specified using a sinusoidal waveform, it varies in magnitude and phase relative to frequency What is an amplifier''s Intercept Point? Intercept Point is the fundamental output power where the specified distortion term (2nd, 3rd, or 3rd order intermodulation) is equal in power to this fundamental value power What is an amplifier''s Specified Supply Range? The Specified Supply Range describes the power supply voltages required to power the op amp My amplifier design works OK on a single 5V supply but if I try to put 4 volts on the input, the output will not go above about 3.6 volts. What''s wrong? In the device datasheet, look for the specification labeled Input Voltage Range or Input Common Mode Voltage Range. This specification tells how close to the upper or lower supply voltage the amplifier can operate. Most amplifiers cannot operate with inputs closer than one to two volts to the supply rail voltage. Some op amps can go to the negative supply rail but not the positive rail. If you need the input to go very close [within 20 to 200mv] to the supply rail, select a Rail-to-Rail Input amplifier, or one that allows the input to desired supply rail. If the output must also go very close to the positive rail, select a Rail-to-Rail Input/Output (RRIO) amplifier. You can use Amplifiers WEBENCH to identify and select op amps that can meet your input and output voltage range requirements. I selected a Rail-to-Rail Input/Output amplifier but the output does not go all the way to the negative rail or all the way to the positive rail. What am I doing wrong? The term "Rail-to-Rail" is misleading. To be completely accurate, it should be "almost Rail-to-Rail" or "very nearly Rail-to-Rail." The output for most R-R amps is from 20 to 200mv from either supply rail, almost never all the way to the rail. And as more load current is required, the output will "pull" further away from the supply rail voltage. Most amplifiers will provide their maximum output voltage swing with a load of 100k Ω or greater. The Electrical Characteristics table and the Characteristic Curves in the product datasheet will specify the output voltage swings that can be expected. In addition, when creating a design using Amplifiers WEBENCH, the user can request a preference for rail-to-rail output amplifiers. The op amps presented in the selection table will meet the output swing requirements of the design, with the rail-to-rail output devices showing up higher in the table if that preference is selected. Where can I find models, tools and information for National''s amplifier products? Information about National''s Amplifier products can be found at the Amplifiers home page, http://amplifiers.national.com. This web page provides access to highlights of new op amps, comparators, and buffers; selection guides, application notes & articles; SPICE models & evaluation boards; plus relevant online seminars. Also available here is an entry to Amplifiers WEBENCH, an online tool for designing op amp-based circuits. What''s the difference between GBW, Unity Gain Bandwidth, Gain Bandwidth Product, and the -3dB frequency? For many op amps, the open-loop gain drops with frequency at a steady rate, -20dB/decade. At any point on this downward slope, the product of the gain and the frequency at that point is constant, and is known as the Gain Bandwidth Product or GBW. If the op amp has been stabilized to operate at unity gain, then the Unity Gain Bandwidth, or the frequency at which the open loop gain is unity (gain of one) is usually equal to the Gain Bandwidth Product. This is shown on the "Open Loop Gain and Phase" plots as the frequency where the gain crosses through 0dB. Some op amps do not have a steady GBW, especially those not stabilized to operate at unity gain. In these cases, the GBW will be different from (usually higher than) the Unity Gain Bandwidth. The -3dB frequency is a measurement of the bandwidth of a closed-loop application of the op amp. The -3dB frequency is the point where the overall closed-loop gain drops by 3dB. The frequency at which the closed-loop application gain is unity can be calculated using BW=GBW/Av. Both the -3dB frequency and the unity gain bandwidth of the application depend on freedback gain setting, output swing, load, and circuit layout. What is the typical input capacitance of an op amp? The input capacitance is typically about 2-3 pf. About half of the capacitance is in the chip and half is in the package. How do I protect the amplifier inputs from possible voltages above/below the supplies? You must either clamp the input, or limit the current into the device, or ideally, both. The simplest way to limit this current is with a resistor selected such that the maximum voltage applied to the circuit input creates a current on the input pin that is less than the maximum pin current rating. A 1K to 100K resistor in series with the input pin is usually enough. The series resistor of an inverting configuration usually serves this function fairly well. However, a non-inverting amplifier may require this input protection resistor because the signal is usually applied directly to the non-inverting input pin. For low-impedance circuits that cannot contain a large resistance, a pair of clamping diodes between the negative rail, the input, and the positive rail, along with a small series resistance, will protect the device. For high-impedance circuits, use a larger resistor and/or low-leakage diodes While the SPICE models for the bipolar (LMxxxx) op amps work fine, the CMOS (LMCxxxx) op amp models do not converge. Is there a SPICE option that needs to be changed? To correctly model the input bias current, the CMOS op amp SPICE models require a change to the default GMIN option setting on most SPICE packages. There is a comment about this on the op amp SPICE models web page, and also a few lines down in the model file. Set .OPTIONS GMIN=1E16 to model the ultra-low input bias current and improve convergence. In the simulation environment of Amplifiers WEBENCH, this option statement is correctly set.. How should a very low frequency (<1Hz) differentiator be designed to minimize output noise? The traditional differentiator uses a series Rs-Cs input and a parallel Rf-Cf feedback around an op amp. But there is no simple solution, no "one size fits all" solution. Try more Rs or Cf to minimize the noise. The only reason the output of the differentiator is noisy is because there is a lot of gain, and the INPUT is noisy. Adding more Cf or Rs reduces the gain. Miracles are not expected... Besides, just because the differentiator''s output is noisy, does not mean it is doing harm. It is just magnifying the noisy signal that is there, as well as amplifying the signals! If you are closing a loop, the differentiator''s output noise may be HELPING make the loop quiet and stable. If the differentiator''s output is rather noisy, and if that is because the input is too noisy, try to figure out what is really creating the input noise. (From Bob Pease''s Online Seminar, "The Use and Abuse of Amplifiers", 12/5/02.) What''s a good way to minimize 1/f noise while amplifying low-level DC signals? For a high signal-to-noise ratio, the circuit must be well-engineered. This includes choosing the best amplifier for (a) the bandwidth of interest, and (b) the impedance level of the input signals. Choosing a low voltage noise amplifier will not be helpful if the input source has rather high impedance, and the amplifier has high current noise. An amplifier with extremely low 1/f noise can be made using the LM394 transistor pair (as described in LB-52), better than most integrated op amps. (From Bob Pease''s Online Seminar, "The Use and Abuse of Amplifiers", 12/5/02.) Can op-amps be purchased with a selected offset voltage? Not unless you plan to pay a premium price and order 100,000 units. Small scale special testing of this sort is very expensive. If you want parts with consistent offsets, you will have to screen them first. Our advice is to redesign your circuit to be less sensitive to offsets. This will also prevent trouble in the future if devices need to be replaced in the field, as they can throw in any "off-the-shelf" device, instead of a "selected" device. There are ways to add/force an external offset. Take a look at the fourth page of application note AN-31, "Op Amp Circuit Collection", for some circuits for adjusting the offsets of various op-amp configurations. Why do some amplifiers oscillate with capacitive loads? Oscillations caused by capacitive loads are a result of interactions between the op-amp''s output impedance and the capacitive load. The output impedance and the capacitive load form an R-C pole in the output stage, causing additional phase lag in the feedback signal. CMOS amplifiers have a higher output impedance that causes the pole to come near, or below, the op-amp''s unity-gain frequency. The additional phase lag of the pole will erode the phase margin of the op-amp, causing the total amplifier phase lag to increase to more than 180 degrees at the unity-gain frequency, resulting in oscillation due to a total feedback phase shift of greater than 180 deg at unity gain. CMOS parts can have an output impedance between 100 and 500, causing the pole frequencies to be relatively low. Similar speed bipolar op-amps have output impedances in the 1 to 100 range, resulting in pole frequencies much higher than the CMOS op-amp, keeping the pole away from the unity-gain frequency of the part. The capacitive load drive of CMOS parts can be increased with the use of output resistors and external "feed-forward" capacitors placed on the output. The datasheets for CMOS op-amps have a section on how to compensate for capacitive loads. Some op-amps have special accommodation for high capacitive loads. See these products listed in this filtered view of the Catalog online selection guide. Do you need to carefully bypass the supply pins of a high speed (>200MHz) op amp if you are only using it at 1MHz? Absolutely! If it''s not bypassed, it may ring at frequencies between the signal and the amplifier''s bandwidth, causing unexpected errors. Ringing at 11Mhz would bum you out! If you have to use a 200 MHz op-amp, it is only fair to put in pretty good bypassing! (From Bob Pease''s Online Seminar, "The Use and Abuse of Amplifiers", 12/5/02.) What is the difference between an amplifier''s Output Current and Short Circuit Current? "Short Circuit" current is the current that the device will deliver if the output is directly tied to one of the supply lines. Depending on the device''s design, this represents the current limiting of the output stage. However, short circuiting current does not represent the true output driving capability of the output stage. Due to output stage resistance, the maximum output current is determined by the output voltage swing under load. The output will have higher swings with lighter loads, and less swing with heavier loads. The "Output Swing vs. Load" or "Vout vs. Iout" graphs in the device datasheet should be consulted to determine if the op-amp can safely drive the load to the correct levels. Don''t forget to account for feedback resistor load, as it can be significant with high-speed or micropower circuits If the output of the op amp is stuck at a voltage close to one of the supply rails (i.e. the output is railed), what''s the cause? There are many ways to "rail" an op amp. The hard part is keeping it from railing. If the input exceeds the input voltage range, the output will generally go to one supply voltage rail. If the output would theoretically go beyond the actual supply voltage, given a hypothetically higher voltage supply, then again the op amp will rail. If the feedback around the amplifier is either nonexistent or in the wrong polarity, again the op amp will rail. Also, if the positive input is higher than the negative input, the op amp will rail. The op amp application should be analyzed to be sure the input voltage(s) and gain are appropriate for the supply voltage used, so that in normal operation the input voltage is within the operating ratings, and the output voltage is within its normal bounds. Why do application schematics involving op amps seldom show supply connections? Power supply connections are often omitted to simplify the application schematic. This was done historically when op amps were first being used, and the tradition is perpetuated itself, mainly to keep the application schematic from looking overly complicated. Drawing the supply connections to all op amps can be especially confusing with multiple (e.g. dual or quad) amplifiers in one package. In such cases, only one supply pin per package needs to be connected, and the supply is routed internally to all the op amps in the package. Even if there is no supply connection shown on the application schematic, a power supply must be connect to the amplifier for it to operate. Consult the device datasheet for appropriate power supply voltages. When developing circuits using Amplifiers WEBENCH, the choice of supply voltage is incorporated into the design requirements. What is the difference between a single supply op-amp and a dual supply op-amp? Generally, "single supply op amp" means the op-amp has an input common mode range that includes V- (Gnd). However, since there is no "ground" pin on an op-amp, there may no difference in the actual circuitry, layout, or behavior of op-amp itself that allows it to operate on a single supply vs a dual or split supply. The only difference one might find upon a thorough examination is in the op-amp data sheet. An op-amp that is designated as a dual supply op-amp usually references the output load with respect to ground, whereas a single supply op-amp usually references the output load with respect to the midpoint voltage of the single supply. Although op-amps that are designated as single supply op-amps generally operate at lower voltages, this is not a requirement. So whether you operate your op-amp from a single 5 volt supply and ground, or from +2.5 and -2.5 volts, makes no difference to the op-amp. All the op-amp cares about are the relative voltages of those supplies with respect to each other, and with respect to the input and output voltages. Amplifiers WEBENCH circuits may be developed using either a single supply (e.g. 5V or 3.3V) or a dual supply (e.g. +/-5V, +/-3.3V, +/-12V), or custom supply voltages may be selected. What are the advantages of National''s VIP50 process for precision products? The VIP50 process combines vertical bipolar NPN and PNP transistors, 0.5µm CMOS and highly accurate thin film resistors. Laser trim capability to further enhance circuit accuracy is an integral part of the process. VIP50 enables new high precision amplifier solutions. For more information on National''s VIP50 process, technology and products, visiti : http://www.national.com/appinfo/amps/vip50.html. How do you check stability of an op amp circuit? To check the stability of a control loop such as an op amp circuit, apply a pulsed load and observe the output voltage change. The pulsed load can be a pulse or step change in load current, connected to the op amp circuit output by a series R-C network (such as 10k/0.01µF). The greater the ringing or oscillation, the less the stability of the circuit. This process is often referred to as "banging" the output. (source: Online Seminar Q&A, "The Use and Abuse of Amplifiers" by Bob Pease, 12/5/02) . What is an LMH® amplifier? What benefits does the customer enjoy with National''s LMH High Speed Amplifiers? LMH is National''s brand of high-performance, high-speed amplifiers. A high-speed amplifier is an amplifier that has a bandwidth (i.e. speed) of >= 50 MHz In the receive path, the LMH amplifiers offer best-in-class low-noise performance, keeping the noise floor of the end product very low. In the transmit path, the LMH amplifiers offer great output drive with low distortion for extended transmit signal reach and fidelity. National has optimized the LMH family for best-in-class speed/power consumption performance.