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Audio amplifier frequency compensation in op

The solution is to compensate the amplifier in terms of frequency response by using a frequency compensation circuit across the operational amplifier. The stability of an amplifier is highly dependent on different parameters. Simple 60 Watt Power Amplifier. Audio power amplifier frequency compensation. Simply take the lowest pole to hand p1 and make it dominant i e so much lower in frequency than the next pole p2 that the total loop gain i e the open loop gain as reduced by the attenuation in the feedback network falls below unity before enough phase shift accumulates to cause hf oscillation.


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WATCH RELATED VIDEO: Op-Amp: Gain Bandwidth Product and Frequency Response

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Introduction: Numerous opinions and preferences exist regarding op amp selection for hi-fi audio circuits. The question we attempt to answer is "Which op amp is the best for audio applications? We read reviews, check out forums, and in our frustration we seek advice from the guy on the right -.

Actually, the LM is about the worst device imaginable for hi-fi audio use. When choosing an audio grade op amp, some opt for high loop gain devices as it is well known that high loop gain makes for a more precise amplifier and lower THD specifications. Others decry the use of high gain devices, claiming the high gain has audible affects, and none of which are desirable. Others want high speed in the form of fast slew rates and wide bandwidths following the old adage that "faster is better.

Op amp compensation techniques, and their affects on speed, gain, and bandwidth will be examined. Short comings and pitfalls will be exposed. Geek out level 9 will be attained. Lastly, a vastly superior alternative to the monolithic IC op amp will be revealed.

Background: There is a thing that ties gain, slew rate, and bandwidth together, and that thing is the op amp's compensation. All op amps have it, and the ones that don't are oscillators. In order to fully understand compensation, I must now provide some background for the background.

Op amps always operate with some amount of external feedback where some of the output signal is fed back into the input. If we start looking at op amp circuits, we can see how this is being done with feedback resistor RF.

A better way to describe feedback action is that it is used to hold the op amps output to a mirror image of input signal, multiplied by the closed loop gain.

Exactly how tightly it holds it to it is determined by the op amps OPEN loop gain. The thing we must be leery of when using feedback, is the phase delay phase shift, or time lag that the op amp has. It can't hold its output to a copy of its input if there is a time lag between the two.

Unfortunately, all op amps will have some phase lag associated with them, and it is caused by the small parasitic capacitances associated with the transistors that the device is fabricated from. There are several of these stray capacitances, and they will stack up quickly causing a rapid phase shift of hundreds of degrees to occur at the transition frequency of the internal transistors inside the op amp.

If the phase lag in the op amp is severe enough, it will oscillate as the delayed feedback signal causes the op amp to chase its own tail, resulting in an oscillation. This condition is called Nyquist stability criteria, and frequency compensation is employed to ensure it.

Compensation works by employing a single, dominant pole in the device that hits first, and will come long before the phase delays that come from internal parasitic capacitances have any affect.

This single pole will systematically burn off or reduce the gain, and push the gain below zero before the subsequent poles come into play, and stability will be maintained. This is what compensation does. Refer to the plot on the right to see an uncompensated amplifier, and one that uses compensation. This is the characteristic of a single pole roll off.

It is created by a single capacitor purposefully placed inside of the device to roll off and reduce the gain to zero before the poles from the parasitic capacitors internal to the op amp come into play.

It has the same effect as a single pole low pass filter, and in a sense, that is exactly what it is. Every op amp ever made uses this technique to maintain stability. An example of this appears to the right. Three hypothetical amplifiers are presented, with unity gain bandwidths of 1, 10, and MHZ. This plot shows the maximum theoretical gain that any of them can have at any frequency. A given device may well have less gain in the low frequency 10 — Hz region where loop gain typically tops out.

The point here is to illustrate how unity gain bandwidth sets an inherent cap on how much gain is possible given a single pole compensation scheme. For example it is impossible to find a 10MHz single pole compensated op amp with 80dB of gain at 10KHz. The nature of the compensation scheme would limit said amplifier to around 60dB of loop gain at that frequency. The fact that the gain drops at 6dB per octave is assumed to be known by the reader, and often goes unmentioned by the author of the device datasheet.

The point is that a 10MHz op amp with a specified gain of dB in the datasheet will still only have 80dB of gain at 1KHZ, 60dB at 10KHz, and so on, whether the datasheet spells this out or not.

Well, because high bandwidth devices are finicky creatures. If you read the datasheets of high bandwidth devices, you will begin to notice all sorts of extra things and special precautions that one must start paying attention to in order to successfully use them in your circuits without oscillating. For example, they usually require extra power supply bypass capacitors on their power pins.

Some manufacturers recommend an array of them, comprised of tantalums, ceramics, and a small NPO for high frequency decoupling. None of these high speed devices are recommended for use in DIP sockets, as the inductance of the socket is enough to upset the delicate balance that must be maintained to ensure stability.

Most of these high bandwidth devices specify a maximum value of feedback resistor that can be used, and its typically less than 1K.

This is because the feedback resistor will form an extra pole and add extra phase shift with the input capacitance of the device. The parasitics of the board layout must also be taken into account. A few tenths of a PF of stray capacitance on the input pins are enough to send a MHz op amp into a tizzy. In short, high bandwidth devices do not drop into audio circuits with DIP sockets well. When the bold do attempt to put such a device in their system, more often than not the device will have a low level oscillation in the MHZ region, yet still play audio signals.

It will just sound like crap due to the oscillation. To see what this condition looks like on an oscilloscope, refer to the picture on the right. This is why high gain devices receive minimal praise in some audio circles.

The oscillation from the high bandwidth is what causes the terrible sound. Its not the slew rate, either: Fast slew rate devices are a favorite among audiophiles as well, and fast slew rates are usually associated with wide bandwidth devices.

There is an equation that can be used to calculate the slew rate requirement of a device if we know the maximum frequency and the peak amplitude of the signal that the device must amplify. This equation is —. If we omit the division by 1 million, our result will be in volts per second. Slew rates are typically specified in volts per microsecond, so we shall divide by one million to obtain a result in those units.

Most op amps in line level circuits process signal amplitudes of a few volts peak. So how much slew rate is required of an op amp to reproduce a 10V peak signal at KHz? Pretty slow, right? If we get more reasonable, and calculate a slew rate requirement for 5V peak at 50KHz, we only need a paltry 1. Under mathematical inspection, it would appear that excessive slew rates, just like excessive bandwidths, are not needed for audio signal amplification. So where do we go from here?

Seems like we are stuck, Right? If you want high loop gain, you will only get it with high bandwidths and slew rates, and we will just have to deal with the cantankerous nature of such devices. Can we have it all? Can we get radically high loop gains with reasonable bandwidths? Suddenly, much more gain would be theoretically possible at any frequency, which is nice, because that is what we are after anyway.

So what would this look like? The loop gain plot on the right shows 6 hypothetical op amps with bandwidths of 1, 10, and MHz compensated with a single pole and a two pole scheme each. It can be seen that for a given bandwidth device, there is far more loop gain that is theoretically possible using two pole compensation due to the steeper gain roll off.

Below 10KHz, a two pole device has more gain still than its single pole compensated counterpart. So can we do this? Can we get an op amp with 2 pole compensation and have massive loop gain and a reasonable bandwidth?

Monolithic IC's pretty much all use single pole compensation because the capacitor required to implement it is small and there is only one of them required. Putting capacitors inside of IC's is a tricky business, as the capacitor consumes a large amount of the space inside of the IC.

Its capacitance value must be kept small in order to make it fit. Two pole compensation is nearly impossible to implement in an IC, because it requires at least two capacitors for compensation, and the capacitance value tends to be larger than the value required for single pole.

The only chance we have of getting a two pole compensated op amp, is to build one ourselves from discrete components. Such devices exist, fabricated from tiny surface mount components and are physically compatible with popular IC packages like the DIP8.

Such a device can be found here —. To the right are loop gain plots comparing a single pole compensated op amp to the same op amp compensated with a two pole scheme. Red flags might be popping into the mind. If this can be accomplished, the Nyquist stability criteria will still be met.

This is happening and can be seen on the prior graph by how the two pole gain plot overlaps the single pole plot starting around the 1MHz region of the curve. This ensures that there will be ample phase margin to ensure stability. To see the two pole compensated phase plot, refer to the graph on the right. This can be seen in the following plots that illustrates what happens as the loop is closed. It can be seen from the plot that as the loop is closed with external feedback, the poles move outward.

This little gem of a truth often goes unrealized, which accounts for a lot of the apprehension associated with feedback and high loop gains. Conclusion: As one navigates the field of op amps in their quest for the perfect device, here are a few things to keep in mind. Get all the Sparkos Labs good good.

Right in your inbox Your email [email protected] Submit. Discrete Op Amps, and why they are superior to ICs. We read reviews, check out forums, and in our frustration we seek advice from the guy on the right - Actually, the LM is about the worst device imaginable for hi-fi audio use.


Operational Amplifier Frequency Response

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Use in operational amplifiers[edit] · The open loop gain of the amplifier is ≥ 1 and · The difference between the phase of the open loop signal and phase.

Audio Power Amplifier Frequency Compensation


There are few audio frequency designs today that do not use operational amplifiers op-amps, or just 'opamps'. Over the years, the poor opamp has been much maligned, with mainly spurious claims about 'audibility', distortion, and other so-called defects. There are even people who will compare the bass performance of opamps, which is IMO lunacy - all function perfectly to DC, and none will be found to be lacking in low frequency performance i. Note that although the term 'audio' is used throughout this series, it doesn't necessarily mean audio in the traditional sense. Countless industrial processes operate within the same frequency range, so when you see the term 'audio', it generally means 'audio frequency' - and covers the range from DC up to perhaps kHz or so. Some of the more basic opamps do have limitations which make them less than desirable in some cases, but most of the new breed are unsurpassed for linearity, with total harmonic distortion figures as low as 0. This can be important for industrial processes as well as hi-fi, because very high linearity also means the potential for very high accuracy. Even the most basic types still have their uses in simple low-speed control circuits and other non-demanding applications.

Hifiman Arya Review (headphone)

audio amplifier frequency compensation in op

An operational amplifier op-amp is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. Operational amplifiers had their origins in analog computers , where they were used to do mathematical operations in many linear, non-linear and frequency-dependent circuits. Characteristics of a circuit using an op-amp are set by external components with little dependence on temperature changes or manufacturing variations in the op-amp itself, which makes op-amps popular building blocks for circuit design. Op-amps are among the most widely used electronic devices today, being used in a vast array of consumer, industrial, and scientific devices. The op-amp is one type of differential amplifier.

Circuit Stability Precautions : Power Supply Decoupling — Feedback along supply lines is another source of op-amp circuit instability.

Inverting op-amp


As noted in the previous section, general-purpose op amps contain a compensation capacitor that is used to control the open loop frequency response. The signal developed across this capacitor will be amplified in order to create the final output signal. In essence, this capacitor serves as the load for the preceding stage inside of the op amp. Like all stages, this one has a finite current output capability. Due to this, the compensation capacitor can be charged no faster than a rate determined by the standard capacitor charge equation:. By definition, this parameter is called slew rate SR.

Inactive Datasheet Archive

Introduction: Numerous opinions and preferences exist regarding op amp selection for hi-fi audio circuits. The question we attempt to answer is "Which op amp is the best for audio applications? We read reviews, check out forums, and in our frustration we seek advice from the guy on the right -. Actually, the LM is about the worst device imaginable for hi-fi audio use. When choosing an audio grade op amp, some opt for high loop gain devices as it is well known that high loop gain makes for a more precise amplifier and lower THD specifications. Others decry the use of high gain devices, claiming the high gain has audible affects, and none of which are desirable.

Frequency Compensation of Op-amp and its types | Circuit frequency modulation, such as radio broadcasting, of an audio signal representing voice or.

Op Amp Slew Rate: details; formula; calculator

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5.4: Slew Rate and Power Bandwidth

RELATED VIDEO: Frequency compensation Techniques -- Pole Zero Compensation in Op-Amp -- LICA U-2-9

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Designing With Opamps - Part 1

Q: ADI has published a lot of information on dealing with capacitive loading and other stability issues in books, such as the amplifier seminar series, in earlier issues of Analog Dialogue , and in some design tools. But, I need a refresher—NOW. Although some capacitive loading is inevitable, amplifiers are often subjected to sufficient capacitive loading to cause overshoots, ringing, and even oscillation. The problem is especially severe when large capacitive loads, such as LCD panels or poorly terminated coaxial cables, must be driven—but unpleasant surprises in precision low-frequency and dc applications can result as well. As will be seen, the op amp is most prone to instability when it is configured as a unity-gain follower, either because a there is no attenuation in the loop, or b large common-mode swings, though not substantially affecting accuracy of the signal gain, can modulate the loop gain into unstable regions.

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