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What will happen if an amplifier's gain is not stable? I can understand it may drive the amplifier into saturation, any other reason?

What if the gain is changing in such a way that the amplifier stays in active region, in that case will any distortion occur?

Likely not. But then what kind of problem may arise in the second case?

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  • \$\begingroup\$ I think you need to give a concrete example here. Certainly if the gain changes quickly enough there'll be distortion -- in fact, an RF mixer (not audio -- "mixer" means something different there) effectively changes it's "gain" at the local oscillator frequency, and that's exactly how frequency shifting is effected. \$\endgroup\$
    – TimWescott
    Jan 8, 2021 at 20:22
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    \$\begingroup\$ If you were watching a video, would you like the brightness and volume to change outside of your control? Unwanted changes is the amplitude of a signal is a form of distortion, even if it is not harmonic distortion. \$\endgroup\$ Jan 8, 2021 at 20:24
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    \$\begingroup\$ Sometimes the gain is dependent on the signal level; and that is deliberate, in the case of AGC, or compressors, limiters and noise gates. \$\endgroup\$
    – user16324
    Jan 8, 2021 at 20:54

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I'll just try and answer the main thrust of your question, as I understand it. But first let me add that I believe you are asking about a gain that varies with the signal itself, as you mentioned "staying in active region" and "drive the amplifier into saturation." I also suspect you are talking about a single-stage case (or, at least, a grouping that can be treated similarly), but I'll try and broaden out the answer to include a small nod to a larger system. Finally, I think you are referring to the common case of voltage gain variation with signal.

A single amplifier stage almost always has some variation of voltage gain. Sometimes, the design goal for a single stage is to minimize that variation. But there are also times where the design goal is to maximize the voltage gain at almost any cost, because the designer also intends on using global negative feedback to perform two key purposes: establishing a precision overall voltage gain as well as correcting distortions in the signal caused by gain variations in individual stages.

The classic case of a well-designed amplifier stage where the voltage gain varies quite widely with the signal, and therefore highly distorts the signal -- but acceptably so when global negative feedback is applied across multiple stages in order to correct that distortion -- is this AC-grounded BJT amplifier stage with single-ended input and output:

schematic

simulate this circuit – Schematic created using CircuitLab

The voltage gain itself varies substantially with the signal because the emitter current varies significantly and \$r_e^{'}\$ itself varies inversely with that same emitter current. Even designed well, any significant, finite input signal will be noticeably distorted at the output.

However, that type of stage is still used because it also provides high voltage gain and it can be made to function well within a larger amplifier system if appropriate global NFB is included in the final amplifier design.

But please note that this type of amplifier should be only considered in cases where global NFB is intended or else where the distortion itself is acceptable for the purposes.

Even in a full amplifier system, there will be signal-dependent voltage gain variations. But these can always be reduced to an acceptable level, with attention to details.

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in that case will any distortion occur? Likely not.

Definitely yes.

"Distortion" is anything that's not a linear, stable amplification. So, by definition, gain that varies is distortion.

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    \$\begingroup\$ "Gain that varies is distortion"....or is it better (more accurat) to say: Input-output relationship that is nonlinear creates distortion? \$\endgroup\$
    – LvW
    Jan 9, 2021 at 12:12
  • \$\begingroup\$ @LvW yes, that is a very sensible way of putting it, but it neglects that time-variance is also. \$\endgroup\$ Jan 9, 2021 at 12:16
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What will happen if an amplifier's gain is not stable?

If the gain changes this can cause instability (if you are switching gains or have an active gain) for this reason the majority of gains for amplifiers are fixed to known values or unchanging.

Stability goes hand in hand with amplification, especially in the case of feedback and negative feedback.

If you have positive feedback then this can drive the output of the amplifier to the rails.

Another problem is positive feedback at certain frequencies. If an amplifier is placed in negative feedback, the phase of the output (when the gain is more than one when the phase is 180deg) the feedback loop can also become unstable.

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I have some problems with your wording "stable gain". To me, a circuit can be stable or unstable. Perhaps the gain can "drift" due to temperature or other influeces, but I suppose that is not what you have in mind. And - because "linearity" is mentioned: There are no amplifiers which are strictly linear.

Hence, for amplification we select a certain operational point which - together with a limited input signal - can allow "quasi-linear" operation (amplification). When the input signal is so large that we can detect/measure some distortions at the output, the gain has NOT changed - because the gain is defined using the slope of the transfer curve (input-output) at the fixed operational point.

However, I am aware that this point can be discussed because it involves the question (and definition) of the term "instantanious gain". This is - more or less - a question that touches the "philosophie of electronics".

But it is interesting to ask "What happens for positive feedback"? There are three cases to be considered:

  • Loop gain still below unity: We still have a stable gain (somewhat larger than without feedback)

  • Loop gain equal to unity: Oscillation condition

  • Loop gain larger than unity: Unstable biaspoint, saturation (even without any input signal) - the definition of "gain" is meaningless.

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