Exactly - and will the answer be Av=-2 as required ......
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Op amp not respecting the gain?
- Thread starter rougie
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Some clarification might help.
Firstly, you indicated in an earlier post you are interested in the gain from input V1 to the output. You stated you weren't interested in the differential gain V1-V2 to the output.
I would define the voltage gain for this situation [i.e. gain relative to changes in V1] as ....
Av1=ΔVout/ΔV1
where ΔV1 indicates a specified change in the input and ΔVout the observed change in the output.
Experiment (thought or otherwise):
With V2 fixed at 5V, change V1 from 1V to 2V [ΔV1=+1 volt]. What is the corresponding change in Vout? What is the corresponding gain [Av1] as I indicated above?
Firstly, you indicated in an earlier post you are interested in the gain from input V1 to the output. You stated you weren't interested in the differential gain V1-V2 to the output.
I would define the voltage gain for this situation [i.e. gain relative to changes in V1] as ....
Av1=ΔVout/ΔV1
where ΔV1 indicates a specified change in the input and ΔVout the observed change in the output.
Experiment (thought or otherwise):
With V2 fixed at 5V, change V1 from 1V to 2V [ΔV1=+1 volt]. What is the corresponding change in Vout? What is the corresponding gain [Av1] as I indicated above?
Yes - correct. However, as mentioned before, the method using diff. and common mode separately is more complicated. I always prefer (for opamp circuits) separate gain expressions in case of two variable input voltages.
Sorry - it is a "sloppy" name for the classical two-transistor differential amplifier. With a 3rd transistor as a current source in the common emitter path, this configuration has a common mode gain that can be neglected in most cases.
Sorry took so long. work, work, work...
So, if nobody's said yet, differental input is the difference between the two inputs: vd = Δv2 - Δv1 and common mode input is the average of the two inputs, thus vcm = 1/2*(Δv1 + Δv2)
If v2 was held steady at 5V and v1 was increased by 1v,
vd = -1v
vcm = .5v
Each of these components has different gian, equations were given in the blog. If you add them up, they would make would make a gain of -2 for the given senario, just as we calcuated earlier.
I'd probably rather state ...
Δvcm = 1/2*(Δv1 + Δv2)
i.e. the change in common mode voltage.
If V2 was 5V and V1 was 2V, I don't see how the common mode voltage for vd could only be 0.5V. Wouldn't it be 3.5V in that case?
Re-visiting my amplifier in the box proposal, I would offer a second challenge to rougie to devise a set of tests to determine the differential and common mode gains purely from external measurements and no knowledge of what's in the box - other than some form of a linear amplifier.
There is an important point to keep in mind regarding any proposed test of the amplifier in the box with respect to the linear output range. One would have to ensure any input signals were not forcing the output to a limit condition. A possible approach is to make sure the output stays in a sensible range for any inputs applied.
Having made that qualification it is possible to find both common mode and differential gains by experiment. The key is to apply inputs V1 & V2 in such a way that either the common mode value is held fixed or the difference voltage is fixed while the other term is varied as the output is monitored. Can you you think how you could do this?
Having made that qualification it is possible to find both common mode and differential gains by experiment. The key is to apply inputs V1 & V2 in such a way that either the common mode value is held fixed or the difference voltage is fixed while the other term is varied as the output is monitored. Can you you think how you could do this?
O.K. I understand your dilemma.
Perhaps as Brownout suggested in post #34 this might be a good time to take a break and put the matters of common mode and differential mode gains on the "back burner" for the time being.
You initial question about the true gain relative to changes in input voltage V1, has hopefully been addressed.
I'm not sure exploring my challenge any further will actually aid your understanding or simply create further confusion.
Are you studying op-amps as part of a formal course or is this something you are trying to teach yourself independently?
Perhaps as Brownout suggested in post #34 this might be a good time to take a break and put the matters of common mode and differential mode gains on the "back burner" for the time being.
You initial question about the true gain relative to changes in input voltage V1, has hopefully been addressed.
I'm not sure exploring my challenge any further will actually aid your understanding or simply create further confusion.
Are you studying op-amps as part of a formal course or is this something you are trying to teach yourself independently?
You didn't let me down. You can only advance your understanding with further study and experimenting. Hopefully you will have success.
A final comment from me:
The schematic / amplifier we have all been discussing is probably of limited practical use. It might find use as an amplifier with a DC level shifted output. It certainly wouldn't fill the requirement of a true differential amplifier - given it's poor common mode rejection and unipolar operation.
Your idea of studying a range of typical realistic examples might be far more productive in the long term. While you will receive excellent help on AAC forums, it's possibly worth considering attending a formal class on the subject which includes practical workshops as a class activity. Not knowing your circumstances, this may be impractical for you. Just a suggestion.
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Hi rougie, may I give you one recommendation?
Because you try to become familiar with opamps and their applications, I think it is good and necessary to study the effect of feedback (instead of starting with rather complex stuff like common mode and diff. mode expressions).
As you probably know - the gain expression for an inverting opamp with feedback G=-Rf/R1 is an approximation that applies to a frequency range only with very large open-loop gains Aol (Aol approaching infinite).
Thus, it is important to know the correct formula based on the feedback theory and to see WHY and under which restrictions the simplified formula can be used.
As one of the next steps, the question of stability could be studied because negative feedback - in spite of many, many advantages - has one disadvantage: It reduces the stability properties of an amplifier.
Good luck
LvW
During your investigations of non-inverting amplifier configurations it might be interesting to find the bandwidth of the gain stage - in relation to the designed gain value. By doing this you will gain some new insights in "opamp secrets".
