Differential amplifier with zener diodes

[Femme_physics]

Ah...so basically those formulas I wrote don't apply since this is a closed loop gain, yes? Gotcha.Technician, can you write me the formulas you applied to solve it? I don't appear to have them written. Thanks a bunch, you're a world of help

I'd like to emphasize the distinction between open-loop-amplification and feedback-amplification.

The output voltage is then Vout = A(V1 - V2). A can be very high, almost always greater than 105, and typically closer to 106. The actual gain from chip to chip, even in the same batch of chips can vary quite a bit. But it's so large that it usually doesn't matter since op-amps are rarely used open-loop fashion, except as comparators.

Why not? What's the practical difference/advantage of closed loop to open loop?

 

[technician]

I think I have misread the various Vs... I will go through again

 

[Femme_physics]

I think I have misread the various Vs... I will go through again

What do you mean? I got the correct solution by copying your text, but I'm unsure what formulas you've used. I'm just trying to reverse-engineer the logic

 

[technician]

FP
everything is OK The Vout for the differential amplifier is = A(V1-V2) when the ratios of Rf/Rin and R2/R1 are equal.
In my last post I should delete that you must add on V+...that works its way through with the equal ratios. I will go back and edit that post so there is no confusion.
If the ratios are not the same then you cannot use the simplified formula
Vout = A(V1 - V2)

 

[Femme_physics]

Then it's all sorted out. Thanks , everyone!

 

[gneill]

The output voltage is then Vout = A(V1 - V2). A can be very high, almost always greater than 10⁵, and typically closer to 10⁶. The actual gain from chip to chip, even in the same batch of chips can vary quite a bit. But it's so large that it usually doesn't matter since op-amps are rarely used open-loop fashion, except as comparators.

Why not? What's the practical difference/advantage of closed loop to open loop?

Well, as I stated in the next paragraph the greatest advantage is precise control over the circuit gain and thus the circuit's behavior. Imagine how difficult it would be to manufacture something if no two circuits behaved exactly the same way because the gain of the components could not be controlled. You'd probably have to reject half the products that come off the assembly line.

Another reason to "close the loop" is that you can do some clever things by putting more than just resistances in the feedback path. Some powerful filters can be made by placing reactive components (capacitors, inductors) in the loop. You may cover this in your course. It's also possible to make circuits that take square roots or logs of the applied voltage! The op-amp with feedback is the building block of analog computing.