I have a question about the gain of opamps in different configurations.

I've seen these 3 times of opamp compensation around but I do not know what the difference is.
How does each configuration affect the gain of the opamp at different frequencies?
I would prefer an answer with as little math as possible
Thanks.

 

First and second config freq response -

 

It's actually U1 and U3 I'm most interesting in.
To my novice eyes, U1 appears to be purely open loop with HF attenuation and U3 appears to be set at a specific gain with HF attenuation, but in spice U3 has more loop gain.
What makes U3 different from U2?
Why doesn't U1 provide the most gain?

 

None of the diagrams represent an open loop situation.

 

I meant the loop gain.
The U1 ppears to have all of its gain available for use at lower frequencies because if you took away the capacitor it would be open loop and the capacitor should attenuate high frequencies.
U3 to me seems functionally the same as U2 but with more HF attenuation.
Why am I wrong and why does U3 offer more gain then U1?

 

Third item -



U3 vs U2, it adds a zero in the transfer function.

Regards, Dana

 

The capacitor does not attenuate high frequencies -- it passes them. It blocks DC and LOW frequencies. The formula for reactance of a capacitor is:

\(X_C\;=\;\frac{1}{2\pi fC}\)
As

\( f \rightarrow \infty , \;X_C \rightarrow 0\)
And as

\( C \rightarrow \infty , \;X_C \rightarrow 0\)

 

U1, U3, are open loop at DC. So G = OpAmp Aol.

Regards, Dana.

 

U1, U3, are open loop at DC. So G = OpAmp Aol.

Regards, Dana.

Even the smallest difference above or below ground for the input will cause the output to saturate and the capacitor will charge(discharge) through the output impedance of the amplifier.

 

If the low frequencies are open loop then why does U3 have more gain?

Also can someone explain in laymens how attenuating low frequencies increases opamp stability ?
Does it have to do with the phase shift that seems to be apparent in U1?

 

At lower f the effective feedback factor is lower so there is more
G, at that lower freq. But at high f where cap is a short G should
converge to ratio of resistors.

Regards, Dana.

 

Ah I see now So with U3 the feedback ratio holds throughout the entire bandwidth.

 

No U1 fdbk ratio is fixed, its U3 that changes.

Regards, Dana.

 

Correction. U1 G is constanlty decreasing with f.

U3 has higher initial G because the Xc contributes to higher
fdbk Z, then drops when the zero breaks at w = 1/RC.

Regards, Dana.

 

Slew rate issues can often resemble gain issues even though they are different things, just muddling the issue, op amps are fun to play with.

 

Slew rate issues can often resemble gain issues even though they are different things, just muddling the issue, op amps are fun to play with.

 

Also can someone explain in laymens how attenuating low frequencies increases opamp stability ?

Where did you read that?

 

The capacitor does not attenuate high frequencies -- it passes them. It blocks DC and LOW frequencies. The formula for reactance of a capacitor is:

\(X_C\;=\;\frac{1}{2\pi fC}\)
As

\( f \rightarrow \infty , \;X_C \rightarrow 0\)

Increasing the cap value usually makes thing more stable right?

 

Increasing the cap value usually makes thing more stable right?

Whoa! There is nothing in the three circuits that even hints at "unstable" behavior. Unstable means the output increases without limit, usually exponentially. Mathematically it means the roots of the characteristic equation are located in the right half-plane. Increasing the capacitor value just reduces the reactance in the same way as increasing the frequency.

 

But in an unstable circuit increasing the cap value improves the phase margin right?

 

An integrator is a single pole roll off and such a system is inherently
stable. But throw in an OpAmp with one or two poles and thats a whole
new story.

https://www.analog.com/en/analog-di...-to-avoid-instability-capacitive-loading.html

Its not always throw in a cap, sometimes an R to use with
a C to change phase margin.


Regards, Dana.