Question about opamp gain

heyheyletsgo

Joined Nov 21, 2018
24
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.

Joined Mar 10, 2018
4,057
First and second config freq response -

heyheyletsgo

Joined Nov 21, 2018
24
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?

Last edited:

Papabravo

Joined Feb 24, 2006
15,700
None of the diagrams represent an open loop situation.

heyheyletsgo

Joined Nov 21, 2018
24
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?

Joined Mar 10, 2018
4,057
Third item -

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

Regards, Dana

Papabravo

Joined Feb 24, 2006
15,700
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

$$C \rightarrow \infty , \;X_C \rightarrow 0$$

Last edited:

Joined Mar 10, 2018
4,057
U1, U3, are open loop at DC. So G = OpAmp Aol.

Regards, Dana.

Papabravo

Joined Feb 24, 2006
15,700
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.

heyheyletsgo

Joined Nov 21, 2018
24
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?

Joined Mar 10, 2018
4,057
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.

heyheyletsgo

Joined Nov 21, 2018
24
Ah I see now So with U3 the feedback ratio holds throughout the entire bandwidth.

Joined Mar 10, 2018
4,057
No U1 fdbk ratio is fixed, its U3 that changes.

Regards, Dana.

Joined Mar 10, 2018
4,057
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.

Wendy

Joined Mar 24, 2008
22,358
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.

Wendy

Joined Mar 24, 2008
22,358
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.

crutschow

Joined Mar 14, 2008
26,963
Also can someone explain in laymens how attenuating low frequencies increases opamp stability ?
Where did you read that?

heyheyletsgo

Joined Nov 21, 2018
24
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?

Papabravo

Joined Feb 24, 2006
15,700
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.