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Current source feedback capacitor

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OM222O:
That fixed the simulation problems but the main point still remains:

I have no idea what these numbers mean, which defeats the purpose of the simulation  :-DD
What are we looking for here? what is the frequency range to simulate over (i.e: why only 10Hz to 1Meg, not 1GHz for example?)
What does V(b)/V(a) show? aren't we just adding an AC source to the output of the op amp?
a small explanation as to what this all means would be very very helpful since regardless of how much I searched, this makes little sense to me. thanks!

Jay_Diddy_B:

--- Quote from: OM222O on May 22, 2019, 10:49:37 am ---That fixed the simulation problems but the main point still remains:

I have no idea what these numbers mean, which defeats the purpose of the simulation  :-DD
What are we looking for here? what is the frequency range to simulate over (i.e: why only 10Hz to 1Meg, not 1GHz for example?)
What does V(b)/V(a) show? aren't we just adding an AC source to the output of the op amp?
a small explanation as to what this all means would be very very helpful since regardless of how much I searched, this makes little sense to me. thanks!

--- End quote ---

Let me take a much simpler circuit:


This is an non-inverting amplifier with a gain of 2.

I have added a disturbance source V3. The op-amp will try an maintain the output at 2V, 2x the voltage on the +ve input.

If I look at the signals V(a) and V(b) with respect to ground I see:


That most of the disturbance signal appears on V(b), it is trying to reject the disturbance.

There is a very small amount of the disturbance on V(a).

If the opamp had unity gain, equal amounts of the disturbance will appear on V(a) and v(b).

Note that the phase shift between v(a) and v(b) is 90 degrees.

The gain is the amplitude of the at v(b) divided by the amplitude of the signal at v(a). Hence the expression:

Gain = v(b)/v(a)

This is a single frequency measurement take in the time domain.


If I move to the frequency domain, I make this measurement at many frequencies:







This is loop gain.

Since it is the 'loop' gain it can be measured at several place in the loop.

Regards,

Jay_Diddy_B

OM222O:
This makes a lot more sense now! so if I plot and for each range I look at the point where the gain is 0db (i.e gain = 1) and the phase margin is not near -180 degrees, then my circuit is fine, but if it is near or below -180 degrees at that point, then the circuit oscillates. correct?

Jay_Diddy_B:

--- Quote from: OM222O on May 22, 2019, 04:33:08 pm ---This makes a lot more sense now! so if I plot and for each range I look at the point where the gain is 0db (i.e gain = 1) and the phase margin is not near -180 degrees, then my circuit is fine, but if it is near or below -180 degrees at that point, then the circuit oscillates. correct?

--- End quote ---

Yes.

As shown in the examples above the phase axis is phase margin. You should have a phase margin greater than 45 degrees when the gain is 0dB.

Gain margin is the negative gain (attenuation) when the phase margin is 0 degrees. It should be -6db or more attenuation.

If you have a single dominant pole both of these conditions are met.

In the example I gave the dominant pole comes from the opamp.

The opamp gain phase response is given in the datasheet:



The gain is -4dB at 1MHz.

The simple circuit, in my previous message, measures -9dB at 1 MHz. The divider, R1 and R2 provides -6dB of loop gain.



There is a 1dB difference between the model and the datasheet.

If I introduce capacitor C1:



There is now a pole at the frequency

F = 1/(2 x pi x (R1 in parallel R2) x C1)

= 31.8 kHz

If I measure the response:



I get -6dB at 31.8kHz, 0db because the RC filter is at 31.8kHz and -6dB from the divider for a total of -6dB.

In this case the circuit behavior is determined by the passive parts, not the opamp.

Regards,

Jay_Diddy_B


OM222O:
I ran the analysis with the final values of the components and I'm pretty happy with the results (phase is about 94 degrees on most ranges and about 80 degrees on the 10uA / 6th range)

also attenuation is better than -46db when the phase is at 0 degrees (I was not aware of the -6db rule / gain margin before). Again the schematic and the maxim part are attached. can you double check the results so I can be sure I haven't made a bone head mistake somewhere?
Thank you!

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