Little GEM

[AndrewT]

got it to run.
Thank you.
The square wave is applied to the normal audio input and passes the normal RF attenuator. This slows down the step response of the sq considerably.
To stress the circuit more, the RF attenuator should be opened up a lot, maybe a factor of 10, or maybe a lot more. Changing C2 to 22p may give a different result, But it may be that 2pF is more appropriate. But don't go to far. An ultrafast step of the squarewave may be too stressful. How do we adjust the pulse parameters to change the slew of the pulse?

[AndrewT]

@ 22pF I see some ripples in the square wave? Is this feedback leaking past the input transistor affecting the pulse signal?
@ 2pF the negative going has gone haywire. Is the pulse now too fast? I see a rise time of ~20ns?

[dadod]

Originally Posted by AndrewT

got it to run.
Thank you.
The square wave is applied to the normal audio input and passes the normal RF attenuator. This slows down the step response of the sq considerably.
To stress the circuit more, the RF attenuator should be opened up a lot, maybe a factor of 10, or maybe a lot more. Changing C2 to 22p may give a different result, But it may be that 2pF is more appropriate. But don't go to far. An ultrafast step of the squarewave may be too stressful. How do we adjust the pulse parameters to change the slew of the pulse?

You can change the generator rise and fall times.

[wahab]

In principle such a computation is made without the input high frequency
attenuation capacitor and without output LR filter, this to check if the amp
is unconditionaly stable.

Indeed , most amps rely on theses filters to be stable as conditionnaly
stables amps allow to reduce THD by the virtue of greater HF loop gain.

[dadod]

Originally Posted by wahab

In principle such a computation is made without the input high frequency
attenuation capacitor and without output LR filter, this to check if the amp
is unconditionaly stable.

Indeed , most amps rely on theses filters to be stable as conditionnaly
stables amps allow to reduce THD by the virtue of greater HF loop gain.

What you suggest to be rise and fall time, 10nsec is to short? If I use 200nsec and remove input filter all is OK.
dado

[wahab]

10ns should be enough , it s about the value of average real world generators.

So far the few sims i made with your amp show a tendency to oscillate
with relatively small capacitive loading , i ll recheck using the models you
did provide with the LT Spice file.

[dadod]

Originally Posted by wahab

10ns should be enough , it s about the value of average real world generators.

So far the few sims i made with your amp show a tendency to oscillate
with relatively small capacitive loading , i ll recheck using the models you
did provide with the LT Spice file.

I've got better square wave behaviour with change in LTP CCS, I moved C8 from base-emitter to base- collector (Q1) and set for 150pF.
I am not sure what capacitance value is standard for that kind of test. For big capacitance value there is L//R to protect from oscilation. Already small resistance(0.22ohm) in the output will cancel most of oscilation. JLH used that in his 80W mosfet amp.

[Waly]

Originally Posted by wahab

In principle such a computation is made without the input high frequency
attenuation capacitor and without output LR filter, this to check if the amp
is unconditionaly stable.

Indeed , most amps rely on theses filters to be stable as conditionnaly
stables amps allow to reduce THD by the virtue of greater HF loop gain.

What do you mean (analytically) by "unconditionally stable" and "conditionally stable"?

I don't think the input filter has much to do with the overall stability (in the small signal sense, OLG, etc...). Some large signal effects may happen during very fast transients, but these are beyond the scope of the current discussion.

The output filter effect on stability is relevant only in the context of reactive loads.

[dadod]

Originally Posted by Waly

What do you mean (analytically) by "unconditionally stable" and "conditionally stable"?

I don't think the input filter has much to do with the overall stability (in the small signal sense, OLG, etc...). Some large signal effects may happen during very fast transients, but these are beyond the scope of the current discussion.

The output filter effect on stability is relevant only in the context of reactive loads.

This amp, of course, is not unconditionally stable. It is not stable at the gain of one, as this is not an opamp it is not needed to be. It is design to work with the gain of 28 dB with good gain margin, and it is very stable, but I like to hear different opinion and suggestion how to test it, and to improve it.
dado

[Waly]

Originally Posted by dadod

This amp, of course, is not unconditionally stable. It is not stable at the gain of one, as this is not an opamp it is not needed to be. It is design to work with the gain of 28 dB with good gain margin, and it is very stable, but I like to hear different opinion and suggestion how to test it, and to improve it.
dado

If we accept this definition of "unconditionally stable" (an amp stable at any closed loop gain), then a "conditionally stable amp" like yours will always have some pulse response overshoot (such a "conditionally stable" amp is necessary an at least second degree underdamped system - see the phase response with a dip before ULG, this phase dip is what creates instabilities at e.g. low CLG). This overshoot can be tamed using an input filter, but then it is not realistic to expect, in simulation or practice, perfect step response without one.