Thank you.
Is this a high-pass filter? (When the cap is on the positive side of the output, right? Or not?)
And which formula is used to work out the phase shift?
Thank you for your time.
Kind regards,
Marius
Don't get too concerned about the capacitor on the output, but you just want to test that the supply is still stable with a capacitor on the output. I haven't worked out any phase shifts. I have built similar circuits before, so I just used seat of the pants reasoning. The transistor circuit has a gain of 22, and I know the opamp is stable when the total loop gain is unity. So the first thing is to say at high frequencies, I want the total gain to be one, so I want the opamp to have a gain of less then 1/22. So I set the resistor in the compensation circuit for a gain of 2k2/100K = 1/45. This is a lot less then 1/22, so I am happy with that.
Next the capacitor in the compensation circuity sets the frequency the gain of the op-amp circuit wants to fall off. The bigger the capacitor, the slower the feedback circuit will work, so the idea is to make it as small as possible. The transistor circuit will start to add extra phaseshift somewhere near 1MHz, so you want the opamp to be at a gain less then 1/22 by then. 220pF sounds about right, but if you are not sure. make it a bit bigger. try not to let it get above 1nF or it will degrade the regulation.
Does that make sense? It is probably easier to understand if replace the dc voltage source with a 100mV AC source with a 10V DC offset and do an AC analysis. You can then see what is happening with phase shifts around the circuit.
Richard.