How works the feedback with 220u Cap to earth?

[Progg70]

Hi I'm trying to build a power amp. I'm now at the feedback coupling.

In the picture; what is the cap, 220u to earth for in the feedback? Is it necessary?

What is the theory for the most common feedbacks, like the one in the picture?

[djk]

Reduces amplifier gain to unity at DC, used to reduce
DC off-set.

If your amp has say 10mV off-set with the cap, and say 32dB of gain, it would have 400mV off-set if you removed the cap.

[dhaen]

Without that cap, the AC feedback will be 100% leading to an unstable amp with a signal gain of only 1.

The cap does not affect the DC feedback, meaning that a servo mechanism is achieved to keep the DC operating point stable. The cap decouples some of the AC feedback to the ratio off 22K/499.

[dhaen]

djk is assuming you shorted the cap (I think).
I am assuming removing the cap.

[djk]

Correct. It looks like he has a DC gain of X45 (33dB) with the cap removed (shorted).

[Progg70]

Thanks I'm learning.

And what’s happened if I don’t have the caps and no earth connection…

[djk]

Unity gain

[janneman]

Originally posted by Progg70
Thanks I'm learning.

And what’s happened if I don’t have the caps and no earth connection…

With these situations, rule of thumb is that the signal on each side of the input pair (base) is equal. The amp is set up to try to get to this situation. So, if you have no cap (removed, not shorted), as the feedback base of the input pair is Vin, and there is a direct connection to Vo, without any divider, Vo is forced to Vin as well. Unity gain indeed.

Whenever you start to divide the feedback down, you need more Vo to get Vin on both sides of the input pair so up goes your gain (as long as the amp gain itself is sufficient of course).

If you use the cap in the divider, you block DC, so for DC it looks as if there is NO divider, so for DC Vin = Vo -> unity DC gain.
Since the cap starts to come into play well before DC, you would measure a decreasing gain from a frequency where the cap impedance = R value of the cap series resistor. That point, where Zcap = R series, is the lf 3dB point.

Jan Didden