M.Joshi ) writes:
Hi,

I was recently in a job interview and was asked to describe an OP-Amp
circuit.

The OP-Amp configuration consisted of negative feedback with a resistor
and capacitor connected in series to ground from the inverting input.

I haven't seen this configuration before and I was asked the purpose of
the capacitor?

What purpose does any capacitor have? To pass AC and not let DC through.
SO when it's used as a coupling capacitor, it lets the AC signal pass
between stages, but blocks the DC from the first stage from getting into
the second stage where it would upset bias. When used as a bypass capacitor,
it in effect shorts the AC signals to ground, without shorting out
the DC voltage.

Of course, a capacitor's ability to pass AC varies with frequency and
capacitance. A small value capacitor won't do well at passing low
frequencies, while a high value capacitor will.

Now given that information, that should have been covered in the books,
why do you think the capacitor is there? Think about why it might
be different from having the resistor going directly to ground.

Michael