# Measure Operation amplifier problem.

B

#### boki

Jan 1, 1970
0
Dear All,

We often simulation op-amp plus a load capacitor, but in theory, the
AC signal will pass capacitor, if we connect load capacitor at op-amp
output, will the AC output signal pass to ground? or how to explain it
?

Thanks!

Boki.

J

#### John Popelish

Jan 1, 1970
0
boki said:
Dear All,

We often simulation op-amp plus a load capacitor, but in theory, the
AC signal will pass capacitor, if we connect load capacitor at op-amp
output, will the AC output signal pass to ground? or how to explain it
?

Thanks!

Boki.

Often a coupling capacitor is put between an opamp output and its
load, to block the DC voltage from the opamp from being delivered to
the load, while the AC component of the output signal is passes to the
load with little attenuation or phase shift. This means that the
capacitor is carrying the load current but has such a low impedance
that it drops a very small fraction of the total output swing (charges
positive and negative very little voltage by the load current in the
half cycle time of the signal frequency).

If that same capacitor were connected directly between the output and
ground, the opamp would see a very much larger load (have to deliver a
larger current swing to produce the same voltage swing on its output)
because the low impedance capacitor is no longer in series with a
relative higher load resistance. The signal current, has to be larger
to charge the cap positive and negative the whole signal voltage swing
in each half cycle, rather than charging it only a small fraction of
that voltage in that time.

B

#### boki

Jan 1, 1970
0
Thank you very much.

Best regards,

Boki.

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