How does a precision rectifier work? the one with a single diode.
When the input goes positive, the output of the op-amp is a large positive signal which turns on the diode and completes the feedback path. what i don't understand is what happens to the initial output of the op-amp which was the signal multiplied by the large gain A.
Thanks in advance
Nothing happens instantaneously - everything takes time - as the output of the amplifier rises the diode starts to conduct and feeds back the signal to produce a unity gain output.
So what happens after the op-amp produces a unity gain output, how does the diode conduct .
Assuming that the peak of the input signal is less than 0.7V.
Diodes conduct some current at all voltages other than zero. However, in the forward direction, the current depends exponentially on the voltage. See for example Current vs. voltage properties of a diode
The feedback voltage appears at the cathode of the diode, not at the anode (opamp output). So it doesn't really matter what appears at the opamp output provided that the voltage at the diode's cathode matches what is fed into the positive terminal of the opamp. ie unity gain. You have to consider the diode is part of the opamp system, rather than as a separate discrete component.
adarsh94:
So what happens after the op-amp produces a unity gain output, how does the diode conduct .
Assuming that the peak of the input signal is less than 0.7V.
The opamp always act to keep the inputs equal (assuming feedback path sense
is negative). If the input is 0.1V, the output will rise until the cathode also
equals 0.1V, the opamp output/diode anode will be about 0.7V of course, but we
aren't interested in that.
If the input goes negative the output can no longer feed back since a reverse biased
diode is effectively open circuit - in other words the circuit is effectively different
and no feedback operates. The opamp input voltages diverge.
Its worth noting that circuits like this where the opamp inputs can be very different
don't suit every opamp, some require that the voltage difference between inputs is
limited (in the abs-max specs). IIRC some actually draw (relatively) large currents
through the inputs when the input voltage is large (via protection components in the
input stages). So read those datasheets!
Description:
Why you should use another diode to speed up the circuit:
