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Im trying to understand how a super diode works. I understand that when the input is negative, the opamp output is positive, this forward biases the diode, and sends all of the current to the opamp output. What i dont understand is the simulation results i am seeing. It takes a while for the negative swing to be rectified. This happens regardless of frequency or input voltage. Does someone know why this is happening? I am using an ideal OpAmp in this simulation.red is output at top node. Blue is input.

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up vote 2 down vote accepted

Answered my own question as i looked deeper into it. From the text book:

The rectifier circuit shown above has the property that for positive inputs the operational amplifier output saturates in the negative direction.

– The op-amp output voltage is required to change instantaneously from this saturated voltage, (Vs-) to +0.6 V when the input waveform passes through zero.

– Because of the limited slew rate of real operational amplifiers this cannot occur, and the output waveform will not be a precisely rectified version of the input waveform as the frequency of the input sinusoid is increased.

Ill leave this here in case someone has the same question in the future.

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As an FYI, the time for an OP-amp to come out of saturation can be much, much longer than would be predicted from the slew rate. – Spehro Pefhany May 6 '14 at 6:50
Fair enough, but 2 msec? What the bleep kind of model is being used? – WhatRoughBeast May 6 '14 at 15:02
WhatRoughBeast, i think Sphero answered this discrepancy in his comment above – scordova88 May 6 '14 at 16:02

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