Kevin Gaughan wrote:
> I'm using a textbook op amp precision rectifier circuit( see below)
> to half wave rectify an input signal.
> With a reasonably fast TL084 OP amp the output starts to look bad at as
> low as 10kHz input. The problem being that at zero crossing the opamp has
> to slew from + a diode drop to - a diode drop. This gives a kink in my
> waveform around zero. I am going to try using schottky diodes and a
> faster op amp but does any one know a better circuit?
> (If the picture looks funny try lining it up so that every line starts
> with an x)
> x                --------[RRRR]--------@---- to buffer
> x               |                      |
> x               |       |\            _|_
> x               |       |  \          /_\
> x in--[RRRR]----@-------|-   \         |
> x               |       |      )-------@
> x               |   ----|+   /         |
> x               |  |    |  /           |
> x               |  |    |/            _|_
> x               | _|_                 /_\
> x               |  -                   |
> x               | _____________________|
> x
I have built one for a limiter at the radio sation where I work.  It
worked well up to 20kHz.

Add a diode on the output of your circuit to match the drop of the diode
in the feedback loop.  Also the input resistor needs to be slightly
larger than the feedback resistor to match the resistance of the diode. 
I put a variable resistor in the feedback loop to adjust for unity
gain.  If you want to get extremely high prescision, the load after the
output diode needs to match the load of the feedback diode.  Place a
variable resistor from the output diode to ground and drive the
non-inverting input of a second op-amp in a unity gain config.  You
should be able to achieve rectification tracking error of less than


Date: Mon, 18 Nov 1996 19:11:58 -0800

Original Subject: Re: Precision rectifier circuits.

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