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Wave Shaping

# Precision Limiter

The precision limiter frequently used in the analog preprocessing circuitry is shown in the figure below. In this circuit, the high open-loop gain of the operational amplifier (op-amp) is used to reduce the effect of the diode nonlinearity and the temperature sensitivity. Using the usual op-amp relationships, the circuit can be analyzed.

For *v*_{3} > 0 (*v*_{i} < 0), the current
*i*_{3} will be zero because *D*_{2} is reverse-biased.
If the infinite amplifier gain is considered, all input current
*i*_{1} flows through *R*_{f} to *D*_{1}
(forward-biased) and *R*_{L}, generating an output voltage

The effect of the diode forward voltage *v*_{D1}
is reduced by the loop gain of the closed-loop circuit. The
"rounding" of the turn-on region therefore virtually disappears.

For *v*_{3} < 0, diode *D*_{1} no longer conducts
and all the input current *i*_{1} flows through *D*_{2}.
Theoretically, the output voltage is then exactly equal to zero.
Thus, the precision limiter provides a good approximation of ideal diode
behavior, reducing the diode nonlinearity, temperature sensitivity,
and forward voltage drop by a factor equal to the loop gain of the amplifier.
The transfer curve for the precision limiter is given in the figure below.