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Wave Shaping

Precision Limiter

The precision limiter frequently used in the analog preprocessing circuitry is shown in 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.

Precision limiter
Basic circuit of the precision limiter.

For v3 > 0 (vi < 0), the current i3 will be zero because D2 is reverse-biased. If the infinite amplifier gain is considered, all input current i1 flows through Rf to D1 (forward-biased) and RL, generating an output voltage

Output voltage

The effect of the diode forward voltage vD1 is reduced by the loop gain of the closed-loop circuit. The "rounding" of the turn-on region therefore virtually disappears.

For v3 < 0, diode D1 no longer conducts and all the input current i1 flows through D2. 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 figure below.

Transfer curve of precision limiter
Transfer curve of precision limiter.



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