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.

Basic circuit of the precision limiter.

For v₃ > 0 (v_i < 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 R_f to D₁ (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 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 the figure below.

Transfer curve of precision limiter.