The classic peak detector in Figure 1a lacks the necessary performance to drive an audio-level indicator. Because of the amplifier’s slew-rate limitations, this circuit suffers a reduction in detected output with increasing frequency. The amplifier acts as a comparator. The output is low most of the time but slews up to "catch" the peak of the input signal when the voltage at the positive input exceeds that at the negative input. The amplifier output is, thus, a slew-rate-limited voltage spike. As frequency increases, this catch point occurs after the peak has passed; as a result, the held output demonstrates a frequency-dependent error.

You can add an additional diode (D_X) and resistor (R_X) (Figure 1b) to eliminate the slew-rate error. This circuit eliminates the error by keeping the amplifier output active during the positive half cycle of the input signal. Now, circuit error is due to the different conductances of the diodes, and only the frequency response of the amplifier limits the circuit’s bandwidth.

A transistor replaces both diodes in the actual tested circuit (Figure 1c). This transistor, which functions both as a switch and two back-to-back diodes, eliminates the conductance error and reduces the number of components. Note that the input-voltage divider prevents the negative portion of the input signal from causing the amplifier output to go to the supply rail. Figure 2 shows the improved frequency response. (DI #1815)