The frequency response of the preamplifier circuit is shown in Figure 8-3. The –3-dB frequencies are 15.87 Hz and 181.1 kHz, which meet the design requirements. The gain within the passband of the circuit is 18.9 dB, slightly less than the design goal of 20 dB. The reduction in gain is a result of the voltage division between the output resistors of the piezo preamplifier circuit and the input impedance of the microphone preamplifier. The A-weighted noise of the circuit (referred to the input) is 842.2 nV_RMS or –119.27 dBu.

Figure 8-3 Frequency Response of the Preamplifier Circuit for a 8-nF Piezo Element