Analogue Electronics Design (3) Case Study #1 - Section 3: Preamplifier response optimization and trade-offs

摘要

If you are new to this series, reference should be made to the previous installments [1],[2]. For all readers to benefit from, the circuit of the microphone preamplifier we’re about to discuss was extracted from the Bat DetectorPLUS full schematic and is shown separately in Figure 1. Flashlights on and let’s do some more design analysis on it. Because of the very high fixed gain and the low supply voltage of the preamplifier, there is a chance that the output will saturate and clip, and when signals are too small, and/or frequency dependent, the gain may be insufficient. The dynamic range is determined by the maximum undistorted output signal and the smallest usable signal well above the noise. Unlike ADCs and DACs, S/N is not the same as dynamic range. The S/N of microphones is usually specified at 94 dB sound pressure (1 pascal), but for most microphones there is also a linear relationship between sound pressure and output voltage, usually up to 120 dB. Then there is still minimal distortion and therefore the dynamic range is considerably greater than the specified S/N. For the bat detector, it is obvious that adding a fast, automatic gain control adapted (perhaps even limited) to the source signal would be a good addition to further increase that range. This could be something quite simple, like the classic circuit with a JFET as variable resistor. This will also improve the detection of the modulation. There is hardly any danger of feedback (howl) if the high frequencies are filtered out properly at the power amplifier, preventing the loudspeaker from reproducing them, if that should be a problem at all. Unfortunately, despite the frequency separation, both harmonics in the output signal of the power amplifier and the reproduction by the loudspeaker can result in feedback from the loudspeaker to the microphone. This is a mechanical problem, because even the best loudspeaker is not ideal and will distort the sound. In the case of the bat detector, there are of course no requirements for really small signals as with an audio amplifier. A signal-to-noise ratio of 20 dB in the signal going to the comparator is likely to suffice.