Can you spot any corrections or errors in the given circuit below:

enter image description here

This is the circuit image for "Transistorized Microphone Preamplifier With Band Pass Filter".

This circuit is a simple and effective solution of the problem with low noise microphone preamplifier with limited bandwidth by passive RC filters.

  • \$\begingroup\$ It is what it is so, what do you intend it to be I.e. what is the design spec that it intends to meet? \$\endgroup\$
    – Andy aka
    Dec 27 '16 at 9:57
  • \$\begingroup\$ To generate low-noise sound amplification once it is out of the microphone. \$\endgroup\$ Dec 27 '16 at 10:05
  • \$\begingroup\$ No, that's just what it does functionally. What performance criteria was it designed to meet? Input impedance, output impedance, pass band, roll-offs, gain, power supply range, output clipping level, distortion etc etc.. \$\endgroup\$
    – Andy aka
    Dec 27 '16 at 10:10
  • \$\begingroup\$ As Andy says, it is what it is. It's pretty crude, it's got a bunch of unnecessary noise sources, its VCC supply appears to be labelled "+VEE" which is ... strange ... yet it'll probably "work". As it's apparently designed to connect to an electret microphone, its presumably not intended to be high quality so none of this matters as long as it meets its specification, whatever that is. \$\endgroup\$ Dec 27 '16 at 11:59
  • \$\begingroup\$ Can you help me modify this circuit? What all are the changes required? \$\endgroup\$ Dec 28 '16 at 10:55

The 220 ohm in series with base of T4, the 100 ohm in emitter of T4, and the rbb' of T4 (internal resistance from bondwire to the actual base-emitter junction) are setting the noise floor. I'd estimate Rnoise (sum of those 3, all affecting the noise across the BE function) to be 1K Ohm, which is 4nanoVolts per rootHertz. [ The DC-feedback 330Kohm is in parallel with 220Ohm Rin, thus we ignore it]

The bandwidth is 250 Hz to 2500Hz. Ignore the 250, and call BW 2500Hz. The total input referred noise voltage is NoiseDensity * sqrt(Bandwidth). Which is 4nanovolts/rtHz * sqrt(2,500) = 4 * 50 nanoVolts, or 200 nanovolts. Input Referred.

Voltage Gain is Rc / Re = 3.3K / 100 = 33. Thus the output noise is 200nV * 33 or 6.6 microvolts, over 250 Hz to 2500Hz. That 33X is also 31dB gain.

The 100 Ohm emitter resistor helps linearize T4's amplification.

I do not see any errors.

  • \$\begingroup\$ The passive filters are so droopy that the highpass and lowpass both combine and reduce the levels of mid frequencies. The scribbled nF looks like uF so the lowpass filter will ne -9dB at 3636Hz and people listening to the chipmunk will hear no important consonant sounds in speech. \$\endgroup\$
    – Audioguru
    Sep 19 '20 at 18:20

The preamp has a very low input impedance and would load down the electret mic. It would be a good preamp for a low impedance dynamic mic (coil and magnet type).

The filters are not active so they simply add to create a droopy response.

1) The highpass filter cuts 242Hz to -24dB and begin cutting frequencies at much higher frequencies. If it was a good active filter then it would make adult male voices sound like chipmunks, but it would reduce low frequency rumbling noises.

2) The lowpass filter cuts all audio frequencies because the capacitor values are way too high. For the 20k resistor your 2.2uF capacitor cuts 3.6Hz and all higher frequencies. If the capacitor is 2.2nF then it cuts 3.6kHz to -3dB. The filters add and cut many lower frequencies and cut important consonant sounds of speech. All the capacitor values in the lowpass filter should be 1000 times less.


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