For a moment, consider the circuit without R3 and C3. Q1 operates in common emitter, its collector voltage directly-coupled to Q2 and sets the bias for Q2. Output is taken from Load resistor R5 and the emitter voltage of Q2 is developed across R6. The emitter voltage is used to bias Q1 base fed via R4. This feedback provides DC stabilisation of the circuit. Q1 is operated at a low collector current of 215uA for has low noise. If R5 did not exist and output was taken from the emitter of Q2 the circuit would be identical to the ECM Mic-Preamp circuit. The combined gain of both stages is high, about 60dB.
The inclusion of R3 and C3 sets an additional feedback loop from Q2 collector to Q1 emitter. R3 takes the collector voltage of Q2 and feeds it back to Q1 in series. This is series-voltage feedback and sets the overall gain of the circuit to:
|AV = 1 +||R3|
With R1=1k and R2=22 this is a voltage gain of 23x or 27.2dB This figure can be seen in the bode plot below. All plots and analysis are made with LTspice. C3 rolls off the high frequency response and sets the high frequency cut-off point.
The BC549C is a plastic package version of the BC109C. Alternative low-noise transistors could be used like BC650C for even better noise figures. The noise figures calculated by Spice assume a perfect power supply. In the real world, any noise on the power supply is likely to be higher than the preamplifier. A 7812 regulator is quoted as having 76uV of noise on its supply rails, this is 1000x greater than the preamp.Bode Plot
Fourier components of V(out) DC component:-0.0350794 Harmonic Frequency Fourier Normalized Phase Normalized Number [Hz] Component Component [degree] Phase [deg] 1 1.000e+03 2.689e+00 1.000e+00 0.31° 0.00° 2 2.000e+03 1.976e-03 7.350e-04 91.94° 91.63° 3 3.000e+03 1.980e-03 7.363e-04 0.24° -0.07° 4 4.000e+03 4.749e-04 1.766e-04 -89.84° -90.16° 5 5.000e+03 2.891e-04 1.075e-04 -1.50° -1.82° 6 6.000e+03 4.255e-04 1.583e-04 146.14° 145.83° 7 7.000e+03 2.219e-04 8.252e-05 -149.83° -150.15° 8 8.000e+03 4.461e-04 1.659e-04 -174.30° -174.61° 9 9.000e+03 8.429e-04 3.135e-04 -162.90° -163.22° Total Harmonic Distortion: 0.113260%Noise
Please remember this circuit is mono, so build two for a stero preamp or four for a quad preamp. I will show details of the simulations used later in a ltspice tutorial and an analysis of this circuit using the hybrid pi model in a later update on Circuit Exchange.
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