High Gain Preamp

Circuit : Andy Collinson
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This is a high gain pre-amplifier made from low noise transistors. The voltage gain is extremely high at 80 dB and therefore requires a very low input signal to avoid overloading. Typical applications include amplifying audio from a crystal radio
hi-gain preamp circuit Notes
Regular viewers to this site will recognise variations in this two stage amplifier as it has been used in my ECM pre-amplifier and intercom systems. The stages are direct coupled and both operate in common emitter mode. The first stage gain Q1 is very high determined by the collector impedance divided by the emitter resistor, R2. The collector impedance is the combination of Q1's load resistor R1, in parallel with the impedance looking into the base of Q2. The gain of Q2 is again very high as its emitter resistor R5 is decoupled by C3. The output impedance is medium about 15k set by Q2's load resistor.

As with any high gain amplifier, instability can become a problem and dc feedback is provided by R4 stabilising the bias against changes in temperature. As the gain is extreme then this circuit must be made in a metal case or on a PCB with large ground plane.

Bode Plot
A quick look at the output voltage versus the frequency response, more commonly known as the Bode plot is shown above. Using LTSpice, the -3dB points can be measured and the bandwidth is flat from 27Hz to 24kHz The roll-off, above 24kHz is caused by the internal junction capacitance between base and collector of Q2.
Transient Plot
Taking a look at the transient plot, shown above the input signal is 1kHz at 100uV peak to peak. The output signal is approximately 935mV +peak and -1.09V -peak. This gives an output signal of 2V pk-pk from an input signal of 200uV pk-pk. This gives a voltage gain of 100,000 or 80dB. The slight difference in positive and negative peaks leads to an overall distortion of 2.7%. AS previously stated, the input signal needs to be very low and after the input increases above 300uV the distortion becomes much more acute.
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