Two-Stage Transistor preamplifier

 
This is a two-stage transistor amplifier using BC547 NPN transistors. The goal is to amplify a low-level input like a microphone or magnetic phono pickup into a stronger output signal suitable for driving an audio amplifier.
 

Schematic Overview
This simulated circuit uses a signal generator to feed a small AC signal into a pair of NPN transistors, each configured as a common-emitter amplifier. The schematic is shown below:
Transistor Amplifier Simulation in Proteus
 

Components List
Label Component Value
R1 Resistor 320 Ω
R2 Resistor 300 Ω
R3, R6 Resistors 1 kΩ
R5, R7 Resistors 68 kΩ
C2, C9 Capacitors 100 nF
C1 Electrolytic Capacitor 470 μF
C4 Electrolytic Capacitor 1000 μF
Q4, Q5 NPN Transistors BC547
 
 

Operation and Signal Flow
The input signal is capacitively coupled into the base of transistor Q4 via C2. Q4 amplifies the signal and passes it to Q5 through another coupling capacitor (C9). The final output signal is taken across R2 and C1. This output shows a significantly larger amplitude than the original signal, confirming the circuit's amplification capability.
 

Simulation Settings

  • Signal Frequency: 1 kHz
  • Amplitude: 27 mV
  • Oscilloscope channels: A = input, C = output
 

Results

The oscilloscope shows that the output signal has a much larger amplitude than the input. Channel A represents the input waveform (sine), while Channel C shows the amplified version. This confirms proper functionality of the two-stage amplifier.

27mV is the maximum input. Any higher will cause clipping.

The output is maxed out at 4.5v

This is roughly a gain of 150x the input voltage, or 43dB. Probably a bit less in the real world.
 

Final circuit diagram
Final circuit diagram of the transistor preamplifier
The circuit works well and I have used it frequently to amplify the audio on home brew RF receivers.

 

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