Experiment: 2 stage common emitter amplifier

I’m almost completely clueless regarding transistor circuits over and above using it as a switch. Before delving into the books and learning the theory and application of transistors, I decided to build a simple amplifier circuit, tweak it, and just test it out to see how it performs. Here follows what I ended up with.

Circuit Description

Circuit schematic
Circuit schematic
An explanation of the circuit from the left:

  • The electret microphone will provide the input signal. It is powered through R1
  • The microphone outputs an AC signal with a possible DC component (or a DC offset). C1 is meant to filter out the DC component and let the AC signal through.
  • R2 and R6 forms part of the 1st stage bias circuit. I adjust R6 until T1‘s output is around 0.8V and T2‘s output is around 4.5V. I want the output of this circuit to swing around 4.5V which is merely the halfway point between 0 and 9V.
  • R3 and R9 effectively forms part of the second stage biasing circuit. I don’t adjust them because I set the 2nd stage output with the 1st stage biasing circuit.
  • R7 is a variable resistor that should limit the current going into T2‘s base. I adjusted this to reduce the amplification until there was no more clipping on the output.
  • I understand that C2 and allows AC signal to move through them without the hindrance of the parallel resistors R9 and R10. This might improve AC amplification a bit?
  • The final stage is meant to drive a speaker with the amplified signal. I’ve tried to build it like this but it doesn’t work: the sound output is too soft and distorts easily. I’m sure there are better ways to drive a small 8Ω speaker and will revisit that part in a future experiment.

I built the circuit on a breadboard and set out to tweak the two potentiometers until I’m happy with the circuit’s performance.

Breadboard layout
Breadboard layout

For experimental reasons I took various volt measurements at different points in the circuit. The next table shows the voltages after tweaking when no signal is applied to the circuit.
To note is the final output voltage is at 4.5V as I wanted it. The base-emitter voltage is roughly 0.6V, as expected from a transistor.
The collector current is calculated from the voltage drop across the two 1kΩ resistors R3 and R4 using Ohm’s Law.

Quiescent Measurements

Vcc 8.96V
Measured Values
1st Stage 2nd Stage
VC 1.51 V 4.50 V
VB 0.90 V 0.80 V
VE 0.25 V 0.16 V
VBE 0.65 V 0.62 V
VCE 1.22V 4.48 V
Calculated Values
IC 7.45 mA 4.46 mA

The following two tables show my findings using rough measurements from an oscilloscope display.
The first table quickly shows the calculated total gain when applying a sine-wave and tweaking the gain just before distortion sets in.

A 440Hz Sinewave Applied Input

Measured Values
Input signal: 10mV p-p
Output signal: 6.5V p-p
Calculated Values
Gain 650

The next table provided more measurements also between the different stages of the circuit.

Electret Microphone Input

Measured Values
Input signal max 20mV p-p
Measured Values – 1st Stage
Signal at base 13 mV p-p
Signal at collector 785mV p-p
Calculated Values – 1st Stage
Gain 60
Measured Values – 2nd Stage
Signal at base 130 mV p-p
Signal at collector 5.0V p-p
Calculated Values – 2nd Stage
Gain 38
Calculated Values – Overall
Gain 346

To note is the loss of signal through the potentiometer R7: From a 785mV p-p output down to 130mV p-p.

Oscilloscope waveforms
Oscilloscope waveforms
Final note: It can be seen that the input and output signals are in phase. This is because the first common-emitter amplifier stage inverts the signal, and the second stage inverts it back.

Feedback

Let me know what you think of this post. Is it useful? Did I miss anything crucial? Leave a comment below. Any input will be appreciated.

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