DESIGN: Making a common-emitter, small-signal voltage amplifier - Part 1

Frequently I have found myself in situations where an output signal from a micro-controller is far too quiet (sometimes less than 1Vpp). What we need is a pre-amp or small signal amplifier. Normally Class A amplifiers are far too inefficient for amplifying larger signals, but due to their simplicity, here they are perfect. I've never really studied amplifier design before so I wanted to go back to basics  and use Bipolar Junction Transistors. Naturally, the maths are essential if you want to do this right so I spend sometime going into the details.

The first part will focus on the specification and design, the second part will be a breakout board implementation for the Arduino-compatible Teensy3.2 micro-controller.

Common Emitter Amplifier configuration

Here is the standard common emitter amplifier design. The transistor can be seen in the center, the capacitors block DC from the input and output signals, and the resistors control the current at various stages of the amplifier. In layman's terms, Rc/Re will control the gain, and R1/R2 will control the position of the signal (the bias) within the amplifiers operating range (determined by the power supply). 

Since I wanted a design that used the most common (jellybean) parts possible I opted to use the 2n3904 transistor. 

Specification

n.b. Vcc and Rc were chosen values, the others come from the 2n3904 BJT datasheet

Calculations

i) Collector current quiescent point

ii) Base current

iii) R2 

n.b. multiplying the base current (Ib) by 10 ensures that enough current is provisioned to prevent loading of the signal.

iv) R1

n.b. multiplying the base current (Ib) by 11 ensures that enough current is provisioned to prevent loading of the signal.

v) Emitter current

vi) Emitter resistor

vii) Input impedance

viii) Zout (output impedance)

Prototyping, testing and adjusting the design

Testing the above design on the breadboard with the calculated values above.

The CH2 signal (top of screen) is the 1Vpp input from a function generator and music source, whilst the CH1 signal (bottom of screen) is the 8Vpp amplified signal from the circuit. Note, the resolution settings: CH1 is 2v/div, CH2 is 1v/div.

Evidence of clipping can clearly be seen on the output signal, and audibly some distortion was noticeable. 

Testing calculated CEA values with test signal. Some clipping evident.

Testing calculated CEA values with music audio signal. Clipping is more apparent.

Clearly some adjustment to the resistor values was required.

After adjustment of resistors, clipping is reduced on the test signal

And this is again confirmed to further improve clipping with audio signal test. 

The final values for the resistors are:

Zin (input impedance)

Zout (output impedance)

Av (voltage gain)

  To be continued....