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Improved Transistor Amplifier

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In order to stabilize the operating conditions of the simple transistor amplifier stage and compensate for variations in transistor parameters and temperature, base bias current (I_B) for the transistor can be derived from the voltage at the collector (V_CE) rather than from the supply (V_CC). The value of V_CE is dependent on the collector current which, in turn, depends upon the base current. A negative feedback path thus exists which provides a degree of self-regulation. The feedback action works like this:

If I_C increases, V_CE will fall and I_B will be reduced. The reduction in I_B will, in turn, produce a corresponding reduction in I_C to offset the original change. Conversely, if I_C falls, V_CE will rise and I_B will increase. This, in turn, will produce a corresponding increase in I_C to oppose the original change.

Typical values are as follows;

• TR₁ = BC108,
• V_CC = 6V,
• R₁ = 470kΩ,
• R₂ = 1kΩ, and
• C₁ = C₂ = 10μF.

Note that, once again, the output signal will be inverted (i.e. 180° out of phase with the input).

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