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Simple Transistor Amplifier

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The signal input (a sine, square, pulse or complex waveform) is 'AC coupled' into the base of the transistor via capacitor, C₁. This removes any direct current component present at the input and also prevents the bias current from flowing back towards the input terminal. Base current is supplied by R₁ which is connected to the positive supply rail.

The collector current, I_C, flows through the collector load resistor, R₂. The AC component of the collector current results in an AC voltage drop ( equivalent to the change in I_C x R₂) which is passed to the output terminal via a second coupling/d.c. blocking capacitor, C₂. The stage gives a voltage gain of about 50 with the following typical values;

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

Note that the output signal is inverted (i.e. 180° out of phase with the input).

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