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ANALOG BASICS

Transistor Signal Amplification

by James Antonakos

Start ý In The Beginning There Was Biasing ý Getting Signals In And Out ý The Big Capacitor ý A Close Look Inside • Loading the Amplifier • Is This the Right Frequency? • Yes, But Op-Amps are Easier • Sources and PDF

GETTING SIGNALS IN AND OUT

To turn the biasing circuit into an amplifier, simply add coupling capacitors. The coupling capacitors allow you to inject an input signal to the base and tap into the output signal at the collector. The input coupling capacitor, C_I, protects the DC bias voltage at the base (created by R1 and R2) by isolating any DC offset on the input signal (V_I). Recall that capacitors act like opens for DC, so C_I prevents any DC offset on V_I from affecting the DC bias voltage at the base. C_I also affects the low-frequency operation of the amplifier, which you will see shortly.

The output coupling capacitor, C_O, performs DC isolation as well, blocking the DC Q-point voltage present at the collector. Together with a load resistance connected to V_O, C_O also affects the low-frequency operation.

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