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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

LOADING THE AMPLIFIER

When a load resistor is connected to the output, the gain of the amplifier drops. Why is that? If you look at Figure 2 again, youýll notice that the load resistor will be in parallel with R_C. Thus, the equivalent resistance of the parallel combination will be smaller than R_C. Because the collector current does not change when the amplifier is loaded (itýs controlled by the base current), less voltage is developed across the equivalent resistance and the output voltage drops. If V_I is held constant but V_O drops when a load is connected, it indicates that the gain has dropped. In fact, the gain of the common emitter amplifier drops from 152 to 105 when a 4.7-kilohm load is attached.

You can come to the same conclusion using a current divider. As soon as the load resistor is connected, the output current must divide between R_C and the load resistor. This means less current for R_C and, therefore, less output voltage and a smaller gain.

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