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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

A CLOSE LOOK INSIDE

The spooky stuff gets spookier when you look at the AC small-signal model of the common emitter amplifier. A simplified form of the model is shown in Figure 2. Two parts of the simplified small-signal model are H_IE and H_FE. H_IE is the input resistance of the transistor (base emitter resistance) and, although hard to predict accurately, is typically several hundred to several thousand ohms. H_FE is the current gain of the transistor. It is a parameter of the dependent current source in the output that creates the collector current. For the 2N2222A transistor, H_FE may be anywhere from 50 to 300.

The strange thing about the small-signal model of the amplifier is that one side of R1 and R_C goes to ground. Even though Figure 1 clearly shows the tops of R1 and R_C going to V_CC, during AC analysis the DC power supply looks like a short to ground (12 VDC is 0 VAC). So, R1 and R2 are in parallel with H_IE in the small-signal model. Together, all three resistors make up the input resistance of the amplifier. (Iýll get to how the input resistance of the amplifier affects its lower frequency of operation soon.)

All current created by the dependent current source flows through R_C, creating a voltage across R_C that is the output voltage of the amplifier and is inverted with respect to V_I. For this reason, the common emitter amplifier is an inverting amplifier. The size of the output voltage depends on V_I, H_IE, H_FE, and R_C. V_I creates I_B through H_IE, which is multiplied by H_FE to become I_C. The output voltage is the product of I_C and R_C. The gain is the ratio of output voltage to input voltage (152 for the amplifier in this case).

If youýre wondering where the capacitors went, the answer is that you assume them to be shorts when the amplifier is operated above its lower cutoff frequency. For example, the 10-ýF coupling capacitors have a reactance of 16 ohms at a frequency of 1 KHz. Compared to the sizes of R1, R2, and H_IE, 16 ohms looks like a short, doesnýt it?

Figure 2ýWhen you look inside, you see the simplified small-signal model of the common emitter amplifier.

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