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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

IS THIS THE RIGHT FREQUENCY?

The small-signal model in Figure 2 shows the amplifier in its midrange of operation. This is when the amplifier is operating between its lower and upper cutoff frequencies and has a steady gain. The gain drops when the frequency is below the lower cutoff because of the coupling capacitors. The gain also drops when the frequency is above the upper cutoff, because of the parasitic capacitance inherent in the transistor.

Each coupling capacitor forms a high-pass filter in combination with the output resistance of the preceding stage in series with the input resistance of the next stage. The input resistance is the parallel combination of R1, R2, and H_IE. When the reactance of the capacitor approaches the same magnitude as the sum of the two resistances, the overall gain of the amplifier begins to drop. The corner frequency of the input high-pass filter will become the lower cutoff frequency of the amplifier, unless the corner caused by the output coupling capacitor is higher.

The output coupling capacitor forms a second high-pass filter with the series combination of R_C and the load resistance. Note that R_C and the load resistor are treated in parallel for the gain calculation, but in series for this calculation.

The components in the amplifier result in a corner of 35 Hz because of C_I or 3 Hz because of C_O. The input signal must have a frequency of at least 35 Hz to generate an output signal.

A third corner is present because of C_E and R_E. You may recall that C_E was chosen to be big so that its effects are present in the circuit at low frequencies. This allows C_I and C_O to control the lower cutoff of the amplifier.

Suppose that 35 Hz is too high. In that case, you may want the lower cutoff to be 20 Hz. This can be accomplished by increasing the value of C_I. You could also change the value of the input resistance, but this is more difficult, because changing R1 or R2 will affect the biasing and the gain of the amplifier.

The parasitic capacitance that shuts down the transistor at high frequencies is the result of the manufacturing process and cannot be eliminated. Every transistor has parasitic capacitance. It takes a high frequency to get parasitic capacitance to do its damage because itýs only 5 to 25 pF. For instance, the 2N2222A has an upper frequency limit of 300 MHz. In actual operation, the gain of the amplifier increases the effect of the capacitance, shutting down the transistor even earlier. In general, as the gain increases, the upper cutoff decreases.

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