Common-emitter circuit

Chapter Common-emitter circuit

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • make an amplifier using a transistor at a frequency higher than its fT, you’ll fail! Thus fTrepresents an absolute upper limit of sorts.The alpha cutoff frequency of a transitor is the frequency at which the gain be-comes 0.707 times its value at 1 kHz. A transistor might still have considerable gain atits alpha cutoff. By looking at the alpha cutoff frequency, you can get an idea of howrapidly the transistor loses gain as the frequency goes up. Some devices “die-off” fasterthan others.Figure 22-8 shows the gain band width product and alpha cutoff frequency for a hy-pothetical transistor, on a graph of gain versus frequency. Note that the scales of thisgraph are nonlinear; they’re “scrunched up” at the higher values. This type of graph isuseful for showing some functions. It is called a log-log graph because both scales arelogarithmic rather than linear.408 The bipolar transistor22-8Alpha cutoff and gain bandwidth product for ahypothetical transistor.Common emitter circuitA transistor can be hooked up in three general ways. The emitter can be grounded forsignal, the base can be grounded for signal, or the collector can be grounded for signal.Probably the most often-used arrangement is the common-emitter circuit. “Com-mon” means “grounded for the signal.” The basic configuration is shown in Fig. 22-9.A terminal can be at ground potential for the signal, and yet have a significant dcvoltage. In the circuit shown, C1 looks like a dead short to the ac signal, so the emitteris at signal ground. But R1 causes the emitter to have a certain positive dc voltage withrespect to ground (or a negative voltage, if a PNP transistor is used). The exact dc volt-age at the emitter depends on the value of R1, and on the bias.