Gain versus frequency

Chapter Gain versus frequency

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • reduced, as shown in Fig. 22-7. Points X and Y in the graph represent the instantaneouscurrent extremes during the signal cycle.Gain versus frequency40722-7Excessive input reducesamplification.When conditions are like those in Fig. 22-7, there will be distortion in a transistoramplifier. The output waveform will not have the same shape as the input waveform.This nonlinearity can sometimes be tolerated; sometimes it cannot.The more serious trouble with overdrive is the fact that the transistor is in or nearsaturation during part of the cycle. When this happens, you’re getting “no bang for thebuck.” The transistor is doing futile work for a portion of every wave cycle. This reducescircuit efficiency, causes excessive collector current, and can overheat the base-collec-tor (B-C) junction. Sometimes overdrive can actually destroy a transistor.Gain versus frequencyAnother important specification for a transistor is the range of frequencies over whichit can be used as an amplifier. All transistors have an amplification factor, or gain, thatdecreases as the signal frequency increases. Some devices will work well only up to afew megahertz; others can be used to several gigahertz.Gain can be expressed in various different ways. In the above discussion, youlearned a little about current gain, expressed as a ratio. You will also sometimes hearabout voltage gain or power gain in amplifier circuits. These, too, can be expressed asratios. For example, if the voltage gain of a circuit is 15, then the output signal voltage(rms, peak, or peak-to-peak) is 15 times the input signal voltage. If the power gain of acircuit is 25, then the output signal power is 25 times the input signal power.There are two expressions commonly used for the gain-versus-frequency behaviorof a bipolar transistor. The gain bandwidth product, abbreviated fT, is the frequency atwhich the gain becomes equal to 1 with the emitter connected to ground. If you try to