Gain versus Frequency 357
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22-7 Excessive input
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Then, dIC /dIB is reduced, as shown in Fig. 22-7. Points X and Y in the graph represent the instantaneous current extremes during the signal cycle in this particular case. When conditions are like those in Fig. 22-7, a transistor amplifier will cause distortion in the signal. This means that the output wave will not have the same shape as the input wave. This phenomenon is known as nonlinearity. It can sometimes be tolerated, but often it is undesirable. When the input signal to a transistor amplifier is too strong, the condition is called overdrive, and the amplifier is said to be overdriven. Overdrive can cause problems other than signal distortion. An overdriven transistor is in or near saturation during part of the input signal cycle. This reduces circuit efficiency, causes excessive collector current, and can overheat the base-collector (B-C) junction. Sometimes overdrive can destroy a transistor.
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Gain versus Frequency
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Another important specification for a transistor is the range of frequencies over which it can be used as an amplifier. All transistors have an amplification factor, or gain, that decreases as the signal frequency increases. Some devices work well only up to a few megahertz; others can be used to several gigahertz. Gain can be expressed in various ways. In the preceding discussion, you learned a little about current gain, expressed as a ratio. You will also hear about voltage gain or power gain in amplifier circuits. These, too, can be expressed as ratios. 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 a circuit is 25, then the output signal power is 25 times the input signal power. Two expressions are commonly used for the gain-versus-frequency behavior of a bipolar transistor. The gain bandwidth product, abbreviated fT, is the frequency at which the gain becomes equal to 1 with the emitter connected to ground. If you try to make an amplifier using a transistor at a frequency higher than its fT specification, you are bound to fail. The alpha cutoff frequency of a transistor is the frequency at which the gain becomes 0.707 times its value when the input signal frequency is 1 kHz. A transistor can have considerable gain at its alpha cutoff frequency. By looking at this
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358 The Bipolar Transistor
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22-8 Alpha cutoff and gain
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bandwidth product for a hypothetical transistor.
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specification for a particular transistor, you can get an idea of how rapidly it loses its ability to amplify as the frequency goes up. Some devices die off faster than others. Figure 22-8 shows the gain bandwidth product and alpha cutoff frequency for a hypothetical transistor, on a graph of gain versus frequency. Note that the scales of this graph are not linear; that is, the divisions are not evenly spaced. This type of graph is called a log-log graph because both scales are logarithmic rather than linear.
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Common Emitter Circuit
A transistor can be hooked up in three general ways. The emitter can be grounded for signal, the base can be grounded for signal, or the collector can be grounded for signal. An often-used arrangement is the common emitter circuit. Common means grounded for the signal. The basic configuration is shown in Fig. 22-9. A terminal can be at ground potential for a signal, and yet have a significant dc voltage. In the circuit shown, capacitor C1 appears as a short circuit to the ac signal, so the emitter is at signal ground. But resistor R1 causes the emitter to have a certain positive dc voltage with respect to ground (or a negative voltage, if a PNP transistor is used). The exact dc voltage at the emitter depends on the resistance of R1, and on the bias. The bias is set by the ratio of the values of resistors R2 and R3. The bias can be anything from zero, or ground potential, to +12 V, the supply voltage. Normally it is a couple of volts. Capacitors C2 and C3 block dc to or from the input and output circuitry (whatever that might be) while letting the ac signal pass. Resistor R4 keeps the output signal from being shorted out through the power supply. A signal enters the common emitter circuit through C2, where it causes the base current, IB, to vary. The small fluctuations in IB cause large changes in the collector current, IC. This current passes through resistor R4, causing a fluctuating dc voltage to appear across this resistor. The ac part of this passes unhindered through capacitor C3 to the output. The circuit of Fig. 22-9 is the basis for many amplifiers, from audio frequencies through ultrahigh radio frequencies. The common emitter configuration produces the largest gain of any arrangement. The output wave is 180 out of phase with respect to the input wave.