An ac amplifier is always used to supply voltage, current, and/or power to some kind of load connected to its output. The load may be a speaker, an antenna, a ‘siren, an indicating instrument, an electric motor, or anyone of a large number ofother useful devices. Often the load is the input to another ac amplifi er. Amplifier performance is analyzed by representing its load as an equivalent load resistance (or impedance). When a load resistance RL is connected to the output of an amplifier, there is again a voltage division bet vee» the output resistance of the amplifier and the load. Figure 5-6 shows a Thevenin equivalent circuit of the output of an ac
amplifier in which the output voltage Vo is produced by a voltage source in series’ with r.: As can be seen in the figure, the load voltage VL is determined by
For a voltage amplifier, ro should be much smaller than RL in order to maximize the portion of Vo that appears across the load. By converting the amplifier output to a Norton equivalent circuit, we can show that
When the effects of both rs and RL are taken into account, the overall voltage gain from source to load becomes
Similarly, the overall current gain
where is is the (Norton) equivalent source current, vs/r
If a signal source has fixed resistance rs, then from the maximum power transfer theorem, maximum power is transferred. from the source to an amplifier when rill = ~. Similarly, if the amplifier has fixed output resistance r«, then maximum power is transferred from the amplifier to a load when RL = r., The amplifier is said to be matched to its source when rin ::::rs and matched to its load when RL == r., (We should also note that if the values of rs and r, can be controlled, maximum power transfer occurs when rs = °and r; = 0, irrespective of the values of rinand RL.)
The Purpose of Bias
In most single-transistor amplifiers, the output voltage must always be positive or must always be negative. When that is the case, it is not possible for the output to be a pure ac waveform, since, by definition,an ac waveform is alternately positive and negative. The purpose of bias in a transistor amplifier is to set a de output level somewhere in the middle of the total range of possible output voltages so that an ac waveform can be superimposed on it. Figure 5-7 illustrates the point. The ac input causes the output voltage to vary above and below the bias voltage, but the instantaneous values of the output are always positive (in this example). In other words, the output is ‘of the form
where VB is the bias voltage, or de component, of the output, and A is the fJ. value of the sinusoidal, accomponent. We have studied this kind of wave’ i-m. Chapter 3 and know that its values range from Vo – A to VB + A. . It is apparent that the values of VB and A must be such that VB + A i~not greater than the maximum possible output voltage and VB – A is not less tha the minimum possible output voltage. If these conditions are not satisfied
output voltage will reach its minimum or maximum extremes before the total ac variation can take place. The result is a flattening of the output waveform called clipping. Figure 5-8 illustrates positive and negative clipping caused by values of VII that are too large or too small and by values of A that are too large. When clipping is caused by the amplitude A being too large, as shown in Figure 5-8(c), the amplifier is said to be over driven.
The purpose of an ac amplifier is to produce an output waveform that is an amplified version of the input waveform. ‘Since clipping defeats this purpose, it is said to distort the signal, and clipping is an example of amplitude distortion. In a transistor amplifier, the minimum and maximum output voltages are typically the voltages at saturation and cutoff, respectively. Thus, the minimum output may be a saturation voltage of a few tenths of a volt, and the maximum output may be a cutoff voltage equal to the supply voltage.
Coupling Capacitors
In many amplifier applications, the source or tire load, or both, cannot be subjected to a de voltage or be permitted to conduct de current. For example, an electromagnetic speaker is designed to respond to ac fluctuations only and may not operate properly
if it conducts a de current. To prevent the de component of an amplifier’s output voltage from producing de current in the load, it capacitor is connected in series with the load. Similarly, to prevent the flow of de current from the amplifier into the signal source (or vice versa), a capacitor is connected in series with the source. These connections are shown in Figure 5-9. The capacitors are called COUj Iii g capacitors, or blocking capacitors, because they block the flow of dc current. The capacitors must be large enough to present, negligible impedance to the ac signals
