To obtain voltage amplification the current from the transistor must be passed through a circuit offering impedance so that an output voltage is developed. The ratio of this output voltage to the voltage controlling the current is then the gain of the amplifier. The circuit across which the output voltage is developed, which is called the load, may be a resistor, a tuned circuit, or even another transistor, depending upon the type of signal that is to be amplified. Of the three electrodes available (base, emitter and collector or gate, source and drain), the electrode to which the load is connected is known as the output electrode, the electrode to which the signal to be amplified is connected is called the input electrode and the remaining electrode is common to both input and output circuits. Thus, there are three possible modes of amplifier as illustrated in figure 5.1. For the bipolar transistor these are the common emitter, common base and common collector (or emitter follower); and for the FET there is the common source, common gate and common drain (or source follower). It is found on comparison between similar modes (for example, common emitter, common source) that the overall amplifier characteristics are similar, the important characteristics being input and output impedances, voltage gain and transfer characteristics. This point will be examined in more detail shortly.
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© Rhys Lewis 1983