In this chapter we discuss the amplifiers used commonly in the front-end and IF stages of receivers and in antenna-mounted preamplifiers. The maximum output power of these amplifiers is typically from 0.01 W to 0.1 W (10–20 dBm). The power amplifiers discussed in Chapter 3 use the full range of the transistor conductance to “push” or “pull” the output voltage to any value from zero to the dc supply voltage(s). Small-signal amplifiers, on the other hand, are class-A amplifiers in which the signal voltages are small, compared with the dc bias voltages. The small ac signals add to dc bias voltages, so the output signal, δVout, produced by an input signal δVin is given by δVout = [dVout/dVin] δVin + 1/2 [d2Vout/dVin2] (δVin)2 + ··· The ac voltage gain is, therefore, dVout/dVin, evaluated at the quiescent bias point. When operated over only a small range of δVin, the higher derivatives of Vout versus Vin make only small contributions, and the amplifier is essentially linear. Key characteristics of these amplifiers are gain, bandwidth, input and output impedances, linearity (those higher derivatives), and internally generated noise.

Linear two-port networks

Small-signal amplifiers are linear amplifiers; the output signal should be a faithful reproduction of the input signal. A general definition of small-signal amplifiers could be that they are amplifiers built entirely of nominally linear elements (which include resistors, capacitors, inductors, transmission lines, and transistors operated over a small differential range), from which it follows that the overall circuit will also be nominally linear.