394 TRANSISTOR AMPLIFIERS
Thus, a biasing signal (current or voltage), in the absence of any other signals, places the
transistor at an operating or quiescent point of itsi–vcharacteristics. Time-varying signals are
usually superimposed on dc biasing signals. Small variations of voltage and current about the
operating point are known as small-signal voltages and currents. While small-signal variations
are just a fraction of the power-supply voltage, the large-scale excursions of a power amplifier may
be comparable to the supply voltage. This chapter is devoted to the study of small-signal amplifiers,
in which the relationships between small-signal variables are linear. Graphical solutions including
the transistor’s general nonlinearity are not considered in this text.
A transistor model, having the same number of terminals as the transistor, is a collection of
ideal linear elements designed to approximate the relationships between the transistor small-
signal variables. While the small-signal model cannot be used to obtain information about
biasing, the ac device model considered in this chapter deals only with the response of the
circuit to small signals about the operating point. The transistor model is then substituted for
the transistor in the circuit in order to analyze an amplifier circuit. Under the assumption of
small-signal linear operation, the technique of superposition can be used effectively to simplify
the analysis.
Amplifier circuits can be treated conveniently as building blocks when analyzing larger
systems. The amplifier block may be represented by a simple small-signal model. A multistage
amplifier is a system obtained by connecting several amplifier blocks in sequence or cascade, in
which the individual amplifier blocks are called stages. Input stages are designed to accept signals
coming from various sources; intermediate stages provide most of the amplification; output stages
drive various loads. Most of these stages fall in the category of small-signal amplifiers.
The subject of amplifier frequency response has to do with the behavior of an amplifier as
a function of signal frequency. Circuit capacitances and effects internal to the transistors impose
limits on the frequency response of an amplifier. Minimization of capacitive effects is a topic of
great interest in circuit design.
After discussing biasing the BJTs and FETs to establish the operating point, BJT and FET
amplifiers are analyzed, and the frequency response of amplifiers is looked into. Advantages of
negative feedback in amplifier circuits are also mentioned.8.1 BIASING THE BJT
A simple method of biasing the BJT is shown in Figure 8.1.1. While no general biasing procedure
that will work in all cases can be outlined, a reasonable approach is to assign^1 / 2 VCCas the drop
across the transistor,^3 / 8 VCCas the drop acrossRCto allow an adequate ac voltage swing capability
in the collector circuit, and^1 / 8 VCCas the drop acrossRE, so thatREis about^1 / 3 RC. With a specified
supply voltageVCC, biasing consists mainly of selecting values forVCEQ,ICQ, andIBQ=ICQ/β,
which define the operatingQpoint. One can then select+−+−VB VCCI RE
2R 1R 2BIBG
ERC
CIEQVEVCEGICCFigure 8.1.1Method of biasing the BJT.