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Introduction to Small-Signal Amplifier Operation !

Introduction to Small-Signal Amplifier Operation !
Introduction to Small-Signal Amplifier Operation !

It is very important to consider that amplifier circuits look very different from the AC point of view than from the DC point of view. For example, the AC load line is basically different from the DC load line. The biasing of a transistor is purely a DC operation. The purpose, however, is to establish a Q-point about which variations in current and voltage can occur in response to an AC signal. In applications where very small signal voltages must be amplified, such as from an antenna, variations about the Q-point are relatively small. Amplifiers designed to handle such small AC signals are called small-signal amplifiers.

A biased transistor with an AC source capacitively coupled to the base and a load capacitively coupled to the collector is shown in the following illustration. The coupling capacitors block DC and thus prevent the source resistance and the load resistance from changing the bias voltages at the base and the collector. The signal voltage causes the base voltage to vary above and below its DC bias level. The resulting variation in the base current produces a larger variation in collector current because of the current gain of the transistor.

Introduction to Small-Signal Amplifier Operation !

As the collector current increases, the collector-emitter voltage decreases. The collector current varies above and below its Q-point value in-phase with the base current, and the collector-to-emitter voltage varies above and below its Q-point value 180° out of phase with the base voltage, just as we saw in the biasing section.

AC equivalent circuits for transistors:

In order to better visualise the operation of a transistor in an amplifier circuit, it is often useful to represent the device by an equivalent circuit, which uses various internal transistor parameters to represent the transistor’s operation. There are several equivalent circuit representations. For example, there is one based on h parameters (hybrid parameters, typically specified on a manufacturer’s sheet), and another on r parameters (easily identifiable with a transistor’s circuit operation). The following tables show the four basic AC h parameters and the circuit shows the general form of the h-parameter equivalent circuit, but specifically there are three hybrid equivalent circuits – one for common-emitter, one for common-base, and one for common-collector configuration, each with differently calculated h parameters.

h parameter

Description

Condition

hiInput resistanceOutput shorted
hrVoltage feedback ratioInput open
hfForward current gainOutput shorted
hoOutput conductanceInput open
 

Configuration

h Parameters
Common emitterhie, hre, hfe, hoe
Common basehib, hrb, hfb, hob
Common collectorhic, hrc, hfc, hoc
Basic AC h parameters and subscripts of h parameters

 Introduction to Small-Signal Amplifier Operation !

The r parameters will also be introduced in the following table. However, the analysis of small-signal amplifiers using their AC equivalent circuits is out of the scope of this course. However, if you need to design circuits, we recommend you to look at a couple of examples inside a text book of how such analyses are carried out for the different common configurations.

r parameterDescription
αacac alpha (Ic / Ie)
βacAC beta (Ic / Ib)
reAC emitter resistance
rbAC base resistance
rcAC collector resistance

h-parameter ratios for the three amplifier configurations

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