The class-A amplifier

Chapter The class-A amplifier

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • The resistor values depend on the application. In some instances, R1 and C1 are notused, and the source is grounded directly. If R1 is used, its value will depend on the in-put impedance and the bias needed for the FET. Nominal values might be R1 680Ω,R2= 10 KΩ, and R3 100Ω for a weak-signal, wideband amplifier.If the circuit is used as a power amplifier, the values of the resistors will be differ-ent. It might be necessary to bias the gate negatively with respect to the source, usinga second power supply with a voltage negative relative to ground.The class-A amplifierWith the previously mentioned component values, the amplifier circuits in Figs. 24-2and 24-3 will operate in class A. Weak-signal amplifiers, such as the kind used in the firststage of a sensitive radio receiver, are always class-A. The term does not arise from in-herent superiority of the design or technique (it’s not like saying “grade-A eggs”). It’sjust a name chosen by engineers so that they know the operating conditions in the bipo-lar transistor or FET.A class-A amplifier is always linear. That means that the output waveform has thesame shape as (although a much greater amplitude than) the input waveform.For class-A operation with a bipolar transistor, the bias must be such that, with nosignal input, the device is near the middle of the straight-line portion of the IC vs EB(collector current versus base voltage) curve. This is shown for an NPN transistor inFig. 24-4. For PNP, reverse the polarity signs.With a JFET or MOSFET, the bias must be such that, with no signal input, the de-vice is near the middle of the straight-line part of the ID vs EG (drain current versus gateThe class-A amplifier43924-4Various classes ofamplifier operationfor an NPN bipolartransistor.