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Transistors in Amplifier Circuits

Authored by: Bogdan M. Wilamowski , J. David Irwin

The Industrial Electronics Handbook

Print publication date:  March  2011
Online publication date:  April  2016

Print ISBN: 9781439802793
eBook ISBN: 9781439802809
Adobe ISBN:


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When a transistor is being used as an amplifier, the typical operation involves a low-amplitude sinusoidal VIN producing a large-amplitude sinusoidal VO. Each sinusoidal voltage and current will oscillate around a DC bias point (e.g., VIN may oscillate ±10 mV around a 0 V bias point), while VO may oscillate ±3 V around a 5 V bias point (see Figure 17.1). The current, IC, performs similarly, oscillating ±0.3 mA around a 1 mA bias point, but note that IC does not go negative. To simplify analysis and design, superposition is applied to the amplifier circuit to isolate the DC component of the signal from the AC component. Therefore, a DC analysis, where all AC sources are turned off, is performed independently of the AC analysis, where all DC sources are turned off. The DC analysis will produce the bias points (otherwise known as DC points or quiescent points or Q-points). The AC analysis will provide an expression of the AC gain. The complete operation of the amplifier, as shown in Figure 17.1, is a recombination of the AC and DC components.

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