Transistors in Amplifier Circuits

When a transistor is being used as an amplifier, the typical operation involves a low-amplitude sinusoidal V_IN producing a large-amplitude sinusoidal V _O. Each sinusoidal voltage and current will oscillate around a DC bias point (e.g., V _IN may oscillate ±10 mV around a 0 V bias point), while V _O may oscillate ±3V around a 5 V bias point (see Figure 17.1). The current, I_C , performs similarly, oscillating ±0.3 mA around a 1 mA bias point, but note that I_C 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. Figure 17.1 Decomposition of amplifier signals. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315218441/57775f29-ae72-4899-a474-bf25d0d8888d/content/fig17_1.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>