7.4.4 High-Frequency Nonlinearity in the Bipolar Transistor

Chapter 7.4.4 High-Frequency Nonlinearity in the Bipolar Transistor

Radio Frequency Integrated Circuit Design Second Edition Book
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Radio Frequency Integrated Circuit Design Second Edition Book

  • 7.4 Linearity in Amplifiers 213the designer has two choices if the current source is not linear enough. They can ei-ther increase the current or increase the output impedance. Also, it should be noted that this relationship only holds true if the transistor does not start to saturate. If it does, the nonlinearity will get much worse.7.4.4  High-Frequency Nonlinearity in the Bipolar TransistorMany frequency-dependent devices can reduce the linearity of a circuit. One of the most troublesome is the base-collector junction capacitance Cm. This capacitance is voltage dependent, which results in a nonlinearity. This nonlinearity is especially important in circuits with low supply voltages because the capacitance is largest at a low reverse bias.This capacitor’s effect is particularly harmful for both frequency response and nonlinearity in the case of a standard common-emitter amplifier. In this configura-tion, Cm is multiplied by the gain of the amplifier (the Miller effect) and appears across the source.The value of Cm as a function of bias voltage is given by: (1/ )( )1onoCC VVµµψ=æö-ç÷èø (7.105)where Cmo is the capacitance of the junction under zero bias, yo is the built-in poten-tial of the junction, and n is usually between 2 and 5. Since this capacitor’s behavior is highly process-dependent and hard to model, there is little benefit in deriving detailed equations for it. Rather, the designer must rely on simulation and detailed models to predict its behavior accurately.7.4.5  Linearity in Common-Collector/Drain ConfigurationThe common-collector amplifier is often called the emitter-follower because the emitter voltage “follows” the base voltage. However, the amplifier cannot do this over all conditions. If the current is constant, vBE is constant and the transfer func-tion will be perfectly linear. However, as vo changes, iout = vo/Rout will change as shown in Figure 7.34. Thus, iE will change, and so will vBE and there will be some nonlinearity.Figure 7.34  Illustration of nonlinearity in the common-collector amplifier.