![]() Thus, if we assume that the common emitter amplifier is properly characterized by these dominant low and high frequency poles, then the frequency response of the amplifier can be approximated by:Īssuming C B = C C = C E = 1 farad and C F = C Π = C μ = 0, and, using a 2N3904 transistor, design a common emitter amplifier with the following specifications: The higher 3 dB frequency (ω H) can be derived as: At high frequencies, C B, C C, and C E can be replaced with short circuits since their impedance becomes very small compared to R S, R L, and R E. Using short-circuit time constant analysis, the lower 3 dB frequency (ω L) can be found as:įigure 3 shows the high frequency, small signal equivalent circuit of the amplifier. R B is the parallel combination of R B1 and R B2. Note that C F is ignored since it is assumed that its impedance at these frequencies is very high. Low Frequency Responseįigure 2 shows the low frequency, small signal equivalent circuit of the amplifier. Miller capacitor C F is a small capacitance that will be used to control the high frequency 3 dB response of the amplifier. Capacitor C E is an ac bypass capacitor used to establish a low frequency ac ground at the emitter of Q1. Capacitors C B and C C are used to block the amplifier dc bias point from the input and output (ac coupling). The schematic of a typical common emitter amplifier is shown in Figure 1. The objective of this activity is to investigate the frequency response of the common emitter amplifier configuration using an NPN BJT transistor.
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