Electronic Devices and Amplifier Circuits

Transistor Amplifier at High Frequencies26242220Gain (dB)18161412108610 0 10 1 10 2 10 3 10 4 10 5 10 6Frequency (Hz) - log scaleFigure 8.7. Gain versus frequency plot for Example 8.3 with MATLAB8.3 Transistor Amplifier at High FrequenciesIn Chapter 3, Section 3.13, we introduced the hybrid− π model for a transistor at high frequenciesand this model is repeated in Figure 8.8 for convenience.i b v' be = vBv be r' br be+ −+ C 2C 1−g m vi cr ov ceCEi eFigure 8.8. The hybrid− π model for the transistor at high frequenciesAn incremental model for the transistor amplifier in Figure 8.9(a) that is valid at medium and highfrequencies is shown in Fig. 8.9(b).The transistor circuit of Figure 8.9(a) is represented by the hybrid− π model of Figure 8.9(b), wherethe resistance R 1 represents resistances R A , R B , and the resistance associated with the source I 1 .The behavior of this amplifier is affected at high frequencies by the parasitic capacitances C eandC c. At high frequencies, these capacitances tend to short−circuit node b' to ground and this resultsin a significant reduction of the voltage v and the current g m v .Electronic Devices and Amplifier Circuits with MATLAB® / Simulink® / SimElectronics® Examples, Third EditionCopyright © Orchard Publications8−9