High frequency equivalent model of transistor

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High frequency equivalent model of transistor

Posted Date: 2024-01-26

Starting from the physical structure of the transistor and considering the influence of the emitter junction and collector junction capacitance, a physical model under the action of high-frequency signals can be obtained, which is called the hybrid π model. Since the hybrid π model of the transistor is consistent with the h parameter equivalent model introduced in Chapter 2 under the action of low-frequency signals, the h parameters can be used to calculate certain parameters in the hybrid π model and be used under the action of high-frequency signals. circuit analysis.

Mixed π model of transistor 1. Complete mixed π model

r. Figure 4.2.1(a) shows the schematic diagram of the transistor structure. r. and r. They are the body resistance of the collector area and the body resistance of the emitter area. Their values ​​are small and often ignored. (C is the collector junction capacitance, /s is the collector junction resistance, r is the base body resistance, (C, is the emitter junction capacitance, r. is the emitter junction resistance. Figure (b) is corresponding to Figure (a) Mixed π model.

In the figure, due to the existence of C, and C, 1. The size and phase angle of and 7. are all related to frequency, that is, the current amplification coefficient is a function of frequency and should be recorded as β. According to the analysis of semiconductor physics, the controlled current / of the transistor is linear with the emitter junction voltage U.

relationship and has nothing to do with signal frequency. Therefore, a new parameter g is introduced in the hybrid π model, where g is the transconductance, describing the control relationship between g, g, and U on 1., that is, I=gU 2. Simplified hybrid π model. r. In the circuit shown in Figure 4.2.1(b), normally, r. It is much larger than the load resistance connected between ce, and r and v are also much larger than the capacitive reactance of ce/ egin{matrix} 7./ egin{matrix} 7.C7.r(x), so it can be considered as r. and rv. open circuit, as shown in Figure 4.2.2(a). Since C. is connected across the input and output circuits, the analysis of the circuit becomes very complicated.

Therefore, for simplicity, C. is equivalent to the input loop and the output loop, which is called unidirectional. Unidirectionalization is achieved through equivalent transformation. Assume that the capacitance between C. and b′c is C′, and the capacitance between cc and ce is C°. Then the circuit after unidirectionalization is as shown in Figure (b).


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