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# Input Resistance in the Grounded Collector Circuit

Author: Leonard Krugman

The general input resistance is defined by equation 3-13:

In terms of grounded collector transistor parameters, the input resistance becomes

 Eq. (4-15)

The variation of input resistance with load is illustrated for the point-contact and junction transistors in Figs. 4-10 and 4-11, respectively. The input resistance for the point-contact transistor is negative from RL = 0 to RL = -r22 = 11,850 ohms. Notice that when RL = -r22, the input resistance is infinite or open circuited. As RL is increased further, the input resistance becomes positive, and gradually decreases to a limiting value of 12,000 ohms. The input resistance of the junction transistor increases from a value of approximately 500 ohms to the limiting value r1 = r11 = 2,000,000 ohms when the output is open circuited.

Fig. 4-10. Input resistance vs load resistance for typical point-contact transistor (grounded collector).

Fig. 4-11. Input resistance vs load resistance for typical junction transistor (grounded collector).

Last Update: 2010-11-17