Input Resistance

Author: Leonard Krugman

Fig. 3-10. Simplified transistor equivalent circuit for analysis of input resistance r1.

The input resistance of the grounded base transistor shown in Fig. 3-9 can now be computed in terms of the transistor parameters and the transistor four-terminal open-circuit parameters. Since the input resistance as seen by the signal generator is r₁, Fig 3-9 may be simplified as shown in Fig. 3-10. This series circuit is expressed E_g = i₁(R_g + r₁) or

Rg + r1 = Eg/i1

[3-9]

Substituting equation 3-3 for i₁

	[3-10]
	[3-11]

then

[3-12]*

and in terms ot the open-circuit parameters

[3-13]*

Fig. 3-11. Input resistance vs load resistance for typical point-contact transistor (grounded base).

The effect of varying the load resistance on the input resistance can be best appreciated by examining Figs. 3-11 and 3-12, which illustrate the r₁ vs R_L characteristics for typical point-contact, and junction transistors, respectively. For the typical point-contact transistor, r₁₁ = 250 ohms, r₁₂ = 100 ohms, r₂₁ = 24000 ohms, and r₂₂ = 12000 ohms. For the typical junction transistor, r₁₁ = 550 ohms, r₁₂ = 500 ohms, r₂₁ = 1900000 ohms, and r₂₂ = 2000000 ohms. Notice that in the case of the point-contact transistor, the transistor input resistance varies from 50 to 250 ohms as the load resistance changes from zero to infinity. The junction transistor input resistance varies from 75 to 550 ohms as the load resistance is varied from short-circuit to open-circuit conditions.

Fig. 3-12. Input resistance vs load resistance for typical junction transistor (grounded base).