Input Resistance for the Grounded Emitter Connection

Author: Leonard Krugman

The input resistance was defined in equation 3-13 in terms of the general open-circuit parameters as:

The input resistance in terms of the transistor parameters in the grounded emitter connection becomes:

Eq. (4-7)*

Fig. 4-3. Input resistance vs load resistance for typical point-contact transistor (grounded emitter).

The effect of the value of the load resistance on the input resistance of typical transistors is illustrated in Figs. 4-3 and 4-4. The input resistance for the point-contact transistor starts at a value of —40 ohms for R_L = 0, and becomes more negative as the load resistance increases. When R_L = -r₂₂, the input resistance is infinite. As the load resistance increases beyond this point, the input resistance becomes positive, decreasing in value to the limiting condition r₁ = r₁₁ - 250 ohms when the output is open-circuited. Negative values of input resistance indicate circuit instability; consequently, the point-contact transistor can be used as an oscillator in the region where R_L is less than -r₂₂. Circuits of this type are called "collector-controlled oscillators."

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

The input resistance of the junction transistor is always positive. In the typical transistor considered, the input resistance decreases from a value of 1,500 ohms at R_L = 0, to 550 ohms for an infinite load.