Output Resistance for the Grounded Emitter Connectio

Author: Leonard Krugman

The output resistance was defined by equation 3-21 in terms of the general four-terminal parameters as:

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

Eq. (4-8)*

Fig. 4-5. Output resistance vs generator resistance for typical point-contact transistor (grounded emitter).

The effect of the value of the signal generator resistance is illustrated for the point-contact and junction transistors in Figs. 4-5 and 4-6, respectively. Notice that the output resistance of the point-contact type is positive at R_g = 0, and decreases rapidly to zero when R_g is slightly less than 50 ohms. As R_g is increased further, r_o becomes negative and gradually approaches the limiting condition, when R_g is infinite, r₁ = r₂₂ = -11,850 ohms. Thus, the point-contact transistor can have a negative output resistance over a large range of generator resistance values, and this characteristic can be used in transistor oscillator design. Circuits of this type are called "base-controlled oscillators."

Fig. 4-6. Output resistance vs generator resistance for typical junction transistor (grounded emitter).

The output resistance of the junction transistor is always positive, and for the typical type considered, r_o gradually decreases from approximately 273,000 ohms to 100,000 ohms as R_g is increased from zero to infinity.

The range in which both the output and input resistances of the point-contact transistor are positive can be increased by adding external resistance in the emitter arm. This increases the effective value of r_e = r₁₂. Notice that if enough external resistance is added so that the effective emitter resistance r_e + R_L is equal to or greater than -(r_c - r_m), the input and output resistance is positive. This stabilizing effect of adding resistance to the emitter load is frequently used in transistor circuit applications.