Output Resistance

Author: Leonard Krugman

Fig. 3-13. Analysis of output resistance r_o: (A) equivalent circuit

The output resistance can be found in a similar manner. Consider Fig. 3-13 (A), which illustrates the equivalent circuit for analyzing the output resistance. The equations for the two loops on the basis of Kirchoff's law are:

Solving the two independent equations 3-14 and 3-15A for the unknown load current,

Looking back into the transistor, the generator E₂ with its internal resistance R_L sees the output resistance r₀. Again the circuit may be simplified as shown in Fig. 3-13 (B).

Fig. 3-13 (B) simplified circuit.

Then

Substituting equation 3-16 in 3-17,

Then

In terms of the open-circuit parameters

Fig. 3-14. Output resistance vs generator resistance for typical point-contact transistor (grounded base).

The latter equation indicates that the output resistance depends to some extent on the value of the signal generator input resistance. The variation of r₀ vs R_g is illustrated in Figs. 3-14 and 3-15 (for the same point-contact and junction transistors considered in the preceding section) . In the case of the typical point-contact transistor, the transistor output resistance varies from 2,400 to 12,000 ohms as the signal generator internal resistance increases from zero to infinity. The junction transistor output resistance varies from 270,000 to 2,000,000 ohms as the signal generator internal resistance increases from zero to infinity.

Fig. 3-15. Output resistance vs generator resistance for typical junction transistor (grounded base).