Grounded Emitters and Grounded Collectors

Author: Leonard Krugman

The design and servicing of the transistor circuit is more complicated than that of the vacuum tube, because transistor input and output circuits are never inherently independent of each other. This makes it difficult for a newcomer to get the "feel" of the transistor. In the long run, however, these same complex characteristics provide for a more flexible device, one capable of many circuit applications beyond the range of the vacuum tube.

This chapter deals with the extension of the four-terminal characteristics developed for the grounded base to encompass the two remaining connections, the grounded emitter, and the grounded collector; a comparison of the major features of the three basic connections; limitations of the transistor; and transistor testing methods.

Introduction

In the following analysis of transistor performance in the grounded emitter and grounded collector connections, the same typical point-contact and junction transistors discussed in Chapter 3 will be used for numerical examples. For the point-contact transistor in the grounded base connection, the parameters are:

r₁₂ = 100 ohms = r_b

r₁₁ = 250 ohms = r_e + r_b; then r_e = 150 ohms

r₂₁ = 24,000 ohms = r_m + r_b; then r_m = 23,900 ohms

r₂₂ = 12,000 ohms = r_c -f- r_b; then r_c = 11,900 ohms

For the junction transistor in the grounded base connection:

r₁₂ = 500 ohms = r_b

r₁₁ = 550 ohms = r_e -f- r_b; then r_e = 50 ohms

r₂₁ = 1,900,000 ohms = r_m + r_b; then r_m = 1,899,500 ohms

r₂₂ = 2,000,000 ohms = r_c + r_b; then r_c = 1,999,500 ohms

Notice that since r_m and r_c are so much greater in value than r_b, particularly in the case of the junction transistor, for all practical purposes r₂₁ = r_m and r₂₂ = r_c.