Limitations, Supply Voltage and Load

Author: Leonard Krugman

As in the case of the vacuum tube, the problem of designing a transistor amplifier is somewhat simplified if the a-c signal is treated independently of the d-c operating point. The first step in the design could logically be the selection of the d-c operating point. (Actually, three separate conditions must be fixed; the operating point, the load line, and the supply voltage. In general, the selection of any two automatically limits the determination of the third.) The d-c operating point may be placed anywhere in the transistor characteristics, limited however by the collector maximums of voltage, current, and power dissipation. The final selection of the operating point is based primarily on the magnitude of the signals to be handled.

Fig. 5-1. Selecting the d-c operating point.

Suppose, for example, a transistor, whose characteristics are illustrated in Fig. 5-1, is to be used with its operating point set at E_c = 10 volts, I_c - 6 ma. Assume, also, that the maximum limits of the transistor are I_c = 18 ma, E_c = 30 volts, and collector dissipation = 100 milliwatts, as shown enclosed by the dotted lines. The supply voltage required is the value at the intersection of the load line and the collector voltage axis. Thus, for a fixed load of 1,670 ohms, the necessary supply voltage is 20 volts. If, however, the supply voltage is fixed, then the load resistance is determined by the line joining both the supply voltage (E_c at I_c = 0) and the operating point. As an illustration, assume the supply voltage is to be E_bb fixed at 30 volts. The resulting load resistance

For any selected operating point there are many combinations of load resistance and supply voltage that will permit the load line to pass through the d-c operating point.

The usual problem is one in which both the load and supply voltages are fixed. The problem then resolves itself into a choice of the operating point. In Fig. 5-1, for the conditions R_L.= 1,670 ohms, and E_bb = 20 volts, the d-c operating point may be placed anywhere along the load line. It is usually desirable to design the amplifier for maximum signal handling capacity. In this case, then, the d-c operating point should be midway between the extreme limits of the base current, namely 0 and 800 microamperes. The choice of I_b = 400 microamperes sets the operating point for maximum signal capacity at I_c = 6ma, and E_c = 10 volts.