Typical Circuit

Author: Leonard Krugman

Fig. 5-14. Transistor phase inverter.

Figure 5-14 illustrates the basic circuit of a transistor phase inverter, which provides a reasonably well-balanced output. The basic operation is as follows: the upper transistor operates as a conventional grounded emitter amplifier except that the emitter is grounded through the parallel circuit, consisting of the lower transistor emitter-base path and resistor R_E.

The emitter-base path has a low resistance, less than 50 ohms, so that practically all of the a-c emitter current of the top transistor flows through this path. Since the emitter current value for each transistor is the same, the collector currents are also equal if the current gains from emitter to collector are equal. For proper operation, the load resistances should be small compared to the output resistances of the transistors, and the emitter-to-collector current gains should be well matched. For the circuit illustrated, the output resistance of each transistor is the collector resistance shunted by R_B. Since r_c is much greater than R_B, the output resistance is equal to R_B. Thus R_B should be about ten times R_L. It is not necessary for the current gains to be exactly matched. Values which fall in the range of .92 to .97 are usually satisfactory. R_L and R_B are selected to provide the operating biases, which in this case are E_c = 10 volts, I_c = 4 ma, and I_b = 400 μa. The value of R_E is particularly important. It must be large compared to the emitter-to-base resistance path of the lower transistor; if it is not, an appreciable portion of the a-c signal will be shunted through R_E and the currents in the emitters will not be equal. In general, a value of R_E that is ten times the emitter-to-base circuit resistance is satisfactory.