One of the drawbacks of a two-transistor amplifier is that the transistors must handle large amounts of current. Figure 11-80 shows an example of a four-transistor amplifier for a DC servomotor. The four-transistor amplifier is commonly called a bridge driver. In this diagram you can see that the bridge rectifier is drawn as a rectangle but its operation is identical to the one shown in the two-transistor amplifier circuit. You should remember that it is easier to see the operation of a bridge rectifier in this configuration when three-phase power supply is used.
The base of each transistor is controlled by a switching circuit. Again the bias of each transistor is a continuous signal that can be varied from minimum to maximum. When the amplifier is set to run the motor in the clockwise direction, transistors Q2 and Q3 are biased on so that positive voltage is applied to the right side of the motor armature. When the motor is set to run in the counterclockwise direction, transistors Ql and Q4 are biased on so that positive voltage is directed to the left side of the armature. The amount of bias voltage will determine the amount of voltage each transistor passes to the armature, which will in turn change the speed of the motor.