This circuit accepts bipolar control inputs of Â± 5 V and provides a phase-chopped output to a dc load (such as a servo motor) of the same polarity as the input. The rms voltage of the output is closely proportional to the control input voltage. N-channel and p-channel TMOS devices', Ql and Q2, are connected in anti-series to form a bidirectional switch through which current can flow in either the forward or reverse direction. Control circuits turn Ql and Q2 on when they are reverse biased, bypassing their reverse rectifier and increasing circuit efficiency. Each device is allowed to turn off only when forward biased.
The Q1-Q2 switch connects the ac power source to the load when its instantaneous voltage is the same polarity and less than the control voltage. Ula is configured as an ideal positive rectifier whose output follows the control voltage when it is positive, and is zero otherwise. Similarly, Ulb is a negative rectifier. Ulc turns Ql on whenever the ac input voltage is lower than the positive rectifier output. For negative control voltages, Ql is turned on only during the negative half-cycle. For positive control voltages, Ql is turned on during the end portions of the positive half-cycle. Similarly, Uld turns Q2 on whenever the ac input voltage is higher than the output of the negative rectifier.