The Paraphase Circuit

Author: N.H. Crowhurst

Another phase-splitting arrangement is the so-called paraphase circuit. It uses two tubes. The output from the plate of one tube is fed by R-C coupling to the grid of one of the output tubes. From this same point, a voltage-divider arrangement cuts down the voltage and applies it to the grid of a second tube, which amplifies the voltage by as much as the resistance divider cuts it down, producing a voltage for driving the second output tube.

The paraphase circuit action

A positive fluctuation of 2 volts at the grid of the first paraphase tube produces a negative fluctuation at its plate of, say, 20 volts, which appears at the grid of one of the output tubes. This 20-volt fluctuation is also divided to provide a negative fluctuation of 2 volts for the grid of the second paraphase tube and becomes a positive fluctuation of 20 volts at the plate, providing positive fluctuation for the grid of the second output tube.

Paraphase circuit and characteristics

For this circuit to operate correctly, the voltage division produced by the resistors feeding the second tube must be exactly in the same ratio as the gain provided by the second tube. In the example given, the tube is multiplied by ten, and the voltage divider divided by ten. As tubes are subject to variation with line voltage, individual samples from production, and other differences, there is no guarantee that the amplification provided by the tube will be exactly the same as the voltage division provided by the resistors. The tube may amplify 9 or 11 times. Consequently this circuit is subject to deviation in its accuracy in a way that the two circuits discussed earlier were not.