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Wave Shaping

RC Differentiator

RC differentiator

The circuit is a variable RC differentiator with emitter followers to isolate both the input and output of the circuit. The input emitter follower is composed of Q1, R1, and emitter resistor R2. The signal is applied through the current limiting resistor R1 to the base of Q1. The differentiator circuit is composed of C1, R3, and variable resistor R4. By varying the arm of R4, the amount of resistance can be increased or decreased, thus changing the time constant of the circuit. Since C1 is relatively small and the sum of R3 and R4 is small, the circuit will have a short time constant which will prevent the passage of low frequency signals.

R5 couples the signal to the base of the output emitter follower while R6 provides a negative bias to maintain Q2 in the linear portion of its operation. The emitter of Q2 is connected through the variable resistor R7 and the resistor R8 to the positive supply. R7 provides a method of adjusting the output DC level with a minimum effect on the AC signal.

R9, C2, and R10, C3 form filtering networks.