Voltage-Controlled Oscillator

TEST & MEASUREMENT 048 VOLTAGE-CONTROLLED OSCILLATOR A1,A2 = 1C2 = TL072 N1...N4 =1C3 = 4071 T 00® T On e5V 894100 • 11 The voltage-controlled oscillator (vco) presented here is based on a Type OP80 operational amplifier. This opamp has an excep- tionally low bias current of, typically, about 200 fA (femto-ampere =10-15amp) and 2 pA maximum, so that any off-set caused by this current is minute. It is, therefore, ideal for use as an integrator, because the operation of that kind of circuit is affected readily by off-sets. The OP80-based integrator in the diagram is used as a vco that is not affected by the polarity of the control voltage. A direct volt- age at the input of the circuit will cause C1 to charge. Depending on the polarity of the input voltage, the potential across Cl, and thus the output voltage of IC1, will be positive or negative. The speed with which Cl charges depends on the magnitude of the input voltage: this characteristic is used to generate a signal at a vo...