Voltage-Controlled Oscillator

Inverted, voltage-controlled oscillator.

The inverted, voltage-controlled oscillator circuit shown in the figure above contains two operational amplifiers (IC₁ and IC₂), a switching transistor (Q₁), a capacitor (C₁), a reverse protection diode (D₁) and three resistors (R₁, R₂, and R₃). The control voltage v₁ can be varied over some range, typically 0.1 to 10 volts. This voltage is fed through resistor R₁ to the inverting input of amplifier IC₁. Transistor Q₁ in the off state allows voltage v₁ to appear at the negative input of IC₁ and in the on state Q₁ shorts the inverting input of IC₁ to common.

The transistor Q₁ is controlled by the output of amplifier IC₁. When the voltage at the inverting input of IC₁ is positive, v₂ is negative; when it is negative, v₂ is positive. This causes the output of amplifier IC₁ to latch in either the positive or the negative state. The output v₂ changes state only when the difference voltage between the inputs changes sign. This sign change is caused by the second amplifier IC₂. Amplifier IC₂ is used as an integrator, with the integration rate controlled by the level of v₂. The output, v₃, of this integrator is fed to the positive input of IC₁. The circuit will now oscillate with the waveforms shown.

The relationship between the input voltage and the output frequency may be seen in the figure below. The graph v₃ shows the output of the integrator as a function of time. Note that the large triangular wave and the small triangular wave have the same slopes. With this in mind, assume that v₁ is equal to V' volts. The integrator will integrate between 0 and +V' volts and will cycle over a period T. Now if v₁ is reduced to V'/2, the period T is reduced by one-half. Thus the period of oscillation is directly proportional to the input voltage v₁; and since the reciprocal of the frequency is equal to the period, v₁ must be inversely proportional to the frequency. Graph v₂ shows the same relationship for voltage v₂.

Output waveforms.