INTRODUCTION
The design of RC operational amplifier oscillators requires the use of a formal design procedure. In general, the design equations for these oscillators are not available; therefore, it is necessary to derive the design equations symbolically to select the RC components and to determine the influence of each component on the frequency of oscillation. A design procedure will be shown for two state variable oscillator circuits that can be used in precision capacitive sensor applications. These two oscillators have an output frequency proportional to the product of two capacitors (C1*C2) and the ratio of two capacitors (C1/C2).

The state variable oscillators have been built using ON Semiconductor’s new family of sub–1 volt operational amplifiers and comparators. The MC33501, MC33503, and NCS2001 operational amplifiers, and the NCS2200 comparator are the industry’s first and only commercially available analog components that are specified at a voltage of 0.9 volts. These components can be powered from a single NiCd, NiMH or alkaline battery cell. The wide operating temperature range of –40°C to +105°C makes these devices suitable for a wide range of applications.