This design resulted from the need for a partial replacement of the well-known 8038 chip, which is no longer in production and therefore hardly obtainable. An existing design for driving an LVDT sensor (Linear Variable Differential Transformer), where the 8038 was used as a variable sine wave oscillator, had to be modernized. It may have been possible to replace the 8038 with an Exar 2206, except that this chip couldn’t be used with the supply voltage used. For this reason, we looked for a replacement using standard components, which should always be available.
In this circuit two opamps from a TL074 (IC1.A and B) are used to generate a triangular wave, which can be set to a wide range of frequencies using P1. The following differential amplifier using T1 and T2 is configured in such a way that the triangular waveform is converted into a reasonably looking sinusoidal waveform. P2 is used to adjust the distortion to a minimum.
The third opamp (IC1.C) is configured as a difference amplifier, which presents the sine wave at its output. This signal is then buffered by the last opamp (IC1.D). Any offset at the output can be nulled using P3.