In a number of cases, an inexpensive source of 115-V, 400-Hz power with low distortion is needed for bench tests or mobile applications. This circuit, which uses the Apex Microtechnology PA25 dual power op-amp IC, can be that source (see the figure).
The first amplifier is connected as a Wien-bridge oscillator tuned to 400 Hz; the second amplifier is a unity-gain inverter. Both amps are biased at one half of the single supply so that the outputs can drive a standard 6.3-V filament transformer in push-pull mode with its primary and secondary interchanged.
Ideal power for test-bench and automotive applications is 12 to 14 Vdc, but standard 28-Vdc aircraft power may be used with the component values given if the transformer is changed to a 12.6-V type. In order to drive synchros and resolvers at 800 Hz, C1 and C2 should be changed to 0.005 mF.
Digging further into the design, bias can be set by letting R1 = R2, R3 = R4, and R10 = R11. The gain of U1A is set slightly greater than 3 by making the value of R5 7% larger than R3 (and R4). The oscillator frequency is determined by F = 1/2pRC, where C = C1 = C2 and R = R9 = R1 in parallel with R2.
To minimize distortion, R6, R7, R8, and CR1 form a "soft" clamp circuit to control the oscillator amplitude. R7 should be adjusted for 115 Vac rms at the output.
The inverter gain is set to one by allowing R12 = R13. Components R14, R15, C3, and C4 are for phase compensation, as recommended by the manufacturer to prevent output-stage parasitic oscillations. C5 is an optional filter that serves to reduce the circuit's high-frequency response in noisy environments. The dual amplifier has internal current limiting and thermal shutdown.
The circuit as shown will handle about 10 W. For higher-power applications, a larger transformer should be chosen and a heat sink should be added to U1.
