To get a sine output, you need to decide what range of angles you will accept--the sine wave keeps on repeating forever. For instance, you could represent 0 to pi/2 (0 to 90 degrees) with an input voltage of 0 to 10 volts. You could then have several feedback resistors in parallel, with each going through a zener diode. The zeners would be different voltages, all less than 10 volts. It's probably easier with a microprocessor, unless you need high speed. Also, with the micro, you can use any value for the angle and convert it to a value where your formula is accurate, say between 0 and 90 degrees, and then use the sine of the supplementary angle, or change the sign. There are several approximations for sine or cosine that are very close with just 3 or 4 terms. You would probably lose more accuracy in the A to D and D to A converters, anyway.