Op-Amp Circuit question

This sounds like the zener is providing a fixed reference voltage to one input, while the other input is a proportion of the 12 V supply. So if the 12V supply voltage goes up /down, the difference between the voltage divider and the zener will develop the input signal to the op-amp. A sort of voltage regulator sensing.
If I understand your schematic, the cap is blocking DC from the op-amp to the base. But for this to work, there would need to be some other feedback path from the op-amp output back to one of the inputs to the op-amp. Then the op-amp will amplify any 'signal' riding on the 12V supply and the DC offset will be blocked by the capacitor. The base of the transistor is biased by the resistor divider and the amplified 'signal' applied to the base gets further amplified by the transistor.
But if there's no feedback from the op-amp output back to the inverting input, then it most likely will saturate to one 'rail' or the other.
That's my guess from what you've provided.
Where is this used?
daestrom

"daestrom" wrote in news: snipped-for-privacy@news3.newsguy.com:
You're correct about the Zener Diode and resistors; it's being used to generate 12-volts from a 24-volt input (at the collector of the transistor).
The capcitor is not in-line with the op-amp output and the base. The base is connected directly to the op-amp's output, one side of the capacitor is connected between the voltage divider, and the other side to the op-amp output and base connection.
The feedback is provided by the emitter and fed back to feed the resistor divider network. With various loads, the op-amp will compensate and maintain a constant 12-volts.