# Op-Amp Circuit question

I have a question about a op-amp circuit.
I saw a circuit where an op-amp is driving a transistor for a voltage regulator circuit.
There is 12-volts feeding both inputs but one input has a zener, the other has a resistor divider, etc...; both circuits are for maintaining a constant voltage.
The 12-volts also comes directly across through a resistor divider and in the middle is a capacitor with the other end connected to the output of the op-amp - which is also connected to the base of a transistor.
My questions are: what is the capacitor doing? If say the 12-volts goes through two equal resistors (for ease of explanation) and there is 6-volts DC on one side of the capacitor, what is this doing for the output of the op-amp?
Another words the the circuit would look like this:
op-amp output >>>>> capacitor >>>>> 6-volts (coming from resistor divider) > > >>>>>>>>>>>>> base of transistor.
TIA
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Steve wrote:

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
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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.