opamp simple question

Hi guys
i have a simple circuit of opamp which gives 1.65v output on 3.3v input (copied from somewhere). I wanna know what is the purpose of 100Ω resistor and a capacitor? Is it for filtration or current limiting? following is my circuit


Thanks

Hi,

Yes, it is a filter.

All is stable when there is DC load. With and without the RC.

But without RC ... when there is pulsed load current it might cause ringing.
It's because
* "pulses" consist of (a couple of) high frequency
* the OPAMP open loop gain decreases with higher frequency
* thus the OPAMP output becomes high impedance with higher frequency
* and thus - every pulse causes ringing in the output signal

Now the RC (output) becomes low impedance at higher frequencies. This stabilizes the output voltage and reduces the negative peak caused by the pulse.
On the other side the Opamp now has more time to compensate for the pulse current.
And the R reduces ringing.
The Opamp feedback connected at the C helps to avoid DC voltage errors caused by the load current (voltage drop across R)

The problem may be...
The RC causes delay, it adds another pole in the Opamp feedback. This reduces phase margin for the feedbacked circuit.
In worst case the whole circuit starts oscillating.
--> with the RC you need to choose a suitable Opamp.

Klaus

Yeah, you might want to look at the circuit design here. External poles might be fine. Or maybe this is a simple model of a dominant-pole op-amp. context is important.

Assuming a standard unity gain compensated OP, the circuit gives near to zero phase margin (= very bad input and load transient response) over a wide capacitor value range. It can be considered quite generally a bad design.

Hi,

to improve the stability I recommend to use a R in the feedback path .. and connect a C directely from OUT to -IN.

R maybe in the range of 10k
C maybe in the range of 1nF, depends on OPAMP and expected regulation speed. (this C stabilizes the OPAMP circuit. The slower the OPAMP the larger the C. But a large C slows down ouput voltage regulation)

Klaus

The general idea is that the output is filtered with the RC which would normally cause voltage drop under load. But by taking the feedback after the R this is compensated out.

It's a good idea, but as mentioned without a couple extra components there are stability concerns with the design.

IT says tremendous gains in voltage (up to one million times ) can be obtained in an op-amps. The feedback resistor filters out the signal input on this device, connected to an analog circuit. Amplifiers. Signal amplifiers. Output is connected to a speaker. Op-amp. Direct coupling.