# calculation on opamp

#### farkhana

Joined Apr 14, 2011
10
can anyone teach me how to calculate the output from op amp?

Thanks

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#### farkhana

Joined Apr 14, 2011
10
i dont mean gain..how to calculate the output voltage from opamp as in the figure..

#### BSomer

Joined Dec 28, 2011
434
The output voltage is the gain * input.

Gain is figured by :
Rf / Ri + 1 = non-inverting op-amp gain
Rf / Ri = inverting op-amp gain
where Rf is the feedback resistance and Ri is the input resistance

The equations shown in the link given by luvv are useful if you know 2 of the following: input voltage, output voltage, gain.

Hope this helps a little.

#### PaulEE

Joined Dec 23, 2011
423
The output can be calculated by multiplying the input voltage by the gain of the op-amp stage.

Since the gain of the op-amp stage has been explained twice, I assume you're okay with that aspect.

The part that nobody explained was that the current source of 4 mA runs MOSTLY through the 10 ohm resistor. The voltage that appears at the +input (in front of the 10K input resistor) is 0.04v.

The GAIN (again) of the stage is that of a non-inverting op-amp stage and is 10K/941 + 1, or 11.62 ish. Multiplying the input of 0.04v by this gain yields the output the meter reads, approximately, 457 mV.

This analysis is only an approximation because the input bias currents, as well as other non-ideal op-amp properties, cause this value to be slightly off; CircuitMaker is o.k. at including these non-ideal parameters into its calculations.

#### farkhana

Joined Apr 14, 2011
10

#### PaulEE

Joined Dec 23, 2011
423

#### farkhana

Joined Apr 14, 2011
10

is it i need to simplify the resistor on inverting input and multiply with Vin on non-inverting?

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#### PaulEE

Joined Dec 23, 2011
423

is it i need to simplify the resistor on inverting input and multiply with Vin on non-inverting?
That one is a bit more work...what's "1K"? What's the actual value of the potentiometer?

#### farkhana

Joined Apr 14, 2011
10
the actual value for potentiometer is 400 ohm..