### Higher Unit 2 - Operational Amplifiers 3 - Gain

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• #### Message 1.

Posted by BitesizePhysicsTeacher (U15165682) on Thursday, 31st January 2013

All amplifiers have the effect of multiplying their input signals, usually to make them larger.

The factor by which the input is multiplied is called the 'gain'.

Gain = Voltage Output ÷ Input Voltage

or

Gain = V(out) / V (in)

You will have seen this before in SG physics - www.bbc.co.uk/bitesi...

For operational amplifiers (both inverting and differential) the gain is given by the ratio of the feedback and input resistors.

Gain = Rf / R1

So, for inverting mode -

Vo = - Rf / R1 x V1

(DON'T FORGET THE NEGATIVE - it's an INVERTING mode amplifier!)

and for differential mode -

Vo = Rf / R1 x (V2 - V1)

In each case the input voltage(s) are multiplied by the gain.

**N.B. the inverting mode op amp can be treated as a special case of the differential mode, where V2 is set to 0V -

i.e. Vo = Rf / R1 x (V2 - V1) = Rf / R1 x (0 - V1) = - Rf/R1 x V1

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