[SOLVED] Non Inverting Difference amplifer

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p5taylor

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Hi
I am trying to derive the transfer function for a Non Inverting Difference amplifer
i have worked it out for a inverted type:

Vout = -V2 (R2/R3)+V1(R4/(R1+R4))((R3+R2)/R3)

Any help is appreciated.

Kind Regards
Paul
 

In differential (Subtractor) amplifier

If V1>V2, output in negative voltage

If V1<V2, output in positive voltage

normally V1<V2...
 

Hi Paul - for my opinion, a "difference amplifier" neither can be "inverting" nor "non-inverting". It just amplifiers the difference between two input voltages (A-B=-[B-A]) .
And - what is your question?
 

Hi LvW
I basically trying to derive the transfer function for Non Inverting Difference amplifer i geuss it will be similar to the inverting type!

Kind Regards
Paul
 

p5taylor said:
Please find circuit below
Click to expand...

Thanks

Firstly, you need to swap the two opamp inputs. As shown, there is positive feedback instead of negative feedback. Then...

(voltage at non-inverting input of opamp) = (voltage at inverting input of opamp)

So...
V2 * R4 / (R3+R4) = V1 * R2 / (R1+R2) + U1 * R1 / (R1+R2)

Rearrange...
U1 * R1 / (R1+R2) = V2 * R4 / (R3+R4) - V1 * R2 / (R1+R2)

Change the colors so I don't get lost in the next step...
U1 * R1 / (R1+R2) = V2 * R4 / (R3+R4) - V1 * R2 / (R1+R2)

Rearrange again...
U1 = [ V2 * R4 / (R3+R4) - V1 * R2 / (R1+R2) ] * (R1+R2) / R1

And again:
U1 = [ V2 * R4 / (R3+R4) ] * (R1+R2) / R1 - [ V1 * R2 / (R1+R2) ] * (R1+R2) / R1

Simplify the bit on the right...
U1 = [ V2 * R4 / (R3+R4) ] * (R1+R2) / R1 - V1 * [ R2 / R1 ]

Finally, rearranging the red and blue parts, we get...
Code: 
            R4*(R1+R2)          R2
U1  =  V2 * ----------  -  V1 * --
            R1*(R3+R4)          R1

(hopefully I got that right)
 
p5taylor said:
Hi LvW
I basically trying to derive the transfer function for Non Inverting Difference amplifer i geuss it will be similar to the inverting type!
Click to expand...

Paul, didn´t you read my post#5 ? There is no "non-inv. diff. amplifier". A diff. amplifier produces an output which is a superposition of inverted and non-inverted signals.
 
Hi Lvw & godfreyl
Thanks i see that now, i some how from the net no doubt got the circuit above and it depends on how you connect the inputs if the +i/p is the greater voltage than the -i/p or vice verses decides if the output is negative or positive thank you all for your help this is now solved.

Kind Regards
Paul
 

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