# [SOLVED]Non Inverting Difference amplifer

Status
Not open for further replies.

#### p5taylor

##### Newbie level 4 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

#### amayilsamy In differential (Subtractor) amplifier

If V1>V2, output in negative voltage

If V1<V2, output in positive voltage

normally V1<V2...

#### godfreyl I am trying to derive the transfer function for a Non Inverting Difference amplifer
Can you show the circuit you are talking about?

#### p5taylor

##### Newbie level 4  Kind Regards
Paul

#### LvW 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?

#### p5taylor

##### Newbie level 4 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

#### godfreyl 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

### p5taylor

Points: 2

#### LvW 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!

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.

• p5taylor

### p5taylor

Points: 2

#### p5taylor

##### Newbie level 4 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

Status
Not open for further replies.