# Non-Inverting Amplifier Question

Discussion in 'Homework Help' started by jegues, Feb 23, 2011.

1. ### jegues Thread Starter Well-Known Member

Sep 13, 2010
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First let,

$R_{N} = R_{N1}||...||R_{Nn}$

$R_{P} = R_{P1}||...||R_{P0}$

I'm trying to find the effect the non-inverting terminal has on the output. I first ground the inverting terminal and call the equivalent resistance there RN.

Now I know that,

$V_{oP} = (\frac{R_{f}}{R_{N}} + 1)V_{in}$

Now that all that's left to do is find "Vin".

The current across Rp1 will be Vn1/Rp1 as follows for the current across Rpn, Vnn/Rpn.

Therefore by KCL the current across Rp0 to ground is the sum of all these currents, i0 = i1+...+in

So the voltage as seen by the non-inverting terminal is simply Rp0 * i0 or Rp0*[i1 +...+ in]

or equivalently,

$V_{+} = R_{P0} * \left[ \frac{V_{p1}}{R_{P1}} + ... + \frac{V_{pn}}{R_{Pn}} \right]$

but I'm having trouble converting this to the following form,

$V_{oP} = (\frac{R_{f}}{R_{N}} + 1) \left[\frac{R_{P}*V_{p1}}{R_{P1}} + ... + \frac{R_{P}*V_{pn}}{R_{Pn}} \right]$

I feel fairly confident I've got the work done right I just can't see how to switch between the two forms.

Any ideas?

EDIT: I found my mistake with currents i1 up to in, I was able to obtain the correct form now.

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Last edited: Feb 23, 2011