# Trouble with superposition.

Discussion in 'Homework Help' started by jegues, Feb 25, 2011.

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

Sep 13, 2010
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43
Hello all, I'm trying to apply superposition to the circuit in the figure.

I've found Vo1 and Vo2 and now all that's left to do is determine io.

I've tried to do so in the following manner,

Vo = Vo1 + Vo2,

Then,

$i_{o} = \frac{V_{o}-V_{L}}{R}$

But if I were to plug in Vo, the VL will not cancel away, the current Io is supposed to be independent of the load ZL.

However, if I solve for V1 and V2 and use this to create an expression for io in terms of VI it works.

Can someone see what I'm doing wrong?

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2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
I think perhaps the problem is that both Vo and VL are dependent on the value of ZL. In essence VL lifts Vo by I*ZL.

The obvious way out is to make ZL=0 & hence VL=0.

Then your Vo1+Vo2 approach will work.

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
By the way - did you take the opportunity to read Bob Pease's useful application note on the Howland Current Pump?