# Voltage at the output/Parallel resistors

1. Aug 30, 2009

### dalarev

1. The problem statement, all variables and given/known data

Reading the voltage taken at the output, Vo.

2. Relevant equations

Resistors in parallel carry the same voltage.

3. The attempt at a solution

The circuit shown in the diagram, the output is taken directly in parallel with Ro, so shouldn't that mean that:

VRL = VRD = Vro ?

In calculating the output, they combine all those 3 resistors in parallel into RL', I don't understand why.

In other words, why doesn't Vo = VRL ?

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2. Aug 31, 2009

### Staff: Mentor

The output voltage is indeed the voltage across those three parallel resistor components. It doesn't matter if they are drawn as 3 resistors or the equivalent single resistor, unless some individual current is needed for some other part of the model calculation.

3. Aug 31, 2009

### rootX

I might be repeating the above post:

Yes

You don't know the voltage across either of those three. But the total current passing through all three resistors is g_m*V_gs which is known as I am assuming. You can go solve the circuit in either way: it's a simple circuit with three resistors in parallel and one current source. Simplest possible method is to combine all three ll resistors and find the voltage.