Transposition for Rg in 5 = 10k (1 + 20k/Rg)

[nothing909]

Homework Statement
Transpose to find Rg:

5 = 10k (1 + 20k/Rg)

The attempt at a solution

5 = 10k (1 + 20k/Rg)
5/10k = (1 + 20k/Rg)
5/10k - 1 = 20k/Rg
(5/10k - 1) / 20k = 1/Rg
20k / (5/10k -1) = Rg
Rg = -20010

This doesn't look right at all, what am I doing wrong?

 

[magoo]

Looks good to me. Did you try checking by substituting Rg back into the original equation?

 

[nothing909]

deleted.

 

[donpacino]

If Rg is a resistance, no that does not look right. But your math seems to be correct. Can you provide us with context?

 

[nothing909]

Sorry, I had the wrong value in. It should have been
Rg = 20k / Vout/10mV - 1
Rg = 20k / (1/10x10^-3 -1)
Rg = ~200ohm

I'm using an instrumentation amplifier and I'm trying to amplify a 10mV input signal to 1-10V.

I used this calculation so I could work out what the gain resistor should be to amplify the 10mV input signal to 1V.

In multisim, I run a transient simulation with the 200ohm gain resistor and I get this:

why is the signal only being amplified to 0.7V and not 1V like I calculated?

 

[donpacino]

All you are doing is creating more questions :)

Please show us the amplifier, circuit layout, and how Rg is used.

 

[nothing909]

sorry, the circuit layout below, with R3 being Rg.

 

[donpacino]

Hmm I'm not sure. Do you have your pos and neg supplies hooked up to all your amplifiers?
Are you simulating this with the common mode gain?
Can you put in the input signal for comparison?

 

[nothing909]

i'm sure the pos and neg power them all by just being connected to one of the amplifiers.

yes, it's being simulated with the common mode gain.

here is the input signals and output:

do you see any other reason why it is not being amplified correctly?