Op_Amp Circuits not tied to ground

Discussion in 'General Electronics Chat' started by PhotoMonger, Aug 5, 2008.

1. PhotoMonger Thread Starter Member

Jan 7, 2008
16
0
I'm having trouble deriving equations for a series of op-amp configurations not tied to ground. The circuits look like typical configurations but instead of being tied to ground they are tied to 1.5V. I have included a picture, any help would be great. I'm assuming it's not as easy as using 1.5V as a reference and adding it to my output voltage. Thanks!

2. mik3 Senior Member

Feb 4, 2008
4,846
70
We assume an ideal op-amp.
Then because the gain is infinite the voltage at the node between R1 and R2 equals 10mV (the voltage on the non-inverting input).

Then, we say that the current through R1 (IR1) equals the current through R2 (IR2) because the inputs have infinite impedance thus they do not draw any current.

Thus:

IR1=(Vss-0.01)/R1

IR2=(Vout-0.01)/R2

then you write current equations at the node between the ressitors:

IR1=-IR2

(Vss-0.01)/R1=-(Vout-0.01)/R2

solve for Vout and you get:

Vout=-[(R2/R1)*(Vss-0.01)+0.01]

3. Wendy Moderator

Mar 24, 2008
21,838
3,049
Just add 1.5 V to whatever answers you get if they were tied to ground. It is a simple offset.

4. PhotoMonger Thread Starter Member

Jan 7, 2008
16
0
mik3, Bill,
Thank you both so much for your replies. Mik3, those were the equations I was following, but didn't come out with an answer that looked correct. Bill, I also tried the method you recommended and again didn't come out with an acceptable answer to match the simulation in multisim which has an output of -1.269 volts. The only thing I can think of is that my simulating is incorrect. Thanks again for the help.

Cheers,
ian