# Opamps -- Unknown values

Poster has been reminded to post schoolwork in the Homework Help forums (moved from EE)
Hi everybody!

This is my first post as I am needing some help in a question I am struggling with. Not looking for answers, just some guidance in the right direction.

## Homework Statement

[/B]
I need to calculate the unknown, highlighted, voltages on the op amp circuits (attached).

Assume all op-amps are ideal

See below

## The Attempt at a Solution

I have done the following so far:

a) Inverting
A=-Rf/Ri =(-100)/10=-10
Vi =I1*R1 =0.00001*100000=1 V
Vo=(-Rf/Ri )*Vi
Vo =1*-10
Vo =-10 V

b) Summing
Vo=-(Rf/R_1 V_1+Rf/R_2 V_2 )
-2=(Rf/R_1 1+Rf/(R/2) V_2 )

c) Non-inverting
V1 =V2
V1 =100 mV
V2 =100 mV

d) Current to Voltage
Vo=-I1*Rf
Vo =-0.0002*10000=-2
Vo =-2 V

I am struggling the (b). I cant see how to calculate given only one value, and also with (c)

Many thanks in advance, happy to discuss the methods.

Joel

#### Attachments

• Op Amps Question.png
19.1 KB · Views: 461
Last edited:

.Scott
Homework Helper
For A, Note that the voltage at both inputs to the op amp are equal, and at ground.
For B, I have 1R-2R+x(R/2)=0
For C & D, we agree.

rude man
Homework Helper
Gold Member
a) Inverting
A=-Rf/Ri =(-100)/10=-10
Vi =I1*R1 =0.00001*100000=1 V
You have R1 wrong.
b) Summing
Vo=-(Rf/R V_1+Rf/R_2 V_2 )
-2=(Rf/R 1+Rf/(R/2) V_2 )
correct (I took out the "Rf" , should be just "R" everywhere)
c) Non-inverting
V1 =V2
V1 =100 mV
V2 =100 mV
Right.
d) Current to Voltage
Vo=-I1*Rf
Vo =-0.0002*10000=-2
Vo =-2 V
Right
I am struggling the (b). I cant see how to calculate given only one value, and also with (c)
The R's cancel. There is nothing more to do with (c) than you have already done.

• j172
Thank you very much for your help

b) Summing
Vo=-(R/R*V_1+R/R*V_2 )
-2=(R/R*1+R/(R/2)*V_2 )

So is there another step to this because this equation doesn't give V_2. do you transpose it ?

Last edited:
rude man
Homework Helper
Gold Member
b) Summing
-2=(R/R*1+R/(R/2)*V_2 )
So is there another step to this because this equation doesn't give V_2. do you transpose it ?
It doesn't?

rude man
Homework Helper
Gold Member
Well let's see - you have one equation with 1 unknown.
If this is difficult I really am at a loss as to what to hint next, so I must perforce cease from further such.

b is 1/2 volt

NascentOxygen
Staff Emeritus