# op amp resistor sizing question

Discussion in 'Homework Help' started by notoriusjt2, Oct 31, 2010.

1. ### notoriusjt2 Thread Starter Member

Feb 4, 2010
209
0

Vin = -Vout(R1/R2)
1=-12(1000/R2)
solve for R2
R2=83.3Ω

correct?

2. ### hgmjr Moderator

Jan 28, 2005
9,030
215
You have made an incorrect assumption.

Notice that the feedback is positive not negative.

hgmjr

Feb 4, 2010
209
0
4. ### hgmjr Moderator

Jan 28, 2005
9,030
215
Section 4 of the article that you have linked to covers the topic fairly well.

hgmjr

5. ### notoriusjt2 Thread Starter Member

Feb 4, 2010
209
0
yes it does, its a great article, and I followed it to a T.

"Since V+ is a voltage divider formed by R1 and R2between Vin and Vout it follows that:
• V+ = (Vin R2 + Vout R1 )/(R1 + R2)

Combining these equations and solving for V+ = 0 yields Vin = -Vout R1/R2.

I get 83.3ohms, but that answer is listed as incorrect

6. ### hgmjr Moderator

Jan 28, 2005
9,030
215
Keep in mind that the problem statement states that the input switches when the +input of the 1K resistor reaches 1V. If you assume that the negative input of the opamp is at 0V then you can calculate the current that is flowing in the 1K resistor at the time the output switches.

hgmjr

7. ### notoriusjt2 Thread Starter Member

Feb 4, 2010
209
0
it has Vin across the two input wires. does this mean that Vin+ is actually +.5v and that Vin- is actually -.5V?

8. ### sharma@vivek New Member

May 2, 2010
9
0
since the input is between the two terminals of the opamp, you need to solve the value for R by superposition theorem.