# op amps

#### THOMAS

Joined Jan 9, 2004
2
<_< I need to draw a circuit for a black box whose output is ten times the difference between its two input signals. The ciruit contains two op. amps and five resistors. I need to give the relative values of the resistors.

Equation give is: Vo=10(V2-V1)

I need help on all fronts please #### Battousai

Joined Nov 14, 2003
141
Use ideal op-amp conditions to solve the problem:

1) No current flows into the inputs of an op-amp (infinite input impedance)
2) the op-amp has infinite gain (drives both inputs to be the same value, that is Vip=Vin where Vip and Vin are the inputs).

So look at my thread about the linear vga: one input terminal is grounded, so the other input terminal must also be grounded: now do nodal analysis at this node:

(Vo-0)/R2 = (Vi-0)/R1 --> simplifying Vo/Vi = R2/R1 --> To make Vo/Vi = 10, make R2=10*R1.

You can realize a wide variety of functions with op-amps: addition/subtraction, integration/differentiation, etc. Search for those type of op-amps if you need more help.

#### Dave

Joined Nov 17, 2003
6,969
There is a superb section here on All About Circuits in chapter 8 about Op Amps.

#### mozikluv

Joined Jan 22, 2004
1,435 hi dave,

this is the 1st time i have browse thru your electronic articles about op-amps if not for your reply here. i like the way the article is presented. frankly i have not gone thru any electronic articles here but seeing the item about op-amps sure had me impressed. B)

as a suggestion, let's try to include as part of a reply about op-amps, it would be a great learning experience. 