# Op Amp resources

1. Nov 6, 2006

### bigaggie

This may be kind of odd, but this is my first post, so I'm kinda new at this.

I need to calculate the output voltage of an op amp with a 2 mA current source inputting into the inverting side, and a ground connected to the inverting input. There is a 1 kiloOhm resistor going across the op amp connected to the inverting side. I hope that made sense.

Now, I'm not really asking for a solution, but rather, is there an internet site or something that has decent examples? Our book is terrible at explaining, gives 2 extremely basic examples, and the prof only confused me further. I obviously need to find something that can help me learn how to do this. Any suggestions?

2. Nov 6, 2006

### Gokul43201

Staff Emeritus
Did you mean "non-inverting input" at the bolded part?

The answer lies in one of the two "golden rules" for an ideal op-amp. If your text doesn't cover these (I'd be surprised, but) look up that phrase.

I don't know any online resources for op-amp examples/tutorials, but I'm pretty certain Berkeman does. He'll come by soon enough. As an extra note, if you have access to a library, see if you can check out "The Art of Electronics" by Horowitz and Hill.

3. Nov 6, 2006

### bigaggie

Oops, yes, that's just exhaustion coming through. The bold part is supposed to read "non-inverting."

The problem with this example is that I have no R1 for the current source, and all the equations I'm finding are dealing with the problem in voltages. This is easily my worst subject, so if the answer is obvious, that's why I'm not seeing it.

4. Nov 7, 2006

### Averagesupernova

If you have a feedback resistor going from the output to the non-inverting input the output will always be at one supply rail or the other.
-
If the resistor goes to the inverting input as you say, then the output will be either -2 volts or +2 volts depending on the direction of the current. Of course this assumes that the non-inverting input is at zero volts.
-
Now, do you know why what I've posted is the way it is? Learning the 'why' is of course the only way to truly understand what is going on.

5. Nov 7, 2006

### Staff: Mentor

I think you are referring to a current-to-voltage converter. The current is routed through the inverting feedback resistor, and the output voltage is the + input voltage plus whatever voltage is developed across the - feedback resistor because of the input current.

http://en.wikipedia.org/wiki/Current-to-voltage_converter

6. Nov 7, 2006

### bigaggie

THAT'S THE ONE!

Why this confused me? Well, I guess it was kinda stupid really. We're told to operate under the ideal op amp model. This states, as I understand it, that the current going into the op amp is zero. However, I forgot...for whatever reason, that it didn't mean the current through the feedback resistor was zero. Oops.

So that problem now makes loads more sense. However, if anyone has any resources to recommend, I'd appreciate it greatly. Thanks.

7. Nov 7, 2006

### Staff: Mentor

Yeah, the current into the inputs of an ideal opamp is zero (and really small in real-life FET opamps as well).

One resource that I recommend often is the book "The Art of Electronics" by Horowitz and Hill. Check it out at your local technical library to see if you like it. It's worth buying as an extra resource, IMO, and well worth reading cover-to-cover early in your study of electronics.