# Op Amps

1. Sep 10, 2009

### uzair_ha91

What's inside an op amp? Is it just an npn transistor used for amplification? So can we explain it in the terms of base and collector current?
Please elaborate simply (It's a high school project)

2. Sep 10, 2009

### waht

Op amps contain many transistors, some are used for amplification, others for current sources.

http://members.arstechnica.com/x/so1os/500px-OpAmpTransistorLevel_Colored_Labeled.svg.png [Broken]

Last edited by a moderator: May 4, 2017
3. Sep 11, 2009

### uzair_ha91

Ok, so if I know what npn transistor, its structure, its working and its formulas like current gain=$$\beta$$=Vo/Vin are and how it can be used as an amplifier and as a switch, will I be able to understand op amps? In other words, does it have the same basics? Is it a combination of many npn transistor amplifiers (as depicted by the image you posted)?

4. Sep 11, 2009

### negitron

Instead of trying to understand op-amps as the sum of their parts, youy're probably beetter off learning op-amp theory as a fundamental unit. Think of an op-amp as a black box; you don't need to know what's inside in order to understand it, you need only know how it behaves given specific input.

http://www.pic101.com/op_amp.htm

5. Sep 11, 2009

### uzair_ha91

ok, that means its a black box which amplifies, that's it, right?

@waht how did you drop the white background in the image you posted?

6. Sep 11, 2009

### negitron

It may not hae been initially clear, but there's a link in my prior post which answers your op-amp question. An op-amp is an amplifier which has the following characteristics:

EDIT: I've bolded an error I just noticed in the previous link. Everything else looks okay, but I'll provide an alternate link with more accurate information:

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampi.html

Last edited: Sep 11, 2009
7. Sep 11, 2009

### uzair_ha91

Oh K.....I checked out the link you gave and I have a question regarding it::

Can you tell why the second equation has Rf+Rin while the first one does not?

8. Sep 11, 2009

### waht

That's because the image has .png extension which is basically a multi layered image, and it does not have a background encoded so it could save memory.

There are two assumptions when analyzing op-amp circuits:

1. The inputs have infinite input resistance.
2. Output has infinite gain.

and when you apply this you are going to get alot of weird equations.

9. Sep 11, 2009

### uzair_ha91

Can you explain the second step of the derivation where i is calculated for both resistors?

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Last edited: Sep 11, 2009