What Components Make Up an Op Amp?

[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)

 

[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

 

[uzair_ha91]

Ok, so if I know what npn transistor, its structure, its working and its formulas like current gain=\beta=V_o/V_in 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)?

 

[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

 

[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?

 

[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:

1) Assume the output impedance as almost zero.

2) Assume the - input impedance as almost zero.

3) Assume the + input impedance as almost infinite.

4) The gain is programmable by the feedback resistor, the resistor from the output pin to the negative input pin, from zero to infinite depending on the device.

5) An opamp can amplify AC or DC voltages, or both.

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

 

[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 R_f+R_in while the first one does not?

 

[waht]

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?

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.

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

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 a lot of weird equations.



http://www.allaboutcircuits.com/vol_3/chpt_8/2.html

 

[uzair_ha91]

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