Integrated Circuit 741

Thread Starter

chrischristian

Joined Feb 22, 2008
43
Plese, can someone help me to understand from the internal strcture of 741 opamp that how possitive input give positive output and how negative input give inverted output, especislly after the differential amplifier section. The internal structure is on http://en.wikipedia.org/wiki/Op-amp!! and plese try to make it discriptive than mathamatical , I know this is too much but plese help me !!!!!!!!
 

Papabravo

Joined Feb 24, 2006
14,239
Your description is slightly off the mark. The inputs do not affect the output independently as you have implied. The absolute magnitude of the inputs is irrelevant as long as they remain within the "Common Mode Range". This parameter is given on datasheets and typically is contained to a range which is smaller than the power supply rails. There are amplifiers whose common mode range includes one or both of the supply rails.

The critcal point, about all the opamps you will ever see, besides all the ones that you won't, is that the output depends ONLY on the difference between the two inputs.

Example #1
Av = 100,000, V+=12 V, V- = -12V, CMR = {-10.5,..10.5}
IN+ = 8.000001 VDC
IN- = 8.000000 VDC
Vo = (8.000001 - 8.0)*100,000 = 100 mV

Example #2
Same as Example #1
IN+ = 6.000002
IN- = 5.999998
Vo = (6.000002 - 5.999998)*100,000 = 400 mV

Example #3
Same as Example #1
IN+ = 8.000000
IN- = 8.000001
Vo = (8.000000 - 8.000001) * 100,000 = -100 mV

Example #4
Same as Example#1
IN+ = -8.000000
IN- = -8.000001
Vo = (-8.000000 - (-8.000001)) * 100,000 = +100 mV !!

It should be clear from these examples that the differential input stage acts only on the difference between the two inputs and not their absolute magnitude.
 

Papabravo

Joined Feb 24, 2006
14,239
You're right, I don't know how to explain it to you. Until you stop considering the inputs one at a time I don't think anyone will be able to get through to you. That's just your tough luck I guess.
 

spar59

Joined Aug 4, 2007
57
I think I am right in this - op amp internal circuitry is often extremely complex since transistors are easy to create and other componenets more awkward - hence a glut of transistors.

T8 (in conjunction with other components) forms a constant current source.
Output to the second stage is taken from the collector of T6.

A positive input at the inverting input increases current through T2 & T4 raising the voltage at T6 collector. Since this current flows through T8 (constant current source) the current through T1 & T3 decreases (not that we are bothered at this stage).

A positive input at the non-inverting input increases current in T1 & T3 and due to the constant current source (T8) the curent in T2 & T4 decreases lowering the voltage at T6 collector !

Steve.
 
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