I have a few questions about feedback. For discussion consider an
inverting op amp configuration with R2 feedback @[EMAIL PROTECTED]
4K ohms and R1
(connected to V-) @[EMAIL PROTECTED]
1K at 1K ohms. The open loop gain to be 200,000 .
Assume that Vin is at 2 volts, according to my calculations using Vout
= (V+ - V-)*200,000 and V- = (Vin-Vout)*R2/(R1+R2)+Vout, V- should be
around .0003999 volts and Vout should be around -7.9980005 volts.
My question is in what fa****on does Vout get to -7.9980005? I know it
happens but it seems like magic. I have looked at the schematic
diagram of a 741 for example. It will probably take decades before I
would be able to understand something like that. Is it instantaneous
or does the op amp spend some time hunting for this value? If it
hunts, does it swing low to the negative rail and move towards the
upper rail or do something else?
I did some experimenting using excel trying to iterate between Vout =
(V+ - V-)*200,000 and V- = (Vin-Vout)*R2/(R1+R2)+Vout to see if the
thing will somehow converge. Here are some examples:
Assume that the op amp guessed the output was -8.1 making V- = -.02
(Voltage Divider), but this would mean that Vout should be 400
(amplifying the difference between V+ and V- , ....
Vout V-
-8.1 -0.02
400 81.6
-1632000 -326398.4
6527968000 1305593602
-2.61119E+13 -5.22237E+12
1.04447E+17 2.08895E+16
-4.1779E+20 -8.3558E+19
Assume that the op am guessed the output was -7.9
Vout V-
-7.9 0.02
-400 -78.4
1568000 313601.6
-6272032000 -1254406398
2.50881E+13 5.01763E+12
-1.00353E+17 -2.00705E+16
4.0141E+20 8.0282E+19
-1.60564E+24 -3.21128E+23
6.42256E+27 1.28451E+27
-2.56902E+31 -5.13805E+30
1.02761E+35 2.05522E+34
Vout for both of these does not seem to converge on anything??? I
don't think that clipping off Vout to stay within the positive and
negative rails would help.
How does an op amp do its magic?
Any help would be greatly appreciated. Thanks
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