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about OPA circuit. simulate in MC11

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cyberjok

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just check the attach file. there are all works fine in MC11 sim software, do u think that is a bug?
 

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  • QQ图片20160608171310.png
    QQ图片20160608171310.png
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I have never heard of "MC11 sim software". It seems that it knows nothing about your simple opamp circuits.
Your first circuit has positive feedback so its output will latch as high or as low as it can go, unless the input level is high enough to destroy the opamp.
Your second circuit is a normal inverting opamp circuit but it uses the lousy old LM324 opamp that has crossover distortion, is noisy and has trouble with high levels above 2kHz.
 

Your simulation package (Microcap) works fine and reliable.
Performing an AC analysis, no simulation package can discriminate between positive and negative feedback.
That is not a bug - it is rather the correct solution.

The reason is that during ac analyses, the simulator assumes that (a) the power supply is available since infinite negative times (no switch-on transient) and (b) that there is no disturbance of the equilibrium.
As a mechanical example: One small ball will ride upon a larger and will be stable if we assume that (a) that both are ideally centered and (b) that there is absolutely no external disturbance.

In the positive feedback example, the instability will be revealed only during transient simulation (time domain) when either the supply voltages or the input siganl are switched-on at t=0.
 

MC11 is micro-cap 11, yes the positive feedback will make the OPA be saturated.

thx for your input.

- - - Updated - - -

LvW said:
Your simulation package (Microcap) works fine and reliable.
Performing an AC analysis, no simulation package can discriminate between positive and negative feedback.
That is not a bug - it is rather the correct solution.

The reason is that during ac analyses, the simulator assumes that (a) the power supply is available since infinite negative times (no switch-on transient) and (b) that there is no disturbance of the equilibrium.
As a mechanical example: One small ball will ride upon a larger and will be stable if we assume that (a) that both are ideally centered and (b) that there is absolutely no external disturbance.

In the positive feedback example, the instability will be revealed only during transient simulation (time domain) when either the supply voltages or the input siganl are switched-on at t=0.
Click to expand...

many thx, Just try the TINA simulate the same circuit, but it shows diff result, positive feedback opa become saturated. :) Maybe I need to learn more about OPA principle... :)
 

cyberjok said:
Maybe I need to learn more about OPA principle... :)
Click to expand...
Basically, the datasheet for all opamps shows the Large Signal Voltage Gain from an input to the output when there is no feedback. For the old LM324 it is typically 100,000 times. Positive feedback adds gain to the input and causes the opamp to amplify a very small input enough to cause the output to saturate as high or as low as it can go. Negative feedback subtracts gain, distortion and noise and can reduce the gain for the opamp to be useable. The ratio of the two negative feedback resistors determines the gain. Your second circuit has a gain of 10k/10k= 1 but since the input is to the (-) input, it inverts the signal.
 

Audioguru said:
Basically, the datasheet for all opamps shows the Large Signal Voltage Gain from an input to the output when there is no feedback. For the old LM324 it is typically 100,000 times. Positive feedback adds gain to the input and causes the opamp to amplify a very small input enough to cause the output to saturate as high or as low as it can go. Negative feedback subtracts gain, distortion and noise and can reduce the gain for the opamp to be useable. The ratio of the two negative feedback resistors determines the gain. Your second circuit has a gain of 10k/10k= 1 but since the input is to the (-) input, it inverts the signal.
Click to expand...


thx,,, I am just curious why that positive feedback OPA still working as negative feedback in MC11 simulator.. :)

as this simple OPA internal circuit, it looks so easy to know which pin is Positive input...
 

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  • QQ图片20160609114717.png
    QQ图片20160609114717.png
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cyberjok said:
thx,,, I am just curious why that positive feedback OPA still working as negative feedback in MC11 simulator.. :)
Click to expand...

Recall my mechanical example.
Under IDEAL condition (which, however, never can be realized in practice) an opamp with positive feedback would also be stable.
And for small-signal ac simulations the simulator assumes such IDEAL conditions. Remember: Before ac simulations (calculations) start the circuit will always be linearized around the corresponding DC bias point.
 
The simulator software that allows an opamp to work when it wrongly has positive feedback instead of correct negative feedback is stupid software.
 

Audioguru said:
The simulator software that allows an opamp to work when it wrongly has positive feedback instead of correct negative feedback is stupid software.
Click to expand...
Audioguru - you have to discriminate between simulations in the time anf frequency domain !
I am sure - even YOUR software (what is it?) will show stable gain values for positive feedback when simulated in the frequency domain.
Perhaps you are not familiar with such simulations?
If you really understand what`s going on during this simulation you will see that the result is correct (but only under ideal conditions).
The simulator does not fail - it is the user who must not misinterpret the results!
 

LvW said:
Recall my mechanical example.
Under IDEAL condition (which, however, never can be realized in practice) an opamp with positive feedback would also be stable.
And for small-signal ac simulations the simulator assumes such IDEAL conditions. Remember: Before ac simulations (calculations) start the circuit will always be linearized around the corresponding DC bias point.
Click to expand...

:-D thank you for the explanation.. Just try to use the idea opa to simulate the same circuit ,got the same result that it works on positive feedback,,, --- TINA 9 simulator.

:bang: it looks not so easy to understand :)
 
Last edited:

cyberjok said:
:-D thank you for the explanation.. Just try to use the idea opa to simulate the same circuit ,got the same result that it works on positive feedback,,, --- TINA 9 simulator.

:bang: it looks not so easy to understand :)
Click to expand...

What is the problem?
Try a calculation by hand - and you will see that you can find a stable output.
(But this calculation assumes absolutely no disturbance and supply voltages which exist since million of years; and that`s not realistic).
 

It is impractical to set up an input DC voltage exactly the same as the input offset voltage and adjust it a tiny amount negative if the output begins to go positive and adjust the input voltage a tiny amount positive if the output begins going negative. If your adjustments are a little slow then the output will speed (snap action) to as high or as low as it can go and stay there.
 

Audioguru said:
It is impractical to set up an input DC voltage exactly the same as the input offset voltage and adjust it a tiny amount negative if the output begins to go positive and adjust the input voltage a tiny amount positive if the output begins going negative. If your adjustments are a little slow then the output will speed (snap action) to as high or as low as it can go and stay there.
Click to expand...
In this thread, who spoke about offset adjustments?
 

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