Hello!

I've got a problem with understanding why the operational amplifier is in saturation state.



I don't know which feedback is the "dominant" here and why the negative feedback doesn't want to stable the input I mean here to stable V- = V+

 

The circuit is saturating because you have a huge amount of positive feedback, no stable DC operating point, and no GND.

The schematic has so many problems that it is not clear what you are trying to achieve. What do you want the circuit to do?

Input signal amplitude

Output signal amplitude

Inverting or not

Bandwidth

Ground node

ak

 

The circuit is saturating because you have a huge amount of positive feedback and no stable DC operating point.

The schematic has so many problems that it is not clear what you are trying to achieve. What do you want the circuit to do?

Input signal amplitude

Output signal amplitude

Inverting or not

Bandwidth

ak

I'm experimenting.
Because the capacitor has a reactance from AC input. So when the negative feedback with resistors has this equation R1/(R1+R2) and positive feedback has R4/(R3+R4) then I thought that when I want to have "dominant" negative feedback I have to increase R1 in this case reactance of this capacitor.

But I don't understand the point "DC operating point" what does it mean? When I don't have the positive feedback I can achieve AC output so as well AC operating point?

Why the negative feedback won't work instead of positive? I know that even positive or negative feedback can work, they can't work at the same time. So I can have a increased AC sygnal or saturated sygnal.

 

The inputs of an operational amplifier are supposed to be at the same potential. This potential must have a DC path to ground for stability. Without that DC path to ground, the actual potential could be anything. You need to get your head out of the dark space it is occupying.

 

The inputs of an operational amplifier are supposed to be at the same potential. This potential must have a DC path to ground for stability. Without that DC path to ground, the actual potential could be anything. You need to get your head out of the dark space it is occupying.

I don't understand, why DC path? I'm using here AC. I also have a negative feedback to create this stability. So why positive feedback "dominates" this circuit and makes it saturated.

 

Your op-amp has no visible voltage supply.

why DC path?

Because a real-world op-amp is not ideal and has input semiconductors which require bias current to operate.

 

Your op-amp has no visible voltage supply.

Because a real-world op-amp is not ideal and has input semiconductors which require bias current to operate.

The supply is 15V and -15V.
I still don't get it why DC path.

Like when I have only a negative feedback without positive feedback, and AC sygnal input, I'll get from the output also AC sygnal, and it is not DC.

 

The supply is 15V and -15V.
I still don't get it why DC path.

Like when I have only a negative feedback without positive feedback, and AC sygnal input, I'll get from the output also AC sygnal, and it is not DC.

I'm guessing that understanding circuits will be a problem for you if you cling to that belief. All points in a circuit must be referenced to a specific reference potential. If thye were not and the voltage could be anything with respect to a fixed reference, there would be no such thing as stability.

Every AC signal that you will ever see, besides all the ones you won't, will have a DC component. That DC component can be the same as the ground reference, but it cannot be absent.

 

I'm guessing that understanding circuits will be a problem for you if you cling to that belief. All points in a circuit must be referenced to a specific reference potential. If thye were not and the voltage could be anything with respect to a fixed reference, there would be no such thing as stability.

Every AC signal that you will ever see, besides all the ones you won't, will have a DC component. That DC component can be the same as the ground reference, but it cannot be absent.

Ok so If I don't have a positive feedback and instead of it is ground. Then I have a DC component. So that's why I can achieve a stability.

So because I have a positive feedback, then I don't have a DC component because both of them can change all the time ?

Bot for example here https://tinyurl.com/ybrv6pc6 I don't have a DC component and the circuit is not saturared.

 

amplifier is in saturation

because the 10uF poses almost a short circuit to the "Phase" terminal of the AC source - it has a connection to the :
no "

no GND

" of the particular Falstad simulation -- the GND is . . . relative to the op amp output (depends how you define it's Vee/Vss/-Vs and Vcc/Vdd/+Vs ) . . . so (in simplified form) the GND comes through to the current source and output impedance of the op amp to the positive feedback chain to the "Common" terminal of your AC source ...
... however you have the near ideal op amp in your hand (means the input bias currents of the op amp are neglible) so it only sees the input voltages which are V.out at +V.in and +V.in + V.ac at the -V.in

your op amp is configured as a voltage comparator with hysteresis

- so the output of the R2R square wave is expected . . .

PS! -- correction ; it seems the AC source in Falstad simulation is not floating but has Common=GND ← that a bit changes the game here

 

" of the particular Falstad simulation -- the GND is . . . relative to the op amp output (depends how you define it's Vee/Vss/-Vs and Vcc/Vdd/+Vs ) . . . so (in simplified form) the GND comes through to the current source and output impedance of the op amp to the positive feedback chain to the "Common" terminal of your AC source ...
... however you have the near ideal op amp in your hand (means the input bias currents of the op amp are neglible) so it only sees the input voltages which are V.out at +V.in and +V.in + V.ac at the -V.in

your op amp is configured as a voltage comparator with hysteresis

In this video there is only positive feedback, but I have here positive and negative.

 

This circuit is your classic hysteretic oscillator. It is a common circuit for many Schmitt trigger gates, such as 555 Hysteretic Oscillator.

 

It's not clear exactly what you want (or expect) the circuit to do(?).
Obviously what you have doesn't do that.

 

Like when I have only a negative feedback without positive feedback, and AC sygnal input, I'll get from the output also AC sygnal, and it is not DC.

The signal of interest is not DC, but it has an average value of some DC voltage. For audio, this is the voltage at the zero-crossings. With dual power supplies, the output DC value probably is 0 V; but it doesn't have to be. If you post your circuit, we can discuss how it really operates.

 

It's not clear exactly what you want (or expect) the circuit to do(?).
Obviously what you have doesn't do that.

This circuit is a generator.
But I am just thinking about why positive feedback is stronger. And why negative feedback will never be stronger than positive feedback.

 

The signal of interest is not DC, but it has an average value of some DC voltage. For audio, this is the voltage at the zero-crossings. With dual power supplies, the output DC value probably is 0 V; but it doesn't have to be. If you post your circuit, we can discuss how it really operates.

The output is average?

 

This circuit is your classic hysteretic oscillator. It is a common circuit for many Schmitt trigger gates, such as 555 Hysteretic Oscillator.

It would be if it weren't being driven by a floating 40 Hz AC source.

ak

 

Why the negative feedback won't work instead of positive? I know that even positive or negative feedback can work, they can't work at the same time.

Yes, they can. But not in a standard amplifier configuration. Having both positive and negative feedback around the same opamp creates a bridge circuit and raises the output impedance. The best case of this is the Howland Current Pump.

https://circuitcellar.com/resources/quickbits/howland-current-source/
https://www.edn.com/op-amp-current-sources-the-howland-current-pump/

ak

 

Yes, they can. But not in a standard amplifier configuration. Having both positive and negative feedback around the same opamp creates a bridge circuit and raises the output impedance. The best case of this is the Howland Current Pump.

https://circuitcellar.com/resources/quickbits/howland-current-source/
https://www.edn.com/op-amp-current-sources-the-howland-current-pump/

ak

Ok so could you tell me why I can find a definition "DC feedback"? This circuit from the beggining has a positive feedback that is DC feedback and negative feedback that is not DC feedback.

What if I add resistor in parallel with capacitor. Will it make negative feedback a DC feedback?

 

// As it turned out (the GND bound AC source case) the Falstad simulates your first presented circuit a bit unexpectedly
Here's an LTspice variant (with the "floating" not ground referenced AC source) but you will need some struggling through the current feedback op amp and voltage feedback op amp and the simplified equivalent model of the op amp to get a better grasp why . . . how exactly the output is dependent on the rest of the circuit
https://www.analogictips.com/what-i...urrent-feedback-voltage-feedback-op-amps-faq/
https://www.analog.com/en/analog-dialogue/articles/current-feedback-amplifiers-1.html