Op-Amp Help

[shaneyj]

As audioguru says in #14 the signal is almost complerely cancelled at the - input. Therefore this can be considered as a virtual earth.
Current running into this position from whatever source has to be provided by the output of the amp. Thus the output current times the feedback resistor is the output voltage.

I must be misinformed and/or confused...
The input (channel 1/yellow) is a smooth sine wave, except for the slight distortion at the zero crossing.
Can you elaborate and explain?

 

[duke37]

Where did you measure the input?
The input to the total amplifier comes from a sine generator which shoud be a pure sine wave if it has zero output impedance.
If you are measuring at the op-amp input, then then the voltage should be the same as the + input i.e. zero. If the voltage of the inputs to the op-amp differs significantly, then the amp is not working properly.
It looks to me that the amp is working properly at the cross over point when clipping is not occurring.

 

[shaneyj]

Where did you measure the input?
The input to the total amplifier comes from a sine generator which shoud be a pure sine wave if it has zero output impedance.
If you are measuring at the op-amp input, then then the voltage should be the same as the + input i.e. zero. If the voltage of the inputs to the op-amp differs significantly, then the amp is not working properly.
It looks to me that the amp is working properly at the cross over point when clipping is not occurring.

I understand, duke37... If you reference post 19, you'll see the circuit. It is supplied from 3 voltage sources. Not sure the integrity because it's a simulator...
So from the info supplied so far, it sounds as though this circuit in particular was for education only and would not be created like this for practical use.

 

[duke37]

This is a common circuit and is used to add several inputs such as microphones so is of great practical use. Since each input goes to virtual ground, there is no interaction between inputs and dynamic microphones do not act as speakers.

The total input must be small enough to avoid clipping.

 

[Audioguru]

What do you mean by "the entire circuit"?

V1, V2 and V3 are the inputs to the entire circuit. You NEVER(!) look at the (-) input of an opamp because its signal is cancelled by the negative feedback when the output is not clipping.

 

[shaneyj]

V1, V2 and V3 are the inputs to the entire circuit. You NEVER(!) look at the (-) input of an opamp because its signal is cancelled by the negative feedback when the output is not clipping.

Where do you suggest channel 1 of the scope be tied to in order to see the input then?

 

[Audioguru]

Where do you suggest channel 1 of the scope be tied to in order to see the input then?

I told you already, at V1, V2 or V3 which are the inputs to the circuit.

 

[shaneyj]

I told you already, at V1, V2 or V3 which are the inputs to the circuit.

I assumed that because V1, V2, and V3 are parallel to each other and they all feed into the input, that meant they are common and the connection point for the scope is the same.
I guess I was looking for something more complicated than "connect the scope further up stream"...
Thanks for your patience.

 

[Audioguru]

Look again. V1, V2 and V3 are not in parallel, they are completely separate from each other and they have different levels. They connect to their own input resistor of the circuit.