Is there anyone who knows why "virtual ground" does not exist when there is no negative feedfack in the OPA circuit?
I also saw some OPA circuits that have both positive/negative feedback, how can I know if the "virtual ground" still good or not?
THANKS FOR HELPING

 

basically when u have negative feedback, the output drives the potential between the two terminals to 0, hence the virtual ground/short..

if u have postive feedback, it will saturate @ the supply limits, and no feedback u dunno the answer unless u know the exact gain (which can vary greatly from device to device)

Hence u always use negative feedback to get the desired gain..

and u can have postive and negative feedback together, and u wont get saturation as long the magnitude of the negative feedback is bigger than the positive...

 

hi,

you said you some circuits using OPAs having both negative and positive feedback. are we talking about integrators or active filters or neutralizing input capacitance to optimize response time or a free running multivibrator or wave generator or pulse width modulator. all of these employ both feedback type.