Hi, I have a question about OPAMP virtual ground.

Yunsik Chung

Intellectual 1150 points

Hi, I was studying about OPAMP and i started wondering about virtual ground concept.

In the lecture, they said virtual ground only occurs in negative feedback OPAMP.

From the equation, Vout = A ( V+ - V-) (A is open loop gain), Vout is finite, A is infinite, so V+ - V- should be 0.

Thus, this is why V+ = V- in negative feedback OPAMP.

But I wonder why this is not true in open loop OPAMP?

In open loop OPAMP, Vout is not finite?

Thanks,

Yunsik

over 5 years ago

0 Ron Michallick over 5 years ago

TI__Guru**** 166906 points

Yunsik,

Vout = A * ( V+ - V-) with Vout limited to VOL and VOH level of the op amp. This is true open loop or closed. In other words, 'A' drops to zero when output get to VOL or VOH level.

0 Bala Ravi over 5 years ago in reply to Ron Michallick

TI__Expert 6625 points

HI Yunsik,

In addition to Ron's comments above, a virtual ground only exist for negative feedback. In case of a positive feedback or open loop the two inputs (inverting input and non-inverting input) are not held at the same potential hence they tend to saturate either to Vcc or Vee. In other words, they behave like a comparator without the negative feedback.

Best Regards,

Bala Ravi

0 Yunsik Chung over 5 years ago in reply to Bala Ravi

TI__Intellectual 1150 points

Hi Ravi,

Thank you for your reply.

I am still little bit confused why V+, V- tend to saturate and behave like a comparator when there is no feedback.

Could you explain in more detail?

0 Yunsik Chung over 5 years ago in reply to Ron Michallick

TI__Intellectual 1150 points

Hi Ron,

Thank you for your reply!

The fact 'A' drops to zero has something to do with virtual ground?

0 Ron Michallick over 5 years ago in reply to Yunsik Chung

TI__Guru**** 166906 points

Yunsik,

The large value for A is the reason that a virtual ground concept exists. When A drops to zero there is no virtual ground at all.

'A' drops to zero because op amp can not supply an output voltage that is bigger than the maximum low and high levels. (VOL & VOH)

0 Yunsik Chung over 5 years ago in reply to Ron Michallick

TI__Intellectual 1150 points

Hi Ron,

Thank you very much.

I understand it crystal clear!

Amplifiers

Amplifiers forum

Hi, I have a question about OPAMP virtual ground.