Virtual ground concept of OPAMP 1

[kunalw]

What are those conditions in which VGC can be applied & which are those where it cannot be applied

 

[GregoryJ]

When dealing with op. amp. schematics we always assume the ideal model. In this scenario, it is given that -(ve) input= +(ve) input. Therefore if there is a voltage applied to any one input while the other is connected to ground, it follows that there is a VIRTUAL GROUND at the other input [since the differential voltage between - and + input must be zero ie., absolute value of -ve input minus absolute value of +ve input].<br>The virtual groung is used to determine the direction of current flow through the feedback and input resistors [If=Ii], given that the input impedance of the ideal op. amp. is infinity and output impedance is small (in order to achieve maximal gain).<br>A situation in which virtual ground is not used is when a voltage is applied to both inputs, hence no input is grounded.

 

[ambertec]

Hello, Kunalw.<br><br>May I suggest looking at it this way. An operational amplifier (op-amp) has two inputs which we call the inverting input and the non-inverting input. With an idealized op-amp, which means one having infinite gain and infinite bandwith, in the response of whatever circuit the op-amp is used, the circuit tries to make the inverting input match whatever voltage happens to be applied to the non-inverting input. If the non-inverting input just happens to be at ground, the inverting input is said to be at &quot;virtual ground&quot;. I prefer to think of it as a &quot;virtual non-inverting input&quot;.<br><br>The rub is that no op-amp really has the ideals of infinite gain and infinite bandwidth. To the extent that gain and bandwidth are actually finite, the inverting input will be not quite equal to the inverting input.<br><br>I hope this helps.<br><br>John Dunn<br>Ambertec, Inc.

 

[kng]

May I ask when will the virtual ground property fail?
As I was setting up the simple inverting amplifier with an opamp (LM324), with the positive node grounded, the negative node only follows the virtual ground property up to a certain voltage. Any increase that voltage, the virtual ground property fail as well as all the point voltages of the circuit.

Any idea? thanks a lot!!

 

[melone]

did you exceed the specs of the device?

 

[kng]

I do not know if it has exceeded the specs. The simple inverting amplifier circuit is setup as such:

http://www.phys.ualberta.ca/~gingrich/phys395/notes/node103.html

where, for example: RI=2.7K Ohm, RF=2.7K Ohm. The maximum value of Vin can only be about 4 V. Any higher value will break down the virtual ground rule

In another instance of the same circuit: RI=2.7K Ohm, RF=5.4K Ohm. Maximum Vin can be up to only 2.13V. Again, virtual ground rule will not hold for any higher values of Vin.

Following is the spec of the opamp I used(LM324):

http://www.national.com/ds/LM/LM124.pdf

does anyone know what the open-loop gain of the opamp is?

Thanks again!!

 

[mleigh]

Just a quick point about virtual grounds - they only apply when you're using negative feedback.

 

[Lewish]

Hello kng, please explain further what you mean by virtual ground will not hold. Do you mean that the output signal level has the wrong gain? Or do you mean that the voltage from the inverting pin to ground is not zero?
Also, what voltages do you have applied to pins 4 and 11 of the chip? If you are running off + and - 5 Volts supplies, then this is about where you would expect the output to stop following the input.
Open loop gain is partly dependent on your power supply, but a reasonable value is listed on page 3 under Large Signal Voltage Gain.