Opamp Virtual Ground

[teknoman2]

Can someone explain the purpose of virtual ground using opamp?
For example I see Brian use it on the Stage Fright project.

[RobA]

Are you asking why the op-amp (IC1B on the Stage Fright) is used to buffer the VB instead of just using the output from the voltage splitter?

If so, there are several reasons. On of the main is the same reason you'd use an output buffer for the audio signal, it provides current drive. It also makes the current used active. You can increase the level of the resistors used in the voltage divider because they just set the voltage level. Then the op-amp actively drives the current that is needed. So, you can reduce the overall current required to drive the circuit. In the Stage Fright, the VB is used in several locations. Buffering the VB with the op-amp will help to reduce the interactions between those points. It also helps to reduce noise into the VB related inputs on the op-amps.

[teknoman2]

Yeap that was my exact question, sorry for not place my question very well,
So if I understand it right buffering the power section using opamp will reduce the required current of the entire circuit and reduce the overall noise of every input stage, so far so good.
If we don't buffer the VB and we will drive the remaining circuit with higher current will that be a serious problem for the overall behavour of the circuit?

[RobA]

...
If we don't buffer the VB and we will drive the remaining circuit with higher current will that be a serious problem for the overall behavour of the circuit?

It depends on the circuit. It could. If you have enough draw from the VB, then you could actually get to the point where the voltage isn't stable and that would seriously degrade the performance of the op-amp.

If there is noise in the circuit, then a stage with much gain could be injecting amplified noise into the VB which would then show up in other stages of the circuit at their VB connection and that could really screw with the circuit. Given that you are usually going to see these in a single power rail design, where you already can't bypass one power rail and that rail is at ground with noise from the circuit sitting on it already, it really is a benefit to get the noise off of the reference voltage.

It's an important enough thing in some circuits, like high end audio, instrumentation, AD converters, etc. that TI makes a part specifically to do the job better than even a buffered op-amp circuit, the TLE2426. The spec sheet for that part goes into some detail about the benefits. Also, in some of TI's op-amp spec sheets and application notes, they give some details about using them.