Understanding VRef

[cranzor]

Hi everyone,

I've recently been playing around with circuits on a breadboard and as I wade a little deeper I'm starting to come across VRef but I'm not completely understanding what it is or its function. Why is it necessary to have a virtual ground as opposed to GND?

For example, in the thread Opamps for Dummies part 2 and 3, https://forum.pedalpcb.com/threads/opamps-for-dummies-part-2.18899/ , by @Chuck D. Bones , what is Vref in relation to the TL072 in the circuit and where is it coming from? is it providing 4.5v to the opamp or is it another ground?

I hope this question makes sense, not sure it does myself haha.

 

[jesuscrisp]

Vref provides basing to half the supply voltage. It's needed because most op amps are basically built for bipolar power, but 9V batteries and standard power supplies only give single supply. Therefore you create Vref and bias the amp in-between 9V and 0V (ground) using a voltage divider.

 

[BuddytheReow]

Vref provides basing to half the supply voltage. It's needed because most op amps are basically built for bipolar power, but 9V batteries and standard power supplies only give single supply. Therefore you create Vref and bias the amp in-between 9V and 0V (ground) using a voltage divider.

Some circuits use a charge pump to create both +9V and -9V to power the opamp. The virtual ground in this case would actually be GND

 

[jesuscrisp]

Some circuits use a charge pump to create both +9V and -9V to power the opamp. The virtual ground in this case would actually be GND

Obviously, but I am talking about the vast majority of pedals.
In that case it wouldn't be "virtual" ground either really.

 

[Robert]

Imagine the opamp sitting there at idle, no signal applied, quiet. (We're assuming it's a single polarity power supply)

With the input of the opamp sitting idle at 0V the negative half of any applied signal will be clipped since the opamp can't swing below it's negative rail (which is 0V).

The solution is to shift the idle voltage up above 0V, to around one half of the supply voltage (4.5V). This allows the opamp to swing both positive and negative (relative to the idle point) while staying within the 0 - 9V limits.

Vref is the 4.5V reference voltage that is used to bias the opamp at 1/2 VCC.

Since VREF is usually decoupled with a fairly large capacitor it is virtually "ground" to any AC signal, but not actually at DC ground potential.

 

[p_wats]

Imagine the opamp sitting there at idle, no signal applied, quiet. (We're assuming it's a single polarity power supply)

With the input of the opamp sitting idle at 0V the negative half of any applied signal will be clipped since the opamp can't swing below it's negative rail (which is 0V).

The solution is to shift the idle voltage up above 0V, to around one half of the supply voltage (4.5V). This allows the opamp to swing both positive and negative (relative to the idle point) while staying within the 0 - 9V limits.

Vref is the 4.5V reference voltage that is used to bias the opamp at 1/2 VCC.

Since VREF is usually decoupled with a fairly large capacitor it is virtually "ground" to any AC signal, but not actually at DC ground potential.

View attachment 78612

I've read many threads trying to understand Vref before and this is the first time it has somewhat clicked, so thanks for that!

 

[DeadAirMD]

@Robert @p_wats 100% totally agree! That was such a perfect explanation Robert, absolutely understand Vref so much better because of that excellent explanation.