Gas station without pumps

2013 July 21

Virtual ground circuits

Filed under: Circuits course — gasstationwithoutpumps @ 15:55

To use single-supply op amps properly, one usually needs to create a “virtual ground” or “bias voltage” between the two power-supply rails.  I have used 3 different circuits for these virtual grounds:

Three different virtual ground circuits I've used.

Three different virtual ground circuits I’ve used.

The circuit on the upper left is the one that I’ve been teaching in the Applied Circuits course, because it uses some of the thematic elements of the course: the voltage divider and the unity-gain buffer. The output is half the power supply voltage and tracks it well. It works well for most of the circuits they do, but has one major problem: any noise on the power-supply line is coupled into the virtual ground. Given that they could be powering their circuits off an Arduino powered from a USB port on a computer, the “5v” power can be very noisy indeed. (See, for example, the glitches observed in capacitor charging).  The unity-gain buffer limits the current from the virtual ground to about ±20mA.

The circuit on the upper right again uses a unity-gain buffer, but uses a TL431ILP  “adjustable shunt regulator” (drawn here as an adjustable zener diode).  This circuit provides a 2.5v output, as long as the power-supply voltage is high enough. The TL431ILP needs 1mA of current to get good regulation, hence the 2.5kΩ pullup resistor.  A somewhat smaller resistor would be safer, as the regulator would then have sufficient current even if the power supply voltage dropped a lot.  I usually use about a 1kΩ resistor, wasting a couple of mA to get good 2.5v output even when the input drops to 3.5v.  Without the unity-gain buffer, the shunt regulator should be able to sink about –100mA, but its source capability would limited (maybe 1.5mA with a 1kΩ pullup). We could get a balanced ±50mA capability with a 51Ω pullup, but there would be a 49mA current through the shunt regulator with no output current, resulting in 123mW of wasted power.

The bottom circuit uses a low-drop-out linear voltage regulator that can source +100mA.  This circuit is limited to sinking about –2.6mA, though, as the only path to ground is through the pair of feedback resistors, and the LDO feedback would cease to provide proper regulation once the output is pulled above the desired output voltage.  Smaller feedback resistors could be used to sink more current, at the cost of having a large current through the feedback resistors.  For example, we could get a balanced ±50mA supply by reducing the resistors to 27Ω each, but there would be a 50mA current through the feedback resistors with no output current (wasting about 116mW of power).

I don’t know of any off-the-shelf chip that provides a good voltage reference that can both source and sink significant current without wasting power in resistors.

Either the LDO or the shunt regulator costs about 40¢ (single-part costs for through-hole parts), so they are expensive parts to add to a design if not needed, but either will provide good noise immunity.

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