How to keep virtual ground stable under load in split-rail analog signal chain - Page 1
Forum section: electronics / projects

author: JOHN2234
author_rank: Newbie
timestamp: September 22, 2025, 10:45:55 pm
content: I'm working on a custom signal generator system that uses a split-rail power supply to generate +Vcc(28V), virtual ground (vg)(14V), and GND(0v). This powers several analog circuits: a trigger circuit, square wave generator, sine wave generator, then a gain stage, followed by a level shifter, and finally a power amplifier.The issue is that when I connect the final power amplifier, the virtual ground voltage starts to drift, especially under load. It seems the virtual ground cannot sink enough current and gets pulled up. I need the virtual ground to stay fixed at around 14 V, but under load it floats.I understand this is likely due to the power supply’s inability to sink sufficient current from the vg node. I’m looking for suggestions to adjust the the power supply portion as is so that the virtual ground can sink up to 0.5 A without drifting. NOTE: I have to use TIP41C npn and/or TIP42C pnp and/or 2N3904 npn transistor and/or 2N3906 pnp transistor to do this and I have to do the voltage division and the voltage regulator needs to be used aswell. There is no limit to the amount of transistors or whcich of these 4 is used. All of them or 2 or 1 can be used.Appreciate any advice

author: Alaezae
author_rank: Contributor
timestamp: September 23, 2025, 03:42:44 am
content: I could think of a few reasons why this might drift:1.) Although 0.5A isn't really a lot (relative, of course), it's enough to heat up those TIPs enough to change the operating point. BJTs are very susceptible to this in practice. You could increase the emitter resistors to reduce the current through the entire output stage, thereby stabilizing the drift due to temperature. This also goes for the input bias network, those 180-Ohm resistors could probably be increased to reduce current through those transistors as well. You can analyze the response to temperature in LTspice with the .TEMP command. Additionally, throwing a heatsink or sizing up the package for the output transistors could increase temperature stability.2.) There's no large output capacitor for the supply. As large gulps of current are taken from the power supply, the amount of energy storage available to source current from is effectively the parasitics of the transistors. Those are very low capacitances, so you end up with a large change in voltage with every turn-on event. Although it's not a switching supply, you still need this to help store energy so that you don't see changes in voltages over time. You can think of a large capacitor as a filter which reduces extremely low-frequency components of your 0 Hz output signal.There is probably more that other people can help you with, but those are my two cents. If you really wanted to be precise with this, I would throw a high-current opamp with an offset correction amplifier to eliminate drift. You'd compensate for temperature offsets and would eliminate mismatches in the transistors. Using BJT transistors makes getting a stable and temperature-independent circuit difficult to achieve.

author: gitm
author_rank: Regular Contributor
timestamp: September 23, 2025, 12:15:42 pm
content: Your virtual ground drifts because the TL081 can only source/sink 26 mA. Current draw above this must cause the vg to "drift". A solution is to have the TL081 drive the vg using one or more pairs of NPN+PNP transistors between what your calling +VCC and -VCC, but remember to have the output of the transistors in the TL081's feedback loop.For higher currents approaching 500 mA, you'll want multiple pairs to share the load _WITH_ resistors on the emitters to draw the power dissipation out of the transistors (i.e. heat) and into the resistors. Spec the resistor values specifically with the power loss in mind and the shared power delivered to the load. Base resistors would help to prevent current hogging, but with sufficient emitter resistors this is not a requirement.ETA: Add output capacitors as Alaezae noted, this is a must.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 23, 2025, 01:04:33 pm
content: I have to question why a virtual ground is used in the first place, and why its used as the return for the power amplifier output. This means any circuit you connect to it must also be referenced to the virtual ground, which would be problematic.edit: well said VVVV

author: MariuszD
author_rank: Regular Contributor
timestamp: September 23, 2025, 08:24:26 pm
content: If the amplifier's output current is flowing into a virtual ground, then the virtual ground must be based on the power amplifier. The requirements for this amplifier are no less than for the output amplifier. High output resistance and slow pulse response can cause fluctuations in Vg and affect the shape of the output waveforms.

author: ThatElectronicsGuy
author_rank: Newbie
timestamp: October 04, 2025, 03:22:30 pm
content: Don't know if they already answered, because I didn't read, but you need to put push pull with transistors on the output of your amp (it maximizes current drive capabilities so your rails are at what they should be). Look for those in the internet.