# Negative Power Supply for Op Amp

I have a 12V (single ended)power supply available for and op amp circuit I am designing. I want it to be very precise around zero volts for an output. I already though about using a virtual ground, but I am putting the output into a Max 10 FPGA, so I do not think that I can accommodate for that. My question is, is it possible to use a second op amp and make the Vcc of it ground and the Vee of it as 12V and use it as an inverting op amp, to then use that as the power supply for the first op amp. The op amps that I have are LM358, but I do not think that it makes a difference for this scenario. I am shifting the input and scaling it with a circuit like the one below, I am just trying to improve its resolution at small outputs.

• <1% error near the rails on RRIO are common issues. Std solution is a charge pump supply inverter IC like those used for RS232 and clamp negative outputs to protect MAX – Tony Stewart Sunnyskyguy EE75 Mar 20 '18 at 17:45
• Thanks for the Responses, I kind of figured that it wouldn't work and would break the op amp based off of the schematic. I just wanted to check, because I didn't see anything here or elsewhere on the internet either way. I was just hopeful for a simple solution – RShuff Mar 20 '18 at 18:02
• It doesn’t take much -Vss to avoid the RRIO error near zero when mV errors count. Otherwise offset the range – Tony Stewart Sunnyskyguy EE75 Mar 20 '18 at 18:08
• I have a little bit more than 1% error. The charge pump looks like the solution that I needed thanks. I believe there is already clamping to protect the chip built in. It was just very important to have precise low voltage readings for my application – RShuff Mar 20 '18 at 18:11
• Make sure Series R on output to limit clamp diode current 5? 10mA? Using BAT54 is suggested – Tony Stewart Sunnyskyguy EE75 Mar 20 '18 at 18:14

Opamps don't produce voltage or current. They let more or less current pass from the powersupply to the output. To get a negative voltage out of an opamp, you have give it a powersupply with a negative voltage.

The output of an opamp is limited to what is available from the supply rails. If you have rails of 0 and 12V, then that's the limits of your output.

• Thanks, I knew this, but was not sure if switching the voltages could work since voltage is relative. Thanks for the response – RShuff Mar 20 '18 at 18:04
• Re, "let more or less voltage and/or current pass." Current, yes. Voltage, no. Voltage does not "pass." Look at the output stage in @Transistor's answer. The op-amp will either allow current to flow from the + rail to the output (Q14 conducting), or it will allow current to flow from the output to the - rail (Q20 conducting). Which one happens depends on the voltages that it senses on its inputs. If the voltage at the output is stable at any point in between the two supply rails, that's probably because of negative feedback provided by the external circuit. – Solomon Slow Mar 20 '18 at 19:19

Figure 1. Internals of the ancient 741 opamp. Source: Wikipedia.

From the internal schematic of the 741 op-amp it should be clear that the output can source current from the $V_{S+}$ rail via Q14 or sink current to the $V_{S-}$ rail via Q20.

It is not possible to generate an output voltage higher than the postive supply pin or more negative than the negative supply pin.

If you are not using the 12 volt supply for anything else, then you can float it and generate a +/- 6 volt supply by connecting what you think of as a virtual ground to your system ground.

This does require that the 12 volt supply is "isolated".

simulate this circuit – Schematic created using CircuitLab

A few notes:

1) The virtual ground op amp should be significantly faster than your application op amp.

2) The virtual ground op amp should have better current output capability than the application op amp.

No, that will just break the op amp. If you truly need -12V, then you need a second power supply to generate it. Something like this might work, but if you need a precise reference then you will want to use a commercial module.

• Thanks, I was pretty sure of that, but it was just a thought that occurred to me since voltage is relative. I didn't see any posts like this around the internet and was hopeful of a simple solution. – RShuff Mar 20 '18 at 18:05

Your question is over 1 year old, so you most likely have a solution already. If not, here is my advice:

• I suggest using the schematic capture tool provided by stackexchange. – Mahendra Gunawardena Aug 22 '19 at 10:04