# Negative voltages in the real world?

I was given the task to design a circuit that has a 5V input and uses a differential op amp to provide an output that oscillates between 5V and -5V. The design I came up with is the following:

simulate this circuit – Schematic created using CircuitLab

Basically, I have a ~50/50 555 timer generating an oscillating output of 0 or 5 volts, and this setup will, in theory, create an output that oscillates between -5 V and 5 V. This output will make the two LED's flash one at a time.

When I ran a simulation of this circuit, it worked swell when I had an oscilloscope in place of the LED's. When I went ahead to build the actual circuit, what struck me is this: I have no idea how to give the op amp chip a -Vcc equal to -5V.

How would I do that? Additionally, is there an alternative setup you'd suggest to accomplish this task?

• Why not use a Vcc of 10V and then create a virtual ground at 5V? – Ignacio Vazquez-Abrams Dec 26 '14 at 1:41
• Could you please add some more information. You say "a circuit that has a 5V input". Does that mean that you have a 5 volt power supply, and you need to make +/- 5 from it? You show a 5V source V1 and a 5V source V2. Are they the same? If not, is the V2 power supply fixed at 5 volts, or can it be more? Does the +/- 5V output need to be a square wave, or is a sine or triangle OK? Is the 5 V input required to be a DC level? – WhatRoughBeast Dec 26 '14 at 1:51
• @WhatRoughBeast I have a 5 volt power supply. V1 is the same as V2, but V3 is what I'm looking for. Currently, it's connected to ground and thus doesn't function properly. – Shahar Dec 26 '14 at 3:43
• I still don't understand. Do you mean you have another 5 volt supply, but it doesn't work? Any good supply can be connected with the + output to ground, and the - output giving a minus voltage. Also, on your schematic you need a current limiting resistor from the op amp to the LEDs. – WhatRoughBeast Dec 26 '14 at 4:32
• Is the real question here "How to generate a -5 volt supply from +5 volts"? Please give us the complete text of the task you were given. What characteristics does the output need to have? What impedance does it need to drive? Depending on what kind of output you need, a large number of different techniques can be used to accomplish the task. You can use capacitor charge pumps or inductive step-up converters to do it. You can invert a voltage if you charge a capacitor to 5 volts and then connect its positive plate to ground, causing its negative plate to drop to -5 volts. What do you need? – PkP Dec 26 '14 at 8:41

## 2 Answers

There are basically three ways to do this. One way is two independent, isolated power supplies. Say, two batteries or two bench supplies. Connect them in series, + to -. The connection between the supplies becomes 'zero', and then one supply gives you + and the other gives you -. The second way is to use a bipolar power supply. This is a power supply that is specifically designed to output equal and opposite outputs around ground. It is basically a single-output power supply with an additional output that is 'mirrored' over the ground connection. In this case, the supplies do not need to be isolated, but you will need to use complementary parts (NPN vs. PNP or NMOS vs PMOS pass transistors, 7805 vs 7905 regulators, etc). The third method is to use a single supply with double the voltage and then synthesize a virtual ground halfway in between. See http://tangentsoft.net/elec/vgrounds.html for many useful details on how to build virtual grounds.

Edit: It is also possible to generate a negative supply from a positive supply with some sort of switching DC to DC converter. An inverter would be the simplest, using either a standard inverter topology with an inductor or a flying capacitor charge pump. An isolated flyback converter would also work. These can be procured as complete modules from various suppliers. A charge pump may not provide very much current, so watch out if you decide to use one. Switching supplies can also create quite a bit of switching noise, both directly coupled into the output and also radiated from the high current switching components. This may require filtering and careful design to prevent interference.

Also, please don't rotate your voltage sources upside down; it makes the schematic very confusing. I would recommend moving V2 and V3 next to V1, one above and one below the ground wire, and both with the positive side up. Then connect them over to the amplifier with long wires. This makes the required power supply configuration very obvious.

Adding a third way to Alex's answer, for situations that two independent power supplies or a single dual polarity supply is infeasible, for example when driven by USB, you can use switch-mode inverting power supplies to generate this negative rail. Check out the 7660 chip that does specifically this using a charge pump technique. However this does mean that you cannot load this negative rail too heavily.