Join Barron Stone for an in-depth discussion in this video Supply dual voltages, part of Electronics Foundations: Semiconductor Devices.
- [Instructor] One of the challenges when building circuits with op amps is that they often need dual supply voltages to draw power from both positive and negative voltage sources. An op amp can only output voltages within the range between its power rails. So, if an op amp is being used to generate an AC signal that has positive and negative voltages, the op amp's power rails will need to be connected to both positive and negative supply voltages. Benchtop power supplies like this one make that easy because it provides output terminals for positive and negative voltages, both relative to a common ground terminal in the middle.
If I configure this power supply to output five volts, the terminal on the left will provide me with positive five volts, relative to that common ground, and the terminal on the right will give me negative five volts. When I use a benchtop power supply to provide dual voltages for my breadboard, I like to connect the common ground bus to each side of the breadboard on its innermost power rails. Then I connect the positive supply line to the red rail on the far left with the plus symbol, and I connect the negative supply line to the black rail on the far right with the minus symbol.
You can organize your power buses on your own breadboard however you want. Do what makes sense for your own circuits. This configuration is just my personal preference for connecting dual voltage supplies because it gives me easy access to the common ground voltage on either side of the board. If you're using a benchtop power supply, then supplying dual voltages for a circuit is simple. But, if you're using another type of power source that only provides a single voltage, like a battery or a wall wart, then you'll have to turn that single voltage into positive and negative voltages yourself.
The easiest way to do that is to use an integrated circuit component that's specifically designed to convert from one DC voltage to another. For example, the TC7660H DC-to-DC converter chip, from Microchip, can generate a negative voltage from a positive source. It takes a positive input voltage between 1.5 to 10 volts and outputs the corresponding negative voltage. If I'm using a single power source, like a five volt wall adapter, I can use the TC7660H chip to generate negative five volts which gives me the dual voltages I need for my circuit.
Now, using these types of DC-to-DC converter chips is usually a bit more complicated than just inserting them into your circuit. You'll almost always need to include a few external capacitors or inductors around the device for it to function correctly. So you should look at the device's data sheet to figure out which additional components you'll need and how to connect them. If you don't have a benchtop power supply or any DC-to-DC converter chips, you're still not completely out of luck. There's a third way that you can generate positive and negative supply voltages for an op amp circuit, and that's to create what's known as a virtual ground.
By dividing a source voltage in half. Virtual ground is a circuit node that's used as a steady reference potential for other voltages. But it's not directly connected to the original ground or common reference potential for the circuit. The easiest way to split a voltage in half is by using a voltage divider with two equal value resistors in series. If I connect that voltage divider to a nine volt battery, it'll cut that nine volt source in half to produce 4.5 volts in the middle.
The voltage values shown here are all positive because I'm treating the negative battery terminal as my common potential reference point. Which, is the normal thing to do. But, if instead I change my point of reference to treat the middle of the voltage divider as zero volts, or my virtual ground, then the voltages at the positive and negative battery terminals will be plus and minus 4.5 volts relative to that virtual ground. And I can use those dual voltage sources to power my circuit.
Now, one downside to this approach is that when I connect an electrical load to the voltage divider that's generating my dual voltage sources, if that load's impedance is too low between the center voltage and either of the battery terminals, then that can change the balance of the voltage divider ratio. I'm still treating the middle of the voltage divider as my zero volt reference, but now the positive and negative supply voltages will be unevenly distributed relative to the virtual ground.
So, what I need to do to fix this is find a way to buffer the output from the voltage divider so that it's not impacted by whatever else is connected to it downstream. And that's easy to do with a simple voltage follower op amp circuit serving as a unity gain buffer. The op amp has an extremely high input impedance, so it won't affect the ratio of the voltage divider. And its low output impedance enables it to maintain a steady output voltage when it's connected to different loads. Since the virtual ground voltage that it's producing is between the original supply voltages from the positive and negative battery terminals, I'll use those to supply power for the op amp.
I've built that buffered voltage divider on my breadboard using a nine volt battery to provide the positive and negative supply voltages. The negative battery terminal is connected to the negative power bus on the far right side of the board, and the positive battery terminal is connected to the positive bus on the far left side. Those will be my plus and minus 4.5 volt power supply lines. These two resistors act as the voltage divider to create the voltage for my virtual ground. I don't want my voltage divider to draw too much current.
It's just there to produce a voltage for reference, so I used a pair of fairly high resistance one megohm resistors. Their output is buffered by a voltage follower op amp circuit and its output, which is the virtual ground, gets shared to both of the inner power rails using this green wire. To check how these virtual supply voltages relate to each other, I'll anchor the black probe of my DMM to the virtual ground, and now I'll use the red probe to measure the positive supply voltage. And I can see that it's a little more than four and a half volts above the virtual ground, and that's half the output voltage from my nine volt battery.
And when I move the red probe to measure the negative supply voltage, I see that it's around negative 4.5 volts relative to the virtual ground. It's very important to remember that the virtual ground created by this circuit is different than the actual circuit ground, or common reference, and that you should never connect the two together. For example, if I was using a grounded power source, like a 12 volt DC wall adapter to power the circuit, and I used a voltage divider to split the 12 volt input into plus and minus six volts, that virtual ground that it produces in the middle of the voltage divider is different from the actual ground of the wall adapter.
If I tried to connect my new virtual ground-based power source to another circuit that included something like an Arduino microcontroller, whose power supply is connected to the actual ground, I've just made a connection between my virtual ground and the actual ground which will create a short circuit between the two and cause unwanted current to flow that'll most likely damage these devices. To avoid that problem, when I'm building circuits with a virtual ground, I always use a floating power source, like a nine volt battery, that's not connected to the actual ground.
The nine volt battery may not be able to provide much power compared to a DC wall adapter, but at least I won't have to worry about accidentally connecting different grounds together. I'll be using this circuit with a nine volt battery to provide dual supply voltages as I build and demonstrate op amp circuits throughout the rest of this course.
- Semiconductor materials
- Diode applications
- Rectifying a signal
- Detecting the signal peak
- Protecting against large signals, reverse current, and flyback voltage
- Special purpose zener diodes, Schottky diodes, and photodiodes
- NPN and PNP bipolar junction transistors
- Using a BJT as a switch
- Field effect transistors
- Differences between BJTs and MOSFETs
- Operational amplifiers
- Op-amp applications
- Comparing signals
- Buffering signals
- Amplifying signals
- Filtering signals
- Combining signals