Join Barron Stone for an in-depth discussion in this video Buffer signals, part of Electronics Foundations: Semiconductor Devices.
- [Instructor] One of the simplest op-amp circuits that utilizes feedback is called a voltage follower. It's created by simply connecting the output of the op-amp directly back into the negative inverting input terminal. The input signal for the voltage follower goes into the non-inverting or positive terminal, and the output signal is simply the output from the op-amp. The reason this circuit is called a voltage follower is because the output voltage follows, or matches, the input voltage. Since the output voltage is the same as the input voltage, the voltage follower circuit has a constant gain of one.
You'll often hear that referred to as unity gain, which means that the signal passing through the circuit is neither amplified nor attenuated. At first glance, a circuit that outputs the same voltage signal that you put into it, that may not seem very useful, but even though the input and output voltage is the same, this circuit is useful because the impedance at the input and output terminals is different. The voltage follower takes advantage of the op-amp's incredibly high input impedance and low output impedance to serve as a buffer between two circuits.
To maximize the voltage that's transmitted from a source to a load, the impedance should be bridged so that the impedance at the load is significantly larger than the source impedance. If the source and load do not already have that impedance relationship, maybe the source impedance is slightly greater than the load, then you'll need to use a buffer circuit to bridge them. The op-amp's impedance is so large, it'll be significantly greater than the source, so that connection will be bridged, and the op-amp's output impedance will be very small compared to most loads, so that connection will also be bridged.
Since this circuit acts as a buffer with a gain of one, it's often called a unity gain buffer, and I'll show you seven common uses for it later in this course. But, how exactly does an op-amp act as a unity gain buffer? By shorting the output from the op-amp directly back to the inverting input terminal, the voltage at those two terminals will always be the same. The inverting input voltage will be equal to the output voltage. Now the op-amp itself doesn't know that those two terminals are connected to each other, but it doesn't care.
The op-amp simply looks at the difference in voltage between its two input terminals, which happens to be equal to the input voltage minus the output voltage, and then the op-amp adjusts its output based on what it sees. When the voltage at the input and output of the voltage follower are the same, there will be zero difference between the two input terminals because of the direct feedback. The op-amp sees that the circuit is in equilibrium, so it'll keep on outputting the same voltage. Now, if the input signal that's connected to the non-inverting terminal goes up, that will create a positive difference between the two input terminals.
The op-amp will see that positive difference and respond by increasing the output voltage until the circuit returns to equilibrium. If the voltage at the non-inverting input drops, that'll create a negative difference between the two input terminals, and the op-amp will respond to that by decreasing it's output voltage until that difference returns to zero again. To demonstrate that behavior, I've built a voltage follower circuit on my breadboard using a variable voltage source as the input signal and a one kilo-Ohm resistor connected to the output terminal as the load.
I've connected a five volt source to the op-amp's positive supply terminal and the negative supply terminal is connected to ground. My oscilloscope is showing the input voltage on channel one in yellow and the output voltage on channel two in blue. Since the output matches the input, those two traces are directly on top of each other. So I'll adjust the position of channel two down a little bit so I can see both signals. The input voltage is currently set at 2 1/2 volts and if I wiggle it around a little bit, I can watch as the output voltage follows it.
However, if I raise my input voltage up several volts higher, the output follows up until it reaches five volts and then it stays there. The op-amp cannot output a voltage that's higher than the positive supply voltage I gave it, which is five volts. Similarly, if I lower the input voltage until it's negative, the output follows it down until it reaches zero volts, and then it stops there. Since I connected the op-amp's negative supply terminal to ground, it'll never be able to output a negative voltage below zero volts.
As long as the input signal to the voltage follower is within the usable range of the op-amp's positive and negative supply voltages, the output signal will look the same, but if the input signal exceeds that threshold in either direction, then the output signal will get cut off at those limits. This is a form of distortion called clipping, and it's not just limited to the voltage follower. Clipping can occur in all op-amp circuits if the output signal tries to exceed the voltages that the op-amp can actually produce.
- 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