Unity gain buffer - Page 1
Forum section: electronics / beginners

author: sy
author_rank: Regular Contributor
timestamp: May 07, 2023, 03:57:33 am
content: Just wondering what is the purpose of this configuration, since the input and output voltages are the same?And would we need to use a separate supply to power the op-amp if we wanted to decouple the signal?

author: james_s
author_rank: Super Contributor
timestamp: May 07, 2023, 04:08:10 am
content: It's to increase the apparent impedance of whatever the buffer is driving so that it doesn't load down the source. For example if you have a voltage divider made of a pair of resistors and you try to use it to feed something that draws significant current the output voltage will be dragged down. If you insert a buffer then whatever you connect to the output won't load whatever is driving the input. Digital systems use buffers too, they serve the same purpose, increasing the drive strength of a signal.

author: sy
author_rank: Regular Contributor
timestamp: May 07, 2023, 04:18:52 am
content: So if my input load isn't getting dragged down, then it must come from the supply rails of the buffer right?

author: dobsonr741
author_rank: Frequent Contributor
timestamp: May 07, 2023, 05:32:10 am
content: Supply rails has no role, only the ground reference matters. Up until now we assumed a single ended signal. We can kick it up a notch and have a differential signal. It won’t even need ground reference. You can apply two unity gain buffers to a differential signal, too.

author: magic
author_rank: Super Contributor
timestamp: May 07, 2023, 08:11:32 am
content: Quote from: sy on May 07, 2023, 04:18:52 amYes, the output current of a unity gain buffer comes from the buffer's supply lines, not from its input.If that's what you asked.

author: sy
author_rank: Regular Contributor
timestamp: May 07, 2023, 09:15:09 am
content: Quote from: magic on May 07, 2023, 08:11:32 amYes that makes sense, but should the power supply of the buffer be separate?For example, I have a power supply for the input terminals, and a separate one for the opamp itself?

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 07, 2023, 09:50:43 am
content: QuotePossibly, it may be much understandable and easier for you if you start with some basic opamp literature. For example, there is great Bible 'The Art of Electronics', by Horowitc and Hill. It covers many subjects, questions and circuits.You'll find there about power supply rejection ratio (and there are several types of PSRR).

author: barshatriplee
author_rank: Regular Contributor
timestamp: May 07, 2023, 03:59:53 pm
content: You can see this write-up:http://www.learningaboutelectronics.com/Articles/Unity-gain-bufferIt has explained the purpose of Unity-gain buffer in a simple way.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 07, 2023, 04:18:39 pm
content: A buffer or follower is a fundamental building block. Now most commonly realised with an opamp but originally as a cathode or emitter follower.The discrete follower is still a useful item if DC offsets are not an issue. eg for Audio.It increases the current drive that would otherwise be unavailable from the signal source.Another way to think of it is as way of isolating the input impedance from from the output impedance.It has current gain but zero voltage gain.The additional current has to come from somewhere, so it can only come from the supply rails.It can share power rails with other parts of the circuit if needs be.Use a simple simulator to identify the current paths

author: tkamiya
author_rank: Super Contributor
timestamp: May 08, 2023, 05:41:39 am
content: I think OP is over-thinking this.Simply, unity gain buffer exists to isolate whatever exists on input side does not get affected by whatever you do on output side.For example, say you have a signal that is 5V. If you short this to ground, you would overload whatever produced that signal and it will be zero. If you had a buffer in between short and signal, then shorting the output makes the buffer itself unhappy but it does not impact what the input side is doing.

author: MrAl
author_rank: Super Contributor
timestamp: May 08, 2023, 08:04:43 pm
content: Hi,You will also find that sometimes a Schmitt Trigger buffer is used. That not only buffers the signal it also squares it up in case the rise and/or fall times are not fast enough for the circuit that follows.

author: TimFox
author_rank: Super Contributor
timestamp: May 08, 2023, 08:08:24 pm
content: To avoid confusion, the Schmitt trigger circuit is only used for binary or pulse applications.The output voltage does not necessarily equal the input voltage, but the output will quickly assume one of two previously-defined voltage levels.A Schmitt trigger gate in a given family of logic gates will have two distinct trigger levels to force the output high or to force it low, both of which levels are within the normal range of the logic signals for that family.

author: Faranight
author_rank: Unknown
timestamp: May 09, 2023, 07:06:17 am
content: Quote from: sy on May 07, 2023, 09:15:09 amThe power supply of the buffer should be well regulated. You do not want to use a high-impedance power supply that is going to be "dragged down" by whatever circuit is present on the buffer output and draws a lot of current. Another potential issue that comes to mind is the stability. For example, if you have some heavy switching (high currents) going on on the positive rail, then that noise is likely to make its way into the buffer output, which could cause stability issues (and in the most severe cases, the circuit to oscillate). So, you ideally want to use a stable low-noise (linear) power supply, but sometimes you can get away with it by using some filtering i.e. put a 100nF decoupling cap close to the opamp, and maybe even a ferrite bead in front.

author: BillyO
author_rank: Super Contributor
timestamp: May 10, 2023, 12:58:37 am
content: The signal source and a simple op-amp buffer (single ended) can have separate supplies as long as the grounds are connected.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: May 12, 2023, 01:22:19 am
content: QuoteWhat does it solve?

author: BillyO
author_rank: Super Contributor
timestamp: May 12, 2023, 01:32:08 am
content: Quote from: dobsonr741 on May 12, 2023, 01:22:19 amI'm not sure how to respond to this.What do you mean, "What does it solve?"?It solves the situation where your signal has to drive an op-amp in a system that has a different supply. Like the OP was asking, and like you also responded. I would think that was .. obvious?Example: Using a function generator to provide a signal to test an audio amplifier that has an op-amp front end. That's one of possibly 5.3 x1013real life examples that could be given.