Signal buffering with single-supply op amp - Page 1
Forum section: electronics / projects

author: loistvan
author_rank: Newbie
timestamp: October 20, 2021, 04:53:20 pm
content: Hi,I have a ~ 20 MHz sine signal with peak-to-peak amplitude of 0.1V that I need to buffer with an op amp. I was thinking about a standard op amp buffer circuit (signal to inverting input, non-inverting input to gnd, feedback resistor = 0).My problem is that I have single-sided supply 0...+5V, and I suppose this circuit won't work because I don't provide negative supply to the op amp, while the output will be negative half of the time.What is the easiest way of overcoming this issue? Shall I just split the supply with two resistors and shift the input signal with the mid-supply voltage (and use a non-inverting configuration)? But won't it introduce noise from the power supply into the output?Many thanks!

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 20, 2021, 04:58:33 pm
content: Look at the "Art of electronics".

author: magic
author_rank: Super Contributor
timestamp: October 20, 2021, 05:12:59 pm
content: AC coupling noise injection can be fixed by filtering the midpoint voltage with a capacitor to ground. And using an opamp with good positive PSRR or RC filtering its supply.If DC level doesn't need to be preserved, wouldn't some PNP emitter follower loaded with a resistor to 5V (bonus: current source) suffice? (That could be followed by AC coupling to re-center at 0V).

author: loistvan
author_rank: Newbie
timestamp: October 20, 2021, 05:44:33 pm
content: I'm interested in any possible solutions. Could you please explain in a bit more detail? I'm beginner in electronics.

author: loistvan
author_rank: Newbie
timestamp: October 20, 2021, 05:50:59 pm
content: Where in particular?Pretty long book.I've checked the chapter of single-supply op amps where it recommends exactly what I've asked (biasing the op amp at mid-supply), but there is no analysis about noise performance. And who knows, there might be better solutions.

author: David Hess
author_rank: Super Contributor
timestamp: October 21, 2021, 01:01:07 am
content: AC couple the input and the output so the operational amplifier can operate between the supply rails.

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 21, 2021, 08:11:14 pm
content: Quote from: loistvan on October 20, 2021, 05:50:59 pmA part about opamp circuits, and amplifiers.It is not a very large book. I've read it several times until became more knowledgeable.

author: TimFox
author_rank: Super Contributor
timestamp: October 21, 2021, 08:46:45 pm
content: First, connect the op amp output back to the inverting input, as you suggested.Then, make a voltage divider (with two equal resistors) between +5 and common to produce +2.5 V with a Thévenin-equivalent output resistance equal to half of the single resistor value. Connect that node to the non-inverting input.If it be too low, it will pull excessive current from the power supply.If it be too high, the input bias current through the resistance will increase the DC offset voltage at the amplifier input.With a Goldilocks' choice of resistors, choose the input capacitor so that it passes the lowest AC frequency of interest into the voltage-divider resistance.(The output resistance of your source should dominate the input noise, but if the capacitor is too small and the input noise current is too high, that will increase the amplifier noise. The safe choice is to make the capacitor's reactance at the lowest frequency of interest less than the source resistance, which is a higher capacitance than in the previous statement.)Similarly, choose a suitable capacitor to couple the output (with a DC level close to +2.5 V) to your load, where the capacitor's reactance is low compared with the load resistance.If your +5 V supply is noisy or has too much ripple, then you can connect a capacitor from the center of your voltage divider to ground to reduce the noise, and then connect a single resistor from that node to the non-inverting input.

author: Zero999
author_rank: Super Contributor
timestamp: October 22, 2021, 07:41:05 am
content: 20MHz is too higher frequency for common op-amps. How about a video amplifier?

author: EPAIII
author_rank: Super Contributor
timestamp: October 22, 2021, 08:16:15 am
content: Assuming that you have an op amp that will have actual gain at 20 MHz, set up the op amp circuit for the gain you will need: feedback and input resistor on the inverting input, whatever biasing is needed on the non-inverting input. For op amp circuits the bias on the non-inverting input usually determines the operating point for the inverting input and output. Generally these three points will have the same average DC level or at least close to it. Then use a capacitor with an impedance that is low compared to the effective impedance of the input to couple the 20 MHz signal in to the input resistor. This allows the DC level to shift from ground to about 1/2 the supply Voltage.And you may (probably) will need to use another capacitor on the output if the signal needs to be brought back to being centered around ground or any other DC level.If the chip you are using is an op amp, then standard op amp formulae can be used to calculate the needed resistors as well as the input and output impedance. If it is described (spec sheet) as a video amp, then there are probably formulae in that spec sheet for determining these values.Breadboard everything and check to see if it operates as expected: gain, response, distortion, etc. Make adjustments as needed.All of the above assumes that you do not need the circuit to pass DC levels. If that is not the case, then other methods will be required. But most higher frequency circuits do not need to preserve DC levels.Quote from: loistvan on October 20, 2021, 05:44:33 pm

author: Terry Bites
author_rank: Super Contributor
timestamp: October 22, 2021, 05:49:25 pm
content: Like this, for instance. Bias the non inverting input to Vcc/2. AC couple the input and the output.

author: TimFox
author_rank: Super Contributor
timestamp: October 22, 2021, 06:47:33 pm
content: I think you posted the wrong drawing. You posted this one as an answer to a different question about a relay circuit.The TL072 with +5 V single supply will not be happy with its inputs at 0.

author: salihkanber
author_rank: Regular Contributor
timestamp: October 22, 2021, 07:41:45 pm
content: Better to add a DC bias and check for single supply amps but 20MHz is a high frequency

author: Terry Bites
author_rank: Super Contributor
timestamp: October 23, 2021, 02:31:13 pm
content: Yes Mr Fox, what a cock up!Glue sniffing again... Lets' hope I havent put a compromising image else where.The MAX4450 is not exactly flying off the shelves so is not hard to source. About 2 euros.You might want to change the termination resistors to suit your circuit. I have assumed 50 ohm source and drive.If your source is high Z, up the 1k's, but dont go crazy.Does no one draw anymore? A picture is worth a thousand words (by a long way).Phones I guess and without cameras it seems, still in the 90's?. Oh well.

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 23, 2021, 06:12:23 pm
content: Yeah, it's annoying when you're already struggling with something and ask, someone drops some theorical answer full of technicisms that only confuses you more.Ex. Just use a dc blocking caps and bias input to vcc/2. Technically correct but it's lacking a lot of information, making it kinda abstract if you are already having issues to understand it.I love taking my 3 minutes making a drawing... even simulating it. It also refreshes you own memory, I don't know you, but after 10 years of not using op amps I barely remembered how to connect them.This is a good read:https://eng.libretexts.org/Bookshelves/Electrical_Engineering/Electronics/Operational_Amplifiers_and_Linear_Integrated_Circuits_-_Theory_and_Application_(Fiore)/04%3A_Basic_Op_Amp_Circuits/4.03%3A_Single_Supply_Biasing

author: Paul Ed
author_rank: Contributor
timestamp: October 23, 2021, 08:56:10 pm
content: Nice reply...Quote from: DavidAlfa on October 23, 2021, 06:12:23 pm...thank you, a nice link - saved for when I've forgotten too