(opamp) Is a dual rail supply required for ... - Page 1
Forum section: electronics / beginners

author: Audiofanman
author_rank: Contributor
timestamp: June 04, 2025, 04:31:43 pm
content: First... I'm a total beginner, learning what I can absorb every day..I could say EE is a 2nd language that I am picking up slowly lol...So I'm (still) developing a high impedance circuit (10Meg min) that reduces a negative voltage from -100vdc to -10, and using a unity gain inverting opamp drives an analog 1mA full deflection meter by using a resistive divider network.The expected range of function is really -6 to -3v, and conversely the output +3v to +6v dc. 10 just was a nice number giving me a bit of leeway.Does an opamp require a negative power supply for the inverting input to work well as some sort of internal reference with the inverting inputoris the negative power supply used most times so the opamp can work well under the 1v or so threshold (i forget the correct term, is it common mode?)I know that it depends is a valid answer applicable to any question with more than one answer lol so I found with ltspice and I ln real life, my non inverting op amp buffer works fantastic with no negative voltage source, but my ltspice sim is getting real complex as the ac power source i want to use is not center tapped and trying to float the output of a bridge in ltspice gives strange results, adding a virtual ground to the bridge with (2) 10k resistors to ground appears to solve most of the trouble i see.Got me thinking that 1, the negative rail model I have is more an ltspice issue and in real life floating the dc output from a bridge shouldn't be a problem..Or2, (probably more true) i know only enough to make myself feel more stupid hahaha..

author: Ian.M
author_rank: Super Contributor
timestamp: June 04, 2025, 04:53:42 pm
content: An inverting amplifier normally*acts to keep the inverting input of the OPAMP very very very close to the same voltage as its non-inverting input, which will be grounded if you don't want to introduce an offset. For single supply operation, you therefore need an OPAMP with common mode input range including its negative supply (and preferably min. 0.3V below that). If there is any risk the negative input voltage will be present with the OPAMP unpowered, you may need a clamping diode from -in to ground to prevent the pin being dragged very far negative.* It fails to do so if its output rails.Edit: I forgot to mention that a single supply OPAMP's negative supply *IS* ground! David's comments below are well worth your attention

author: David Hess
author_rank: Super Contributor
timestamp: June 04, 2025, 06:20:25 pm
content: Quote from: Audiofanman on June 04, 2025, 04:31:43 pmNo, the negative supply limits the negative side of the input common mode range, and the negative side of the output voltage range. For an inverting amplifier, the voltage across the input resistor cannot go negative because feedback will hold the inverting input to the same voltage as the non-inverting input, which will typically be ground (common).Difference amplifiers are sometimes made from a pair of inverting amplifiers to allow input voltages beyond the supply rails.QuoteThere are many reasons to include a negative supply.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 05, 2025, 07:59:14 am
content: With a single supply OP-amp (like LM358) one could get away without a negative supply. The input can work all the way to ground (= neg supply of the OP-amp) and a tiny bit (like 0.1 V) below. It depends on the OP-amp type (the parameter is the input common mode votage range) how for this works, e.g. the classic µA741 would not.One would not need a divider first. So it could be directly some 10 M from the input to the inverting input of the amplifier and than the FB resistor to get an inverting amplifier with a gain of maybe -1/10 or even less. So -100 V are converted to +10 V.The input is virtual ground and the amplifier does not see the high input voltage, as long as the amplifier can provide the compensating current. Unless already part of the OP-amp one wants the already mentioned clamping diode in case the OP-amp power is off, but the input voltage is on.If the meter movement is in series with the feedback path (one of the sensible configurations) one wants a parallel capacitor, as the inductance of the movement can add phase shifts and might cause oscillation.