In my quest to understand more about op-amps (I'm starting that quest from zero) I was reading about single and dual supply op-amps. What I was reading seemed to make sense. In the article there was a reference to AD820 as a single supply op-amp. As any inquisitive person would do, I go and look up the datasheet for AD820. At the very beginning of the datasheet I find the following information about the op-amp:
Thank you.
True single-supply operation
Output swings rail-to-rail
Input voltage range extends below ground
Single-supply capability from 5 V to 30 V
Dual-supply capability from ±2.5 V to ±15 V
Huh? Am I reading this right? A single supply op-amp can go negative? Maybe I misunderstood the article or something, but I thought that single supply op-amps only work in the positive. And how can a single supply op-amp have dual supply specifications? Obviously, I am missing some critical information and/or have misunderstood something that I have read. Is anyone willing to take the time to straighten me out?Thank you.
Some op amp input stage topologies will continue to work (as differential amplifiers) if the input voltages are SLIGHTLY below the negative rail. They never let you go far, normally less than a Vbe below. This allows them to work down to zero volts when the “negative rail” is ground. The old LM324 would do this, but it had other serious problems. For an op amp to truly be “single supply” the output should also be able to swing all the way down to the negative rail, so a “zero volt” output would be possible on a single supply. The old LM324 would sort of do this, but they cheated and created more problems.
The op amp itself doesn’t know or care if it’s on a single or split supply. It only cares if the common mode input voltage is in a valid range for the device, and if the output can swing where it needs to go, relative to supply voltages.
The op amp itself doesn’t know or care if it’s on a single or split supply. It only cares if the common mode input voltage is in a valid range for the device, and if the output can swing where it needs to go, relative to supply voltages.
I wasn't looking at AD820 as options. I'm just trying to learn and understand. I only looked at AD820 as it was referenced in the article that I was reading. It's just confusing to me that the description of the AD820 states that it's a single supply, yet swings negative, and that there are dual supply specifications as well. From the little that I know, and I do mean little, that doesn't make sense.
From a power supply perspective, the opamp really only cares about the voltage difference between the VCC and VEE pins (or V+ and V- if you prefer). As long as you stay within the specified limits the opamp will be happy.
However, its inputs must be within the common-mode range of the opamp. That means that for most opamps, you need to stay some voltage away from the VCC and VEE pins. In audio, that's generally done by feeding the opamp a bipolar or split supply. So VCC = +15 V and VEE = -15 V. The opamp is then powered by 30 V (+15 - (-15) = +30). The input is kept around 0 V.
If you want to operate the opamp from a single +30 V supply you need to lift the inputs such that they're within the allowed common-mode range for the opamp. Typically that's done with a voltage divider that lifts the inputs to around VCC/2.
Tom
However, its inputs must be within the common-mode range of the opamp. That means that for most opamps, you need to stay some voltage away from the VCC and VEE pins. In audio, that's generally done by feeding the opamp a bipolar or split supply. So VCC = +15 V and VEE = -15 V. The opamp is then powered by 30 V (+15 - (-15) = +30). The input is kept around 0 V.
If you want to operate the opamp from a single +30 V supply you need to lift the inputs such that they're within the allowed common-mode range for the opamp. Typically that's done with a voltage divider that lifts the inputs to around VCC/2.
Tom
Member
Joined 2018
The rail-to-rail outputs should be between the VCC and VEE.
But the input signal can be negative if the inverting amplifier rail-to-rail input application and the imaginary short at the GND(=VEE)
CyberPit
But the input signal can be negative if the inverting amplifier rail-to-rail input application and the imaginary short at the GND(=VEE)
CyberPit
Hi,
From what I understood, the negative voltage, when in single supply, refers to input CM (Common Mode) voltage.
See specs below.
If you operate it with single supply (-V=zero and +V) the input common mode can swing from -20V to +V+0,2.
Very unusual to allow CM to go -20V below -V.
Example: with single supply, you could have input (-) as -2V and input (+) as -1.999V resulting in a differential input of +0.001V.
I think this is what they called true single supply operation.
In ordinary opamp, you would need to bias the input (-) and input (+) above zero (some volts) to make the opamp works.
The output will be limited to the power supply. In this case, it will reach -V +5mV and +V -10mV at the output without load.
With load, the voltage drops increase a bit, but still very low drop. See specs below.
If operated with single supply, output will not swing below zero.
All opamp can operate as single or dual supply - it's just a matter of setting the bias.
BUT, most opamps have strict limits on CM and output swing, which sometimes limit the operation as single supply or very low voltage supply.
Other non rail-to-rail opamps will swing some volts above the -V and some volts below +V.
So if your power supply is very low (e.g. 5V) they may not work at all. This AD820 works!
It's an interesting opamp suitable for specific needs.
Assuming the op-amp uses N-Jfets for the input, if the pinch-off voltage of those JFETs was, say, 3V, then the drain and source of those JFETS could be above the negative rail (ground) even when the gate is ~2V (<3V) below that rail. Likewise, the gate voltage of a MOSFET can be above both the drain and source voltage. Never forget that an IC is just a collection of transistors, resistors and small capacitors. If you want to understand op-amps, study their equivalent circuit.
What you are quoting are the absolute maximum ratings, meaning it doesn't go up in smoke over this input voltage range. If you also want it to work, you have to stay between V- - 0.2 V and V+ - 1 V.
I'll give you a purely hypothetical example. Using Tom's convention, I will call the voltage at the positive supply pin of the op-amp VCC and the voltage at the negative supply pin of the op-amp VEE.
There are practical reasons why we usually call one of the supply nodes "ground" and connect any shields to that node, but one is free to choose whether the positive supply node, the negative supply node or something in between (if any) is chosen as "ground". This "ground" node is often also used as a common reference for the voltages in the circuit.
Now suppose you are working on car electronics. The most negative battery terminal is used as the ground node everywhere in the car, so for reasons of compatibility, you do the same. The op-amp is supplied from the car battery, either directly of via a voltage regulator, so its VEE pin will be connected to ground, 0 V.
Suppose you have to amplify the DC output voltage of a sensor that can produce any voltage between 10 mV and 100 mV (with respect to ground) to the range from 500 mV to 5 V. With an op-amp that can handle input and output voltages close to VEE, all you need for that are the op-amp and two feedback resistors with a 49:1 ratio of their resistances.
Many op-amps can't process input voltages less than 2 V or so above VEE. If you had to use one of those, you would need some awkward workaround: generate a negative supply after all, shift up the sensor voltage and shift it down again later, whatever.
So, while in principle any op-amp can be used with a single as well as a split supply, there are cases where it is very convenient for single-supply operation when the op-amp can handle input and output voltages close to VEE. When op-amps are advertised as single-supply op-amps, what is usually meant is that they can handle input and output voltages close to VEE.