basic voltage follower gives output greater than input without resistor( NE5532) - Page 1
Forum section: electronics / beginners

author: basantab
author_rank: Contributor
timestamp: February 19, 2018, 06:39:24 pm
content: hello,the output of voltage follower is more than input voltage without resistor and with resistor it is exactly equal to input.the resistance value does not play much role, meaning i tried with some couple of kilo ohm and with different values and it does not matter much. it is shown in the picture of what i got in putput.any lead would be helpfulthankyou

author: basantab
author_rank: Contributor
timestamp: February 19, 2018, 07:03:46 pm
content: supply = 5 voltinput output1 volt 1.4161.5 1.5532v 2.0503v 3.0504v 4.010the error decreases with increasing input voltage without resistor .but with some resistor (any value like 33kohm ) the output almost perfectly follows input.

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2018, 07:24:36 pm
content: This is normal. The op-amp is doing this, because it's being operated outside of its intended specification.The NE5532 is not specified to run below a total supply voltage of 6V, but you're running it at 5V.It's common mode input range is typically 2V, within either supply rail, but you're expecting it to work with its inputs taken all the way to the supply rails.I you used a bipolar +/-9V supply, then the circuit would work perfectly.Another potential issue is the bias currents, which get multiplied by the source resistance and added to the input voltage, but that doesn't appear to be the case here.https://www.onsemi.com/pub/Collateral/NE5532-D.PDFIf you're stuck with a 5V supply, then you need a rail-to-rail op-amp, such as the MCP602.http://ww1.microchip.com/downloads/en/DeviceDoc/21314g.pdf

author: basantab
author_rank: Contributor
timestamp: February 19, 2018, 07:47:11 pm
content: thanks for replyingThe data sheet does not seems to specify that 6 volt range.http://www.ti.com/lit/ds/symlink/ne5532.pdfstill i checked with 6 volt supply and 1 volt input gives output of 1.423 volts.regarding common mode voltage limitwhere in the data sheet has it been mentioned or is there any way to see it.i tried inputting 0.770 voltage but the output goes high (5 volt output with 6 volt supply)when i further decrease the input the output voltage is just nearer and nearer to supply rail.anything less than 1.2 volt to 0.770 volt the output is is 1.423.... volt..i checked replacing different ICs (just incase ) but still the similar result or 400 milivolt offset at 1 volt input

author: Belrmar
author_rank: Regular Contributor
timestamp: February 19, 2018, 07:56:12 pm
content: Quote from: basantab on February 19, 2018, 07:47:11 pmThe opamp you are using is not a rail to rail opamp , so each time you get close to some of the rails it will either output something larger or either go crazy, try doing the same with a split supply, maybe 2 9v batteryes or a fully charged 2s lipo

author: newbrain
author_rank: Super Contributor
timestamp: February 19, 2018, 08:07:25 pm
content: Quote from: basantab on February 19, 2018, 07:47:11 pmActually you are right about the 6V.The TI datasheet, chapter 7.3 and 10, specifies that the recommended range is +/-5V to +/-15V, that is to say, a symmetrical supply spanning from 10v to 30V from Vcc+ to Vcc-, with center GND.Quote from: basantab on February 19, 2018, 07:47:11 pmChapter 7.5.The whole table is valid for for a +15 0 -15 power supply.Look at rows VICRand VOPP:The former states that the input range is +/-12V as worst case.This means that you cannot expect it to work correctly if the input signal ha less than 3V headroom from a supply rail.In practice, most of the chips will do better, as in your case.The latter shows that the output swing is, still worst case, 24V; also in this case, assuming the output swing symmetrical with respect to GND and the +/-15V supply we have the same 3V limit.Quote from: basantab on February 19, 2018, 07:47:11 pmNice find!This inversion of output is typical of many op amps (especially older designs). I did not know that the 5532 was also subject to this.

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2018, 08:22:17 pm
content: Quote from: basantab on February 19, 2018, 07:47:11 pmNo, it says -5V and +5V, a total supply voltage of 10V, which is much worse. The ON Semiconductor data sheet I linked to says +/-3V is the minimum recommended operating voltage. Different manufactures have different specifications.QuoteAs mentioned above, section 7.5 on the TI data sheet. It's also on page 3 of the ON Semi data sheet.https://www.onsemi.com/pub/Collateral/NE5532-D.PDFQuoteThat's known as phase inversion. In many op-amps, the +input and -input exchange functions, when the common mode is exceeded. The resulting positive feedback can cause the output to saturate as close to the supply rail, as possible and will stay there, unless the input is forced back to a reasonable voltage, by connecting it to a voltage source with a low impedance.

author: rstofer
author_rank: Super Contributor
timestamp: February 19, 2018, 08:26:39 pm
content: If you have to stay 2 or 3 volts below the rails on input and you can't get to withing 2 or 3 volts of the rails on output, there simply isn't anything left for single supply 5V operation. It's a surprise you get anything at all.You either need a different op amp or a different power source. The old dog you are using comes from the era where op amps were supplied with +-15V and operated over +-10V. This left plenty of headroom and life was good.