Opamp Buffer Question - Page 1
Forum section: electronics / beginners

author: tzcczt
author_rank: Contributor
timestamp: July 31, 2014, 01:06:22 pm
content: I was trying to build an opamp buffer using aTL071. My input was a 10hz square wave, 0-5V and the output looked nothing like it, staying at around 5v and dropping to 4.7 or so.On the opamp, I had Vcc+ at 5V, and Vcc- at 0v, which I am starting to think was the problem. However, I've looked at the datasheet, but can't find anything which says it won't work in this configuration. Do I need a negative voltage on Vcc- if my signal is always greater than or equal to zero?I tested the same circuit in LTspice using the ideal opamp and it worked correctly.

author: kfitch42
author_rank: Frequent Contributor
timestamp: July 31, 2014, 01:25:30 pm
content: Please give us some more details, e.g. schematic/wiring diagram/pics... something. At the very least say what each pin is connected to (and in the process, verify they are connected as you expect). In particular how are you treating the "Offset N1" and "Offset N2" pins?You might also want to look at figure 10 in the top left of page 11 of the linked datasheet. It suggests that with Vcc+ of 5V, you can expect a maximum output voltage of 3.5 or 4 volts (just roughly eyeballing the graph).

author: mij59
author_rank: Frequent Contributor
timestamp: July 31, 2014, 01:34:14 pm
content: Hi,Take a look at page 7 , (6.4) of the data sheet.At a supply voltage of +/- 15V the Common-mode input voltage Vicr is -11V to + 11V.So if your input voltage is 0V, Vcc- needs to be min. -4V.

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2014, 01:37:57 pm
content: Neither the input common mode voltage range nor the output voltage range for the TL071 extend to Vcc and ground. Typically with +/- 15 volt supplies, they are limited to somewhere between +/- 10 volts and +/- 12 volts.You will either need supply voltages of +10 volts and -5 volt minimum to buffer 0 to 5 volts with a TL071 or a more appropriate amplifier like a TLV2371.

author: tzcczt
author_rank: Contributor
timestamp: July 31, 2014, 01:43:56 pm
content: Sorry, more details regarding the actual connections:Offset N1 : No connectionIn- : OutIn+ : SignalVcc- : 0vOffset N1: No connectionOut: In-Vcc+: 5vNC: No connectionIt looks like mij59 and David may have helped me solve it though if the Vicr is a direct function of the supply voltages (e.g. the output can only go to within +/- 4v of the supply).Thanks!

author: ivan747
author_rank: Super Contributor
timestamp: July 31, 2014, 01:45:59 pm
content: Another option is using an LM358, which does have a common mode range that extends to ground, on a single (as opposed to dual rail) power supply. Also, it's output goes to ground and up to Vcc-2V or something close to that, I can't recall from memory. With 9V you should have plenty of headroom for both common mode range and output voltage, give it a go, it's a common chip and I like it because of this exact feature (most other generic op-amps that I know of don't do this).

author: tzcczt
author_rank: Contributor
timestamp: July 31, 2014, 01:48:36 pm
content: Quote from: ivan747 on July 31, 2014, 01:45:59 pmThanks for the suggestion. I'll give that one a look.

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2014, 02:12:56 pm
content: There are all kinds of options depending on your circuit requirements. I just picked the TLV2371 as an example which will work in your specific circuit as is and as a general replacement for the TL071 at supply voltages up to about 15 volts.I am not all that fond of rail to rail input operational amplifiers because they have poor common mode rejection but in circuits that do not require precision, that is not a problem.