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I've connected an op-amp (TL071) as buffer, with Vcc+ = 5V and Vcc- = 0V (ground).

When the input voltage (v+) is greater than 1.44V I get the expected result (the output is the same as the input).

However, if the input is below 1.44V, the output saturates.

Anyone knows why this might be and how can I get around it? My goal was to amplify the signal of a LM35 (a temperature measurement).

Thanks.

enter image description here

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I've edited and posted a picture! The negative input is connected to the output directly. \$\endgroup\$ – Francisco Dec 31 '16 at 16:10
  • \$\begingroup\$ Can you use the built-in schematic editor to post a schematic as well? \$\endgroup\$ – ThreePhaseEel Dec 31 '16 at 16:55
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The TL071 is not designed to be used with an input any less negative than 4V above its negative rail.

When powered from +/-15V, the input common mode voltage (from the datasheet) is between -12V and +15V.

TL071 common mode range

In reality, you will probably get away with 2 diode drops above the negative power rail (about 1.4V - there is a huge clue when you see multiples of about 0.6 to 0.7V).

The reason for that is quite clear from the functional block doagram in section 8.2

The saturation you are seeing is due to phase inversion; this is a common issue with JFET input devices.

Most bipolar amplifiers will have a common mode range to the negative power rail (but only up to about V+ -1.4V); you could alternately look for a rail to rail input / output amplifier.

Some possible amplifiers:

LTC2057. Vcm V- to V+ - 1.5V LTC6078

There are numerous offerings from TI, Maxim and ADI.

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  • \$\begingroup\$ Thanks! I've read the datasheet but didn't pay attention to the input values (I'm just getting started with electronics as a hobby). May I ask then what would be a good op-amp to use in this case? \$\endgroup\$ – Francisco Dec 31 '16 at 16:07
  • \$\begingroup\$ I do not know what may be available to you - see my suggestion in the answer as to what to look for. \$\endgroup\$ – Peter Smith Dec 31 '16 at 16:18
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TL071 ... Vcc+ = 5V and Vcc- = 0V

The datasheet says the common mode input range is ±11 V and the output ±12 V with a ±15 V supply. In other words, these opamps require 3 V headroom at each end for the output, and that's with a 30 V supply.

I can't even guess why you think they should do anything useful with just a 5 V supply.

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The TL071 is designed to be a dual supply amplifier or single supply with biased operation and is unsuitable for single supply operation at 5 V. It's designed for a recommended minimum of +/-5 V or 10 V supply.

enter image description here

You can see from the datasheet that the output is a Class B amplifier stage:

enter image description here

You could use an LM324 type op-amp, which will run from a single 3-5 V supply. But even with this device you need to ensure you draw significant current from the output to ground to ensure the output pulls down (you effectively make the output stage an Emitter follower) to ground.

enter image description here

The internal 50 uA sink to ground will ensure the output goes to ground for only very light load resistance (typically a high value gain set feedback resistor of 100k or more). But if you want to drive a lower impedance next stage after your op-amp you may need to include a pulldown resistor from output to ground. Typically I always draw a minimum 1 mA from my highest output signal level.

Notice that the LM324 is the reverse of the TL071 in that the input Common mode range includes ground and not VCC....though that likely won't impact you.

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  • \$\begingroup\$ Nobody has pointed out any such thing, and it isn't true. An opamp can't detect whether it is being used with a grounded or negative bottom rail, as long as the surrounding circuitry is correct. \$\endgroup\$ – user207421 Jan 1 '17 at 21:57
  • \$\begingroup\$ @EJP. You are absolutely correct, the op-amp may no be able to detect it's supply configuration. I fixed in the answer. However the TL071 is completely unsuitable for operation from a single 5 V supply. \$\endgroup\$ – Jack Creasey Jan 1 '17 at 23:37

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