TL081 Op-amp Experiment

[wuiven64]

Hi,

I have done an experiment using TL081 op-amp. The circuit is built based on Figure 1(a) and Figure 1(b), respectively. The supply to the TL081 op-amp is 0 V at V- and +5 V at V+ of the TL081 op-amp. The experimental result is shown in the attachment. When I was comparing the experimental results, I found out that at input voltage of 0.00 V, the output voltage at Figure 1(a) is 4.73 V and the output voltage in Figure 1(b) is 1.35 V.

Can anyone explain why the output voltage in Figure 1(a) is 4.73 V and Figure 1(b) is 1.35 V at input voltage of 0.00 V?

[Dave_C]

TL081 has a minimum common mode input voltage that it works correctly (usually 1 to 2V above the V- terminal, 2V in this case)
If you want to operate an opamp with 5V single supply, then I would recommend referencing the signal to a virtual ground mid supply ( 2,5V ) or using RRIO opamps.
The weird results are due to opamp trying to operate outside of its specifications.

[capt bullshot]

Your circuit will work as you expect if you either use a RRIO OpAmp (e.g. the TLV2371) or use a dual power supply (e.g. +/- 10V) for the TL081.

The TL081 cannot operate with its input voltage at one supply rail, you have to provide a margin of around 2...3V from the input signal swing to the supply rails. So for the TL081 to operate properly with the "+" input at 0V, its negative supply must be at least -3V, and the positive supply must be at least +3V.

If the input signal gets too near to the supply rails, the input stage is driven outside its designed range, causing various modes of misbehaviour. What you can see in your example is called "Inversion" since the output jumps to the opposite side than you'd expect. In this case, it's  dependent of the current that goes into the overdriven input, driving the internal transistors into unwanted states.
Normally, the input shouldn't draw any current at all, but if your input signal exceeds the specified range, internal elements start to conduct current that shouldn't normally, drawing current and driving the internal stages in some unwanted directions.

Many OpAmps are specified this way (with this margin necessary), there are "special" OpAmp called "Rail to Rail", that do allow for the input / output signal over the full supply range. Additionally, these are divided into RRO (rail to rail output) and RRIO (rail to rail input and output). So if you want your circuit to work, you have to use a RRIO OpAmp.

[T3sl4co1l]

Ah yes, good ol' output inversion.

Basically, pulling the input low, causes the input stage to shut down.  No current gets to the output stage, which makes it think it should deliver +MAXVAL output...

Didn't know that a series resistor can "fix" that on TL081, interesting.  I guess what's happening is, the input stage's current gets shunted through the input pin, and with the large series resistor, not enough current can be shunted to make it screw up.

This makes more sense if you look at the internal diagram of the amp (if you can find it -- a lot of datasheets don't bother showing ) and think about diode paths from the input pin.  IIRC, they used P-channel JFETs.  A JFET is a fancy diode, a P-ch being the channel (drain to source) as the anode, and the gate as the cathode.  Pull the gate low, the diode turns on, and the channel-to-gate diode must pull the channel down to ~0.6V above Vin.

P-ch FETs have the drain terminals on the "bottom" (i.e., the negative side), which carry the signal to the rest of the amp.  Probably, it's arranged so that when one JFET's current drops, the output rises (i.e., there is an intermediate gain stage, such that, when less current goes into its input, it makes a higher voltage at its output).  So, pulling that current away, forcibly, causes the output to rise, to +MAXVAL.

Which also means you should be able to measure the input stage's bias current this way: by seeing how much current comes out of an input pin when pulled low.

IIRC, the trim pins control input bias currents as well, so you could play with that too, I suppose.

But yeah, this won't make any sense if you didn't find a diagram, or probably even if you did...

Tim

[Zero999]

Yes, the TL081's input impedance also rises dramatically, once the common mode range is exceeded: as you say the gates become forward biased.

TL081 has a minimum common mode input voltage that it works correctly (usually 1 to 2V above the V- terminal, 2V in this case)
If you want to operate an opamp with 5V single supply, then I would recommend referencing the signal to a virtual ground mid supply ( 2,5V ) or using RRIO opamps.
The weird results are due to opamp trying to operate outside of its specifications.

Yes this is a common  trap for beginners. It should be the first thing mentioned in any good op-amp tutorial.

The TL081 isn't even fully specified at a total supply voltage of 5V, so there's no guarantee it'll work in this circuit, even if its common mode range is not exceeded.

[David Hess]

Modern operational amplifiers tend to have extra circuits to prevent phase inversion.  It is not only a problem with JFET input designs; bipolar and MOSFET input designs can suffer from phase inversion as well although not always because the input stage gets cut off.

[Wolfgang]

... the good old Horowitz and Hill Art of Electronics book has some useful info about OpAmps and maybe also treats this problem.
Just use a newer OpAmp or even better an RRIO one and it will be gone.

[wuiven64]

Hi,

Thanks everyone for the suggestions and ideas. I have repeated the experiment by replacing the supply of the TL081 op-amp from +5 V at V+ and 0 V at V- to +5 V at V+ and -5 V at V-. The experimental result is attached at below. As shown in the results, the TL081 op-amp operates properly.

[wuiven64]

However, why does the output voltage without R1 is high (4.73 V) compared to output voltage with R1 (1.35 V) when the input voltage is 0.00 V as the supply of the TL081 is +5 V at V+ and 0 V at V-?

Can anyone explain why this happen?

[Audioguru]

When an opamp that has the Phase Inversion problem has its input voltage too close to its negative supply voltage then its input transistor or Jfet becomes reverse-biased and draws a lot more current than normally. A series resistor reduces that abnormal input current so the opamp acts differently.