# Electronic – op amp rail to rail voltage follower latching up

operational-amplifier

I have selected a TS922 op amp to use in a voltage follower configuration. The specs for the TS922 in summary are:

• Rail-to-rail input and output
• Low noise: 9 nV/√Hz
• Low distortion
• High output current: 80 mA (able to drive 32 Ω loads)
• High-speed: 4 MHz, 1 V/μs
• Operating from 2.7 to 12 V
• Low input offset voltage: 900 μV max.
• Latch-up immunity

The voltage follower configuration is

Supply voltage is 5 V.

To test I am using a simple 10k pot arrangement (range v supply to gnd) to apply a voltage to the input (pin 3) with a voltmeter measuring the output on pin 1.

My issue is that the TS922 output latches up to 4.7 V if the input voltage rises above 3.6 V. If the input voltage is less than 3.6 V the TS922 behaves as a voltage follower.

To try to identify why this is happening I have tried:

• placing a light load (2k resistor) from pin 1 to ground – no effect
• grounding the inputs to the other dual amp in case there was some interaction – no effect.

Is there something I am missing?? Otherwise I suspect the markings on this device don't match what is inside.

I would ALSO appreciate any advice on other op amps which are close to the TS922. The most important specs are:
– single supply 5 V.
– rail to rail input/output.
– high output current capability > 50 mA.

I know I can search for devices – but I am hoping someone has experience with a device that works well.

A couple of days ago I tested some opamp in voltage follower configuration. All op-amps were bought from a reputable vendor (TME).

The input signal was coming from the function generator, $$\F = 1\textrm{kHz}\$$ and the signal amplitude was $$\10V\$$ peak to peak.

And all Opamp's was also supplied from symmetrical supply $$\+/-5V\$$.

Hence we can easily examine the allowed input common mode voltage range.

The first one was $$\\textrm{LM}358\$$ from ST.

As you can clipping accrue for $$\Vin > (Vcc-1.4V)\$$

And no sign of a famous crossover distortion due to lack of load resistance.

But reducing the input signal amplitude and adding a load resistance reveals the crossover distortion.

The second opamp was $$\\textrm{TS922}\$$ from ST

As you can see $$\Vin = Vout\$$ as expected form rail-to-rail input and output opamp.

No sign of "latching-up".

Hence, if your setup is correct I vote for a counterfeit part from China. Not a big surprise either.

Next one was $$\\textrm{TL}072\$$ from Ti.

As you can see we got a clipping for $$\Vin > (Vcc-0.6V)\$$ or so. And when the input signal approaches $$\V_{EE} - 1.4V\$$ phase inversion phenomenon manifests itself.

And finally the $$\\textrm{NE}5532\$$ from Philips.