# op amp rail to rail voltage follower latching up

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.

• This happens no matter which amp you use? You said there are two on the chip. – Toor Apr 16 at 2:02
• Where did you get the part? I've seen parts from brokers do odd things. – Robert Endl Apr 16 at 2:09
• Yes I have tried both amps and have tried the other chips in the batch of 5 I bought. – IanF Apr 16 at 2:13
• The TSS922 were bought on aliexpress from a supplier in Shenzen. Device marking is TS922IN 90B1432 with the ST Microelectronic logo and MAL (I presume Malaysia) – IanF Apr 16 at 6:20
• @IanF most probably the part is counterfeit, AliExpress is well-known for such surprises. – sx107 Apr 16 at 19:28

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.

• Thanks for the helpful info. Have you tested any of these devices with a single supply? For example the TS922 is optimised for 3 and 5 volts. – IanF Apr 18 at 0:57

Place a .1uF ceramic capacitor between Power and Ground and add resistors in series with both inputs to the op amp. The 2KOhm you added to the output would work.

You bought an electronic IC from a seller in China. Of course it is counterfeit. Here is what Wikipedia says about AliExpress in Google:

• Really, that's the first result you get for searching "AliExpress"? Sounds like fake news. That said, the source of the parts means there is a non-negligible chance they are counterfeit. I doubt it's 100%. – immibis Apr 16 at 22:52
• I have purchased a wide variety of electronic components via Aliexpress over several years. My experience had actually been good up to this particular part. In this case I contacted Aliexpress and they have refunded with no problem. – IanF Apr 18 at 0:42
• The Wikipedia link above does not give the result as shown. – IanF Apr 18 at 0:52

Must be a counterfeit part - there is no other explanation.

• The device is definitely counterfeit. I tested TS922 devices from a reputable supplier and they work as expected in the same test setup - no latch up. Thanks all. – IanF Apr 20 at 20:59