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TL07x, TL08x eBay fakery

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John B:
A few more quick observations:

The fake eBay ones draw very little current at idle with no load on the output, <1mA. The real LCSC one draws around 3-4mA, which is consistent with the datasheet. These are rough values off my power supply.

I also tried to determine the bias currents on the fake TL072. I put a 10M resistor on the input and connected it to ground. I measured about 27mV, so also taking into account the multimeter impedance 27mV/5M \$\Omega\$ = ~5nA? But interestingly the current is flowing out of the input. So it doesn't even sound like a JFET input, or an LM358 either.

magic:

--- Quote from: John B on August 19, 2019, 10:01:32 pm ---The fake eBay ones draw very little current at idle with no load on the output, <1mA. The real LCSC one draws around 3-4mA, which is consistent with the datasheet.
--- End quote ---
Beware that idle current is usually specified with the input tied to ground. It may change if the input is left floating, hits one or the other rail and causes shutdown or saturation of some internal stages.


--- Quote from: John B on August 19, 2019, 10:01:32 pm ---I also tried to determine the bias currents on the fake TL072. I put a 10M resistor on the input and connected it to ground. I measured about 27mV, so also taking into account the multimeter impedance 27mV/5M \$\Omega\$ = ~5nA? But interestingly the current is flowing out of the input. So it doesn't even sound like a JFET input, or an LM358 either.

--- End quote ---
I measure from output to ground, it's simpler that way. Offset voltage needs to be subtracted.
Current flowing out is consistent with 358, though 5nA is below spec. At any rate, it doesn't seem to be JFET input.

John B:

--- Quote from: magic on August 19, 2019, 10:43:51 pm ---
--- Quote from: John B on August 19, 2019, 10:01:32 pm ---The fake eBay ones draw very little current at idle with no load on the output, <1mA. The real LCSC one draws around 3-4mA, which is consistent with the datasheet.
--- End quote ---
Beware that idle current is usually specified with the input tied to ground. It may change if the input is left floating, hits one or the other rail and causes shutdown or saturation of some internal stages.

--- End quote ---

Yep, inputs were tied to ground. Even with the above images I made sure that the other opamp was tied to ground in a buffer configuration to ensure that no amp is saturating. I think it was TI app note which stated that one opamp saturating could result in distortion on other amps on the same chip, so I've always kept that in mind as best practice.

Nominal Animal:
I wonder if it would make sense to create a microcontroller/DSP circuit with a 24-bit ADC+DAC, just to characterize opamps at unity gain with different wave forms down to DC behaviour?

(There are now microcontrollers like Teensy 4.0 with native 480 Mbit/s USB support and I2S interfaces that would allow easy use of audio ADCs and DACs for this.  Say, generate and record 100,000 samples, and do all the processing on a computer.  One channel would be the generated signal as captured by the ADC, and the other the opamp output in unity gain configuration.)

Sounds like a long-term hobby project to me..  ;)

David Hess:

--- Quote from: John B on August 19, 2019, 01:19:04 am ---Below is a comparison of an eBay sourced "TL072CP" and "TL082CP", vs an LCSC sourced TL072CP. Each op amp is powered at +/- 5V, +/- 15V, with a 10k load to ground. All configured as a non inverting buffer.
--- End quote ---

It is interesting that the crossover distortion occurs so far below ground;  I would have expected it to be much closer to ground.  The symmetry between the positive and negative edges indicates no capacitive loading which is consistent with the description of your test.

Is the distortion different in inverting mode where there is no common mode voltage?  Maybe it is not coming from the output stage.

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