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Strange step in amplified signal (with a differential opamp circuit)

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petert:
Hello,

I was making some experiments with a simple differential amplifier circuit, as below.


R1 = 10 Ohm and R2 = 100 Ohm

The result is an amplified sine wave that has a strange step at 0V.


The opamp I use is an LM358N. The positive and the negative supply get powered by 9V batteries, such that there is approx. -9V on the Vcc- and approx. +9V on Vcc+ pins of the opamp. (Verified with a multimeter.)

The input signal is a 4kHz sine wave coming from a function generator, with a 500mVpp amplitude.

The circuit is built on a breadboard, but the signal does not get distorted on the breadboard, when probed with the oscilloscope and bypassing the opamp.

What could I have done wrong?

Edit: When I disconnect the signal generator's ground from the V- signal input of the opamp the distortion disappears. I am still confused what is going on, though.

Andreas:

--- Quote from: petert on August 28, 2019, 01:54:31 pm ---What could I have done wrong?

--- End quote ---

Hello,

the (low power) LM358 is known for cross over distortion under certain load conditions.
(see also LM358 data sheet)

-> use a OP-Amp without "cross over distortion" (e.g. LT1013/TL072)

with best regards

Andreas

nfmax:
You are using low value feedback resistors R1 and R2, which are loading the amplifier output and giving rise to the  'crossover' distortion for which the LM358 is well-known. Try using 1kohm and 10kohm resistors instead: this should give the same gain but with much less load on the OPAMP. I suspect you will find there is still a touch of crossover: adding a moderate-value load resistor from the output to the negative supply will shift the notch to lower voltages. About another 10kohm or so should remove it completely.

Disconnecting the generator ground terminal from the negative input removes the load of the upper R1+R2 in series from the amplifier output. You are left with a circuit that has a gain of about 0.9 and no load, so the distortion vanishes. The return from the signal generator is now going via the common 'ground' (bottom end of lower R2, mid point of the batteries) so there is still a complete circuit.

I hope this helps (and keep learning!)

Old Printer:
There was a thread related to this a couple days ago, down a couple pages now, but here is a link:

https://www.eevblog.com/forum/beginners/adding-filters-to-class-ab-audio-amp/

nugglix:
And of course there is a video:

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