Hi there Audiophiles,
I have built an inverting amplifier with the LM358AP.
The circuit is below. The supply voltage is +/- 15V.
The resistor Rf is the load and Ri is used to programm the current.
The non-inverting input sits at ground potential.
Both resistors have a value of 1,2k which doesn't seem to affect the distortion of the output signal.
A signal generator is used to set the Vin value to 1Vrms, 1kHz perfect sine.
The output signal is then measured by a Tektroniks scope.
Unfortunately the Signal is distorted in certain parts of the sine wave (see picture).
If Vin is set to have a DC offset of about 1,3V, the knee can be shifted and removed (see picture).
Maybe you know why this "knee" is created and how I can eliminate it, without using an extra circuit for the offset. The knee is strongly frequency dependend. Higher frequency results in a stronger knee.
Thank you very much,
all the best
I have built an inverting amplifier with the LM358AP.
The circuit is below. The supply voltage is +/- 15V.
The resistor Rf is the load and Ri is used to programm the current.
The non-inverting input sits at ground potential.
Both resistors have a value of 1,2k which doesn't seem to affect the distortion of the output signal.
A signal generator is used to set the Vin value to 1Vrms, 1kHz perfect sine.
The output signal is then measured by a Tektroniks scope.
Unfortunately the Signal is distorted in certain parts of the sine wave (see picture).
If Vin is set to have a DC offset of about 1,3V, the knee can be shifted and removed (see picture).
Maybe you know why this "knee" is created and how I can eliminate it, without using an extra circuit for the offset. The knee is strongly frequency dependend. Higher frequency results in a stronger knee.
Thank you very much,
all the best
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Simple.
LM358, different from 99.99% of other Op Amps has a Class B biased output, so what you see is crossover distortion.
There's 2 options:
1) use any other Op Amp.
2) Bias it Class A with an added resistor from output to -15V if split supply or from output to ground if simple supply.
From the datasheet:
http://diodes.com/datasheets/LM358.pdf
What value resistor?
Roughly similar to the load impedance you are driving.
So if, say, you are driving a 10K power amp input, use from 10K to 22K to -15V and you'll be fine.
I use LM358 a lot, it's one of my "secret weapons" , cheap, plentiful and .... rail to rail 😀 and it's happy with just +5V , go figure.
Just don't use it for Hi Fi 😉
LM358, different from 99.99% of other Op Amps has a Class B biased output, so what you see is crossover distortion.
There's 2 options:
1) use any other Op Amp.
2) Bias it Class A with an added resistor from output to -15V if split supply or from output to ground if simple supply.
From the datasheet:
http://diodes.com/datasheets/LM358.pdf
What value resistor?
Roughly similar to the load impedance you are driving.
So if, say, you are driving a 10K power amp input, use from 10K to 22K to -15V and you'll be fine.
I use LM358 a lot, it's one of my "secret weapons" , cheap, plentiful and .... rail to rail 😀 and it's happy with just +5V , go figure.
Just don't use it for Hi Fi 😉
Thank you very much that resistor at the output to -15V fixed the problem.
I couldn't find the explanation of A and B class in the datasheet though!
The bias current now always flow from the output to -15V right?
I was worried that the biascurrent configuration could create a small DC current through the load... but that does not seem to be the case.
Thanks
I couldn't find the explanation of A and B class in the datasheet though!
The bias current now always flow from the output to -15V right?
I was worried that the biascurrent configuration could create a small DC current through the load... but that does not seem to be the case.
Thanks
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