# How to eliminate harmonics in operational amplifier circuit?

#### yuesu jin

Joined Aug 22, 2019
20
Hi all,
I made an amplifier circuit using LM675, the schematic diagram is

When the circuit is directly connected to oscilloscope. Signal is heavily distorted by harmonics. The input signal is 5Hz and 2Hz sin wave, Vcc=27V, Vee=0, but the output waveform is distorted like spike. More harmonics component, more distortion. What I want to know is that what is the cause of harmonic distortion in low frequency, and how to remove this distortion? Thanks a lot!!!!!

#### ronv

Joined Nov 12, 2008
3,770
Hi all,
I made an amplifier circuit using LM675, the schematic diagram is
View attachment 186631
When the circuit is directly connected to oscilloscope. Signal is heavily distorted by harmonics. The input signal is 5Hz and 2Hz sin wave, Vcc=27V, Vee=0, but the output waveform is distorted like spike. More harmonics component, more distortion. What I want to know is that what is the cause of harmonic distortion in low frequency, and how to remove this distortion? Thanks a lot!!!!!
View attachment 186632
View attachment 186633
If you have no negative supply you need to bias it differently.
But somehow you show the output going below ground.
What is the input voltage?

#### yuesu jin

Joined Aug 22, 2019
20
If you have no negative supply you need to bias it differently.
But somehow you show the output going below ground.
What is the input voltage?
Hello, ronv!
The input voltage is 200mV

#### ronv

Joined Nov 12, 2008
3,770
Hello, ronv!
The input voltage is 200mV
What is Vee?
What is the dc level of the 200 mv signal?

#### yuesu jin

Joined Aug 22, 2019
20
What is Vee?
What is the dc level of the 200 mv signal?
Vee is the negative power input, you can find it on the schematic diagram. DC offset is 0v.

#### ronv

Joined Nov 12, 2008
3,770
Vee is the negative power input, you can find it on the schematic diagram. DC offset is 0v.
If Vee is zero volts (or ground) as I think you said in your post and the input goes below ground (0 volts offset) the op amp can't work on the negative 1/2 cycle of the input.
So you should make Vee minus 10 volts or so.

#### yuesu jin

Joined Aug 22, 2019
20
If Vee is zero volts (or ground) as I think you said in your post and the input goes below ground (0 volts offset) the op amp can't work on the negative 1/2 cycle of the input.
So you should make Vee minus 10 volts or so.
I don't know why, but LM675 could accept only positive +Vcc with no -Vee, in the datasheet
there are one typical application:

#### ronv

Joined Nov 12, 2008
3,770
What do you want your output to be?
Would it be OK if it were 4 volts pp at 10 volts?
Or do you want it to be 4 volts pp around ground?

#### Ylli

Joined Nov 13, 2015
933
Yes, you need that Vee voltage. In additions, looking at the output waveform, I wonder if your Vcc supply has enough current capability.

#### Audioguru

Joined Dec 20, 2007
11,249
The amplifier with a single polarity supply needs to be designed to work with a single polarity supply, like this:

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#### yuesu jin

Joined Aug 22, 2019
20
That one is OK because the input is always above ground. Your input goes below ground.
Ah, I understand it, Vee is indispensable. Thanks a lot, I will add another battery for -Vee

That one is OK because the input is always above ground. Your input goes below ground.
But there is another question, why the output has negative part?

The amplifier with a single polarity supply needs to be designed to work with a single polarity supply, like this:
Thanks a lot, I understand how it should be! But how can I invert a positive power to negative, is there any circuit can do that?

Last edited by a moderator:

#### crutschow

Joined Mar 14, 2008
24,735
Thanks a lot, I understand how it should be! But how can I invert a positive power to negative, is there any circuit can do that?
Yes, you could use an inverting switching regulator.

#### yuesu jin

Joined Aug 22, 2019
20
Yes, you could use an inverting switching regulator.
Thanks a lot!!

#### DickCappels

Joined Aug 21, 2008
6,312
An ICL7660 is often a good single-chip solition.

#### bertus

Joined Apr 5, 2008
20,461
Hello,

@DickCappels , the ICL7660 is only capable of supplying 40 mA.
I think the OP is using a much higher current.

Bertus

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#### ronv

Joined Nov 12, 2008
3,770
Ah, I understand it, Vee is indispensable. Thanks a lot, I will add another battery for -Vee

But there is another question, why the output has negative part?

Thanks a lot, I understand how it should be! But how can I invert a positive power to negative, is there any circuit can do that?
I don't see any way for the voltage to go negative in your circuit.
Is the picture an actual scope trace or some type of simulation.
If it is scope. Is the scope DC coupled or AC?

#### DickCappels

Joined Aug 21, 2008
6,312
Yes, Thank you Bertus. I see the quiescent current can exceed the ICL7660's rated capabilities. Not your every-day opamp.

Then there is Audioguru's alternate bias arrangement in post #11.