Hi,

I have built a sine wave oscillator with 2 TL 071.
The 100 k potentiometer allows to adjust the amplitude level.
The circuit works fine but … the amplitude of the sine wave goes from 2,5 V to 5,5 V.

I am looking for a solution to have an amplitude level starting as close as possible to 0 Volt.
Please note that I am using a single 9 Volts power supply.

Thank you for your ideas,

Philippe

 

Hi Phil,
You are biassing the Vout of U1b to +9V/2, using those 100k's, try reducing the lower 100k to say 10K as a test.
E

 

Replace R-6 with a 100K-Pot,
Connect the Voltage Divider to the Inverting-Input,
adjust the max-Output with the existing 100K-Pot,
if You still don't have enough Gain, change your existing 100K-Pot for a 500K-Pot.
The Op-Amp may not like such large Resistors in the Feedback-Loop,
You may need to add a small Cap from the Output to the Inverting-Input to prevent Oscillation.

This will give You from zero to what ever level You want to set the maximum-Gain to.
.
.
.

 

Put the level control pot as a voltage divider on the output of the amp to ground.

 

Hi Eric !
Nice to read you gain.
I think you remember our previous discussions about a field strenght meter with LM358 ...
So I have lowerered the value of the resistor. Here are the results :
- 10 k : no more sine wave
- 22 k : same
- 47 k : it works but the shape of the sine wave is strongly deformed ! (Flattened) and the wave starts at about 2 V

Meanwhile there is another answer from LowQcab.
I have to test it too !
Philippe

 

Level control circuit, 0V to max. Use a linear pot.

 

hi Phil,
As you probably know, getting down to 0v on OPA output is only possible when using a dual supply voltage or close to 0v with a Rail 2 Rail OPA.
If you have a 100k pot, replace the lower 100k resistor with that pot.

Using the pot you see how close to 0v you can get on the lower edge of the output waveform

E

 

hi Phil,
This is your original; circuit in LTspice.

E
Added sim showing a dual supply, swings about zero volts

 

Put the level control pot as a voltage divider on the output of the amp to ground.

Hi Mr Chips !
Nice to read you gain.
I think you remember our previous discussions here
I want to be sure to understand you correctly.
I have kept the level control pot as I need to adjust it but added a voltage divider as suggested.
Here is a new schematic : Is that OK ?

So I have just tested it.
Unfortunately the sine wave starts from 0 V but the amplitude level is very reduced at about 0,7 V ! ...

Philippe

 

Since you are using a single supply, put a capacitor in series between the op-amp output and the pot.

 

hi Phil,
Check this Vout swing limit,
With a single supply, I would say +1.5Vout is your lower limit, use a dual supply.
This table is for a dual supply of +/-15v but look at the expected Vout swing limits
E

 

Level control circuit, 0V to max. Use a linear pot.

View attachment 255199

Yes it's exactly how I have connected it !
The amplitude level is 0,7 V ...

 

Since you are using a single supply, put a capacitor in series between the op-amp output and the pot.

I have tested it with a small 10 nF cap
Th sine wave half part is going below 0V
I need it to start at 0 V

 

I have tested it with a small 10 nF cap
Th sine wave half part is going below 0V
I need it to start at 0 V

Sorry !
I was meaning that the half part of the sine wave has to go to 0 V not below 0 V

 

The centerline of a Sine-Wave should ideally be "zero-Volts" only when You have a Split-Power-Supply.

You don't have a Split-Power-Supply,
so the center of the Sine-Wave should be at 1/2 of the "Single-Ended" Power-Supply-Voltage.
in your case that would be ~4.5-Volts.
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.
.
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Sorry !
I was meaning that the half part of the sine wave has to go to 0 V not below 0 V

With the series capacitor, the mid-point of the sine wave will be at 0V (assuming the pot is wire to true 0V).
If the mid-point is not at 0V then you do not have a sine wave.

This is what the series capacitor does. It removes any DC offset. It is a high pass filter.

 

Replace R-6 with a 100K-Pot,
Connect the Voltage Divider to the Inverting-Input,
adjust the max-Output with the existing 100K-Pot,
if You still don't have enough Gain, change your existing 100K-Pot for a 500K-Pot.
The Op-Amp may not like such large Resistors in the Feedback-Loop,
You may need to add a small Cap from the Output to the Inverting-Input to prevent Oscillation.

This will give You from zero to what ever level You want to set the maximum-Gain to.
.
.
.

Good evening LowQcab

Thank you for your message.
One question before testing the modifications :
The output of the 1st TL 071 after R5/C2 has now to be connected to the non inverting input ?

 

hi Phil,
Check this Vout swing limit,
With a single supply, I would say +1.5Vout is your lower limit, use a dual supply.
This table is for a dual supply of +/-15v but look at the expected Vout swing limits
E
View attachment 255209

Eric,

Thank you for the information.
I had the "intuition" that I could not have the lower limit of the sine wave at 0V with such an OpAmp ...

Two questions :
- Have you seen above the idea of LowQCab ? Should it work (I haven't had yet the time to test it)
- I need to keep a design with a single power supply. Do you have some rail to rail and fast OpAmps (min. 100 kHz frequency) to advise me ?

Good evening
Philippe

 

hi Phil,
I will sim LowQ's modification.

You could consider adding a level shifting R2R OPA to the output of the 2nd OPA, also adding level shifting.

The 100k pot on the output will not give you the performance you expect, also it will increase the output impedance, which is not a good idea for a pulse gen driving other loads.

Another option would be to use an OPA to create a dual +/-4.5V from the 9V supply.

E
I will post tomorrow as this evening I am now working on a different computer

 

Beside the "bottom" peak at 0V, how much do you expect the Amplitude (pk to pk value) to be?

Fixed or adjustable?