Sine wave oscillator with TL071 : How to modify the amplitude level ?

Thread Starter

Philexium

Joined Oct 15, 2017
43
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

Sine wave oscillator TL071.jpg

Sine wave oscillator TL071 schematic.png
 

ericgibbs

Joined Jan 29, 2010
15,332
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
 

LowQCab

Joined Nov 6, 2012
1,913
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.
.
.
.
 

Thread Starter

Philexium

Joined Oct 15, 2017
43
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
 

ericgibbs

Joined Jan 29, 2010
15,332
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
 

ericgibbs

Joined Jan 29, 2010
15,332
hi Phil,
This is your original; circuit in LTspice.

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

EG 1140.png


EG 1139.png
 
Last edited:

Thread Starter

Philexium

Joined Oct 15, 2017
43
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.
Sine wave oscillator output modified.jpgUnfortunately the sine wave starts from 0 V but the amplitude level is very reduced at about 0,7 V ! ...

Philippe
Sine wave oscillator TL071 tests 2 with pot on the output.jpg
 

ericgibbs

Joined Jan 29, 2010
15,332
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
EG 1141.png
 

LowQCab

Joined Nov 6, 2012
1,913
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.
.
.
.
.
.
.
 

MrChips

Joined Oct 2, 2009
25,919
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.
 

Thread Starter

Philexium

Joined Oct 15, 2017
43
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 ?
 

Thread Starter

Philexium

Joined Oct 15, 2017
43
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
 

ericgibbs

Joined Jan 29, 2010
15,332
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
 
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