Hello
I searched in Google and found many circuits but no one of them worked properly especially when the output is lowered to 20mVp-p. I have an oscilloscope of 50 MHz BW. I need that circuit to be used as a function generator to examine some low frequency preamplifiers. I have built many circuits but they all had a defect So I hope that one of the members may provide me a proven circuit.
Thank you

 

Post #2 in this thread. Connect a Pot to V(d), and take the output from the wiper. Alternatively, make a fixed two-resistor voltage divider that drops the natural output voltage of any sine oscillator (which will approach a peak-to-peak voltage just less than the dc supply voltage) to 20mV, and buffer it with another opamp, if buffering is needed.

Design equations are Fig 9 in this TI app note

 

I have built many circuits but they all had a defect

What was the defect? The circuit you describe is so basic that the defect is the important qualifying parameter.

ak

 

you could set the frequency with a square wave signal, apply a filter to produce a sine wave, and use an amplifier to set the amplitude. With such a small signal choose a low noise op amp.

 

Hello
I searched in Google and found many circuits but no one of them worked properly especially when the output is lowered to 20mVp-p. I have an oscilloscope of 50 MHz BW. I need that circuit to be used as a function generator to examine some low frequency preamplifiers. I have built many circuits but they all had a defect So I hope that one of the members may provide me a proven circuit.
Thank you

What is it about them that is 'not working properly'? Can you show links to the ones you've tried?

Ken

 

Post #2 in this thread. Connect a Pot to V(d), and take the output from the wiper. Alternatively, make a fixed two-resistor voltage divider that drops the natural output voltage of any sine oscillator (which will approach a peak-to-peak voltage just less than the dc supply voltage) to 20mV, and buffer it with another opamp, if buffering is needed.

Design equations are Fig 9 in this TI app note

Thank you Mike
Can I use LM339 quad comparator?

 

How pure does this signal need to be? If not very exact the project is ridiculously easy, if you need a pure sine wave then it is just easy.

 

Build any oscillator mentioned in TI paper; then filter the output heavily, and you will obtain a pretty pure sine wave. The amplitude depends on many things, but it can be divided down with a couple of resistors to obtain your desired amplitude. Use AGC to obtain a fixed amplitude (remember that the AGC BW must be much greater than the signal frequency.

 

Thank you Mike
Can I use LM339 quad comparator?

Comparators have an open collector (aka digital output). What you want is an operational amplifier with bipolar supplies to give you an analog output.

 

You may try LF356 or LF357 to do the job:
Sinewave Vin → VR(Pot) Voltage divider → LF356 inverter amplifier with negative gain
Sinewave Vin → Fixed Resistor Voltage Divider → LF356 inverter amplifier with negative gain
If you could choose a good value for the voltage divider then maybe the negative gain is no need.

 

Again Samy "What is it about them that is 'not working properly'? Can you show links to the ones you've tried?"

Ken

 

The inexpensive LM324 would be perfect I suspect. Less than a quarter from many sources, will operate at 3VDC.

 

Again Samy "What is it about them that is 'not working properly'? Can you show links to the ones you've tried?"

Ken

I've downloaded these circuits then printed on paper
I do not recall the links
Just remember that the last circuit was from the site "handy dandy"
I finally decided to buy a function generator next month
thank you all