# Simple Function Generator

Discussion in 'Homework Help' started by Mr_Edward, Jul 18, 2014.

1. ### Mr_Edward Thread Starter New Member

Jul 17, 2014
11
0
hello everyone

so we have this laboratory project where we must build a function generator without the use of 555 timers..

This is the circuit that I got.

i've been looking everywhere for an easy to understand circuit for wave generation and this is the best one i could find.

so is there any way for me to vary the amplitude of the resultant wave of the circuit?

i got this circuit here:

"Circuit tutorial - sawtooth generator w/ current sources, diode switches and hysteresis comparator"

Best Regards,
Edward

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2. ### shteii01 AAC Fanatic!

Feb 19, 2010
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function generator

Sine? Square? Triangular?

3. ### Mr_Edward Thread Starter New Member

Jul 17, 2014
11
0
we've been asked to do Sine, Square and Triangular with variable frequency, duty cycle, and amplitude. i've got the duty cycle adjustment from the pots on the current source and drain but i can't seem to find a place to vary the amplitude.

any tips on how?

4. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,800
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Yes. Hints: (a) Consider the purpose of R7. (b) Are you familiar with the use of a potentiometer?

5. ### Mr_Edward Thread Starter New Member

Jul 17, 2014
11
0
i've tried using it on R7 and it just enlarges and shrinks the wave screwing up the frequency..

i shorted the middle terminal of the pot to one of its other legs or am i missing something?

Sep 17, 2013
5,800
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7. ### MrAl Well-Known Member

Jun 17, 2014
2,433
490
Hi,

Just a quick note...

Is that circuit schematic complete? It looks like the second 9v battery has no connection to ground. Is it missing a ground?

8. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
You'll need to add a variable gain stage.

Feb 19, 2010
3,392
497
Pot for R7?

10. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
Unfortunately, that changes the frequency and amplitude. I think the OP wants independent control over the amplitude.

11. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,392
497
Yes. I saw Alec_t suggest it. I was repeating what they said.

About that R7. It is connected to op amp. Is that the amplifier op amp or is it something else? It sounded like one of the functions of that op amp was to amplify the signal, but like OP said, it was changing the signal. Would that mean that OP needs a buffer between amplifier and the rest of the circuit?

12. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
Its wired as a comparator. It just goes from the + limit to the - limit. Changing R7 changes the hysteresis, the switching point, but that won't change the amplitude because its already at the limits.

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13. ### Mr_Edward Thread Starter New Member

Jul 17, 2014
11
0
the circuit works... and the 2nd 9v is connected to a 2nd ground to the right.. I still haven't cleaned this circuit yet so it looks like a jumble of wires..

14. ### Mr_Edward Thread Starter New Member

Jul 17, 2014
11
0
i'm thinking of changing the comparator with a RS latch then connecting the Q' terminal to the diode switches.. a friend of mine suggested it but then again the problem lies on changing the amplitude..

can a non-inverting amplifier be connected after the unity gain op amp?

15. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,503
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Mr Ed,
I would suggest a non inverting OPA, with a gain control, be used to vary the amplitude of the circuits output.

E

16. ### Alec_t AAC Fanatic!

Sep 17, 2013
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Another hint: U2 doesn't have to be a unity-gain amp.

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17. ### ericgibbs AAC Fanatic!

Jan 29, 2010
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hi,
This is one option, the output OPA could also be used to give some Offset.

E

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18. ### Mr_Edward Thread Starter New Member

Jul 17, 2014
11
0
i haven't tried that configuration yet..thank you very much.

can an integrator be connected to produce a sine wave? or do i have to build another circuit like colpitt's or the like?

what did you use for your simulation btw?

19. ### Alec_t AAC Fanatic!

Sep 17, 2013
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An integrator will give only an approximation of a sine wave.

Jan 29, 2010
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