# Sine wave generator

Discussion in 'Misc Electronics' started by Noonie, Nov 25, 2003.

1. ### NoonieGuest

Hi,
I am doing a project on a sine wave generator circuit. problem: when I
simulated it, it gave me triangular wave instead of sine!!!
analysing how it works?
The circuit schematic is on the following site:

2. ### mikemGuest

In the real circuit, the amplitude is stabilized (and distortion is
minimized) by the non-linear resistance of the lamp. As the voltage
across the lamp increases, the lamp's resistance increases, thereby
reducing the amount of feedback, thereby reducing the loop gain such
that the oscillator is just at the verge of dropping out of oscillation,
where the output looks truly sinosoidal. If there is too much feedback
(or the loop gain >1.000), then the distortion gets so bad that the wave
looks triangular or even square.

The nominal resistance of the lamp is 12/0.04 or 300 Ohms. If the
filament is not showing any color (cold), then the resistance is much
lower, like 100 to 150Ohms. I simulated this circuit using LTspice.
In my circuit, the lamp is myR7 (150 Ohms). I only had a model for
2N3904, so I used them instead of BC238.

By playing with the values of yourR5(myR8) and yourR6(myR6), I was able
to get everything from no oscillation at all to a low-distortion sine
wave to trapezoids to square waves. The collector of Q1 consitantly has
a lower distorion output than the emitter of Q2...

I'll leave it as an exercise for someone to come up with a model of the
non-linear resistance of the Lamp to improve the attached simulation.

MikeM

LTspice circuit "oscs.asc" follows:

__________________________________________________________________________-

Version 4
SHEET 1 880 680
WIRE 16 -48 16 -16
WIRE 16 144 16 208
WIRE 16 336 16 368
WIRE 16 368 160 368
WIRE 416 368 416 352
WIRE 224 384 224 368
WIRE 224 368 416 368
WIRE 496 288 496 368
WIRE 496 368 416 368
WIRE 160 320 160 368
WIRE 160 368 224 368
WIRE 160 240 160 192
WIRE 160 128 160 96
WIRE 160 96 80 96
WIRE 256 96 160 96
WIRE 352 160 368 160
WIRE 368 160 368 96
WIRE 496 96 496 32
WIRE 496 208 496 96
WIRE 416 272 416 256
WIRE 416 160 416 96
WIRE 416 96 496 96
WIRE 16 208 96 208
WIRE 16 208 16 256
WIRE 224 208 224 256
WIRE 224 256 288 256
WIRE 368 256 416 256
WIRE 416 256 416 240
WIRE 496 -192 496 -64
WIRE -160 176 -160 128
WIRE -160 -192 16 -192
WIRE 16 -16 16 48
WIRE 16 -192 496 -192
WIRE 432 -16 176 -16
WIRE 272 48 256 48
WIRE 256 48 256 96
WIRE 368 48 352 48
WIRE 288 160 256 160
WIRE 256 160 256 96
WIRE 368 96 416 96
WIRE 368 96 368 48
WIRE 544 96 496 96
WIRE 176 -16 16 -16
WIRE 96 208 224 208
WIRE 16 -128 16 -192
WIRE -160 48 -160 -192
FLAG 224 384 0
FLAG -160 176 0
FLAG 544 96 out
FLAG 160 96 B
FLAG 176 -16 C
FLAG 96 208 E
SYMBOL res 0 -144 R0
SYMATTR InstName R1
SYMATTR Value 5.6K
SYMBOL res 0 240 R0
SYMATTR InstName R2
SYMATTR Value 1.8K
SYMBOL res 144 224 R0
SYMATTR InstName R3
SYMATTR Value 15k
SYMBOL cap 144 128 R0
SYMATTR InstName C1
SYMATTR Value 10n
SYMBOL npn 80 48 M0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL npn 432 -64 R0
SYMATTR InstName Q2
SYMATTR Value 2N3904
SYMBOL res 368 32 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R4
SYMATTR Value 15k
SYMBOL cap 352 144 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C2
SYMATTR Value 10n
SYMBOL res 480 192 R0
SYMATTR InstName R5
SYMATTR Value 500
SYMBOL res 400 256 R0
SYMATTR InstName R6
SYMATTR Value 450
SYMBOL res 400 144 R0
SYMATTR InstName R7
SYMATTR Value 150
SYMBOL res 384 240 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R8
SYMATTR Value 0.1
SYMBOL Misc\\battery -160 32 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 9
TEXT 182 -110 Left 0 !.tran 0 0.06 50m

3. ### Watson A.Name - Watt Sun, Dark RemoverGuest

This is _supposed_ to be a simplified form of a Wien Bridge
Oscillator. Here's what I see. The C1, C2 and R3, R4 are in the
_negative_ feedback loop; they're supposed to be in the positive
feedback loop. The lamp is in the _positive_ feedback loop; it's
supposed to be in the negative feedback loop. So I don't see why the
circuit should work at all. It looks like it's been _too_ simplified!
Long ago I had serious doubts, so I cobbled one together, and it
didn't work for me at all.

--
@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/electronics/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@

4. ### mikemGuest

Nope, the loop gain is reduced as the lamp resistance increases, meaning
that the lamp limits the amplitude of oscillation, and it keeps the
output relatively distorion free. This means that the circuit is a true
Wien Bridge Oscillator.

I further simplified the circuit! In my simulation, I opened the feed
back loop between the emitter of Q2 and R4. I injected a sine wave and
swept it from 10 to 10KHz. The frequency where the phase shift goes to
zero is near 1Khz (which is where it normally oscillates). As the lamp
resistance is varied from 100 to 200 Ohms, the Gain varies from 0.9
(wont oscillate) to about 1.2 (oscillates with distortion); The critical
value for oscillation with no distortion is ~190 Ohms.