Re: LM358 phase shift osc.
- From: Tim Wescott <tim@xxxxxxxxxxxxxxxx>
- Date: Tue, 27 Dec 2005 08:50:41 -0800
Lee Morris wrote:
I have tried three designs for an op amp phase shift oscillator and can't get any to work. Can anyone please assist?
My present circuit uses an LM 358 as follows.
Pin 4 and 5: to earth Pin 8: 9VDC regulated Pin 7: three 0.1uF low leakage electro's in series to pin 6 through 10K (feedback). Positive leads facing pin 7. Pin 6-7: 220K-330K variable for gain control Remaining pins unconnected and there is no load.
What am I doing wrong? Is it the single supply, polarized caps, frequency too low ... ?
Thank you for any suggestions.
Lee Morris
This?
.----.
| |
_V_ |
.----|___|--o----.
| |
| \] \] \] |
o--|]---|]---|]--o
| /]+ /]+ /]+ |
| |
| |\ |
'----|-\ |
| >--------o--------
.--|+/
| |/
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)That's not a phase-shift oscillator, because there aren't enough independent caps for the phase shift to happen. For a phase shift oscillator you need a schematic like this:
_
| \
| \ out
.----------|-K >-----------------o--------
| | / |
| |_/ |
| |
| |
| ___ ___ ___ |
'--o--|___|--o--|___|--o--|___|--'
| | |
| | |
--- --- ---
--- --- ---
| | |
| | |
=== === ===
GND GND GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)where (IIRC) the amplifier has a negative gain that's greater than 29. One very important thing that's different here is that the capacitors are not independent -- each one "talks" to the other through resistors. This could also be done with a series chain of capacitors _as long as_ you have resistors going to ground at the end of each capacitor -- think of the figure above with the resistors and capacitors swapped.
--
Tim Wescott Wescott Design Services http://www.wescottdesign.com .
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