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From: piglet <erichpwagner@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: Random thoughts on sinewave oscillators
Date: Sun, 20 Oct 2024 22:08:47 -0000 (UTC)
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KevinJ93 <kevin_es@whitedigs.com> wrote:
> On 10/20/24 2:55 AM, piglet wrote:
>> KevinJ93 <kevin_es@whitedigs.com> wrote:
>>> On 10/19/24 12:57 PM, piglet wrote:
>>>> KevinJ93 <kevin_es@whitedigs.com> wrote:
>>>>> On 10/19/24 12:13 PM, piglet wrote:
>>>>>> On 18/10/2024 4:46 pm, john larkin wrote:
>>>>>>> On Fri, 18 Oct 2024 11:25:19 -0400, "Edward Rawde"
>>>>>>> <invalid@invalid.invalid> wrote:
>>>>>>> 
>>>>>>>> "piglet" <erichpwagner@hotmail.com> wrote in message
>>>>>>>> news:vetde5$38sbk$1@dont-email.me...
>>>>>>>>> Edward Rawde <invalid@invalid.invalid> wrote:
>>>>>>>>>> 
>>>>>>>>>> The circuit below produces a reasonable looking sinewave but the rise
>>>>>>>>>> time still seems to be slower than the fall time. It may be that the
>>>>>>>>>> amplifier in use is not ideal for this.
>>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> Could that just be second harmonic distortion? You could test the
>>>>>>>>> amplifier
>>>>>>>>> by uncoupling the Wien network and injecting test inputs.
>>>>>>>>> 
>>>>>>>>> Elsewhere I think your amplitude control problems could be simply
>>>>>>>>> due to
>>>>>>>>> too much gain.
>>>>>>>> 
>>>>>>>> Perhaps, but I've not so far been able to get the circuit I posted in
>>>>>>>> response to Bill to produce a sine wave no matter what I do
>>>>>>>> with the control loop gain.
>>>>>>>> It either grows to clipping or dies.
>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> -- 
>>>>>>>>> piglet
>>>>>>>> 
>>>>>>> 
>>>>>>> I do a lot of instant-start LC oscillators as the timebase of
>>>>>>> triggered delay generators. I let them clip just a bit to stabilize
>>>>>>> amplitude.
>>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> Here is an amusing oscillator that has a voltage follower as the active
>>>>>> stage - it has no voltage gain so some people say it cannot work - they
>>>>>> are wrong of course.
>>>>>> 
>>>>> <...>
>>>>>> piglet
>>>>>> 
>>>>> 
>>>>> If you move the ground to the emitter of Q3 and slide R8 through the
>>>>> power supply to the collector of Q3 you can see that it is a
>>>>> conventional phase shift oscillator with feedback from the output of an
>>>>> inverting amplifier via a 3-stage RC network.
>>>>> 
>>>>> kw
>>>>> 
>>>>> 
>>>> 
>>>> What do you do with the cold end of C1 in that scenario?
>>>> 
>>>> 
>>> 
>>> That connects to the collector, as it is now and forms the first section
>>> of the RC network. (voltage sources are zero impedance from the point of
>>> view of AC). C4 is a coupling capacitor - its value and the high
>>> impedance of R4,R5 and Q1 mean it has little effect on the phase shift.
>>> 
>> 
>> It still works if I replace the transistors with an op amp non inverting
>> follower - no inversion in sight. How would you re-describe that?
>> 
> 
> Exactly the same argument applies - any current coming out of the opamp 
> must come from the power supplies and so goes through R8. The gain to 
> the PS pin can be much more than unity.
> 
> Some circuit designs exploit this feature - I think I have seen John 
> Larkin describing one. There are also designs in The Art of Electronics 
> using it.
> 
> 
> 

Bingo! Yes that’s it


-- 
piglet