Positive Feedback

[Andrew Mawson]

For a system to oscillate you need positive feedback and a bit of phase shift.

Our dogs oscillate ! The new 16 week old puppy 'hears something' makes a very gentle 'almost bark' but really a growl, big sister responds with a proper bark, to which puppy responds thinking 'well there MUST be something out there'. So big sister, now hearing a proper bark responds again, and the pair go off into a crescendo of woofing and barking which can be very amusing, but not when you're on the 'phone. Anyone else have this ?

Andrew

ps lucky the nearest neighbour is half a mile away :)

[JimK]

Must be someone else prowling then......:-)

Jim K

[Bill Wright]

On a larger scale you can wake a whole district. When camping in a very quiet and sparsely populated valley dale or glen, I have sometimes let the dogs accompany me outside for a piss in the middle of the night. A butterfly flaps its wings in a nearby bush and the dogs start up. Half a mile down the valley the outdoor farm dogs take up the cry, and you can sometimes hear it relayed right down to the town, where further racket probably ensues.

Bill

[Davey]

Maybe that Rhea on the loose is in your area?

[Brian Gaff]

It can also happen with just on(possibly slightly dim)dog and his or her own echo from houses across the field from us. grin. Brian

[Harry Bloomfield]

It happens that Andrew Mawson formulated :

Yes, we have an 18month old BC, and her 9 year old big brother. She seems to deliberately triggers him, just for the hell of it.

[The Natural Philosopher]

negative feedback and enough phase shift/delay works better. positive feedback just leads in the end to going to infinity...

[meow2222]

NT

[Graham Nye]

As specified by the _Bark_hausen stability criterion:

[Tim Streater]

Yapping bloody dogs, what's the point of them.

[The Natural Philosopher]

Burglars dont like em, and they bark at anything unusual.

[Andrew Gabriel]

I think that's simply amplification feedback, rather than oscilation.

In many countries, it's common for office workers to bring their dogs to work every day. In the Sun offices in California, lots of the offices had a dog curled up under the desk, and normally so quiet and well behaved, you never noticed it. Once in a while, one would start barking, and in a few seconds, the whole office block was barking like mad. It was very difficult to stop once you have several hundred dogs barking.

[bert]

Either something hits a limit - or the whole thing explodes.

[bert]

In message , Andrew Gabriel writes

Bit like PMQs as John Bercow would tell you.

[Cursitor Doom]

Not necesshairily. You gotta have some form of AGC. Remember the Wein bridge oscillators? They used a tiny light bulb for the purpose IIRC. Its resistance changed with the temperature it reached. Nifty.

[meow2222]

You need pfb at the resonant frequency, but no pfb at dc.

NT

[Andrew Mawson]

Still got a Wein Bridge oscillator that I built when I was 19 - 44 years ago. The standard design used an R53 thermistor for AGC

Andrew

[Huge]

Is this why feedback in PA amps is sometimes called "howl round"?

[The Natural Philosopher]

Er that is negative feedback.

And a wein bridge oscillator doesn't use 'positive' feedback, just negative shifted by 180 degrees at the correct frequency of oscillation.

It will still oscillate at the right freq without the bulb, but the output will be approximately a square wave.

The trick the bulb is to trim the gain to exactly one, so that only the sinewave is just enough gain to cause oscillations at all.

[The Natural Philosopher]

You need to understand that pfb at the resonant frequency is nfb, delayed by half a cycle.