despap wrote on Jan 30^th, 2012, 10:48am:


A single inverter with its output connected
to its input, in a CMOS technology, is the equivalent
of a diode connected PMOS in series with a diode
connected NMOS connected between power and ground.

Go look at the schematic, at the transistor level.

@Raj..

No.. even no of inverters in a ring never oscillate.

Please spend time to understand why/how ring oscillators oscillate.. its a large-signal phenomenon. BH is not valid once the signal here reaches rail-to-rail.

Thanks!!

I like the thought experiment, raj.

I doubt that oscillation (with long term stability) is possible with even order. BH does apply in case you are exactly in the metastable point, but the reaction to a step response will always be latching.

There is one thing I would like to share though: suppose you have a large chain of inverters. If you create a pulse somewhere in the chain, that is just long enough to flip the inverter, then you will see that your pulse will live on for some periods.

I wonder if you would have a chain (of even inverters) that is very long, would the oscillation last very long (or maybe indefinitely) if you kickstart it properly?

I simulated it in some generic 018 technology, where from 1 pulse, several pulse could be made. Here's the schematic + simulation results. For the non believers, try it yourself =)

http://imageshack.us/photo/my-images/580/forumrt.png/ (for the large picture)

hello lex,
seems you understood problem clearly. What you are saying is apply pulse at some node for the duration more than one stage delay and it would run along the chain for some time. did i understand correctly? Infact if we can provide the same delay as odd number of inverters oscillations should sustain..what do you say?

@despap: man i don't why you always say 1 come's to this inverter and it becomes zero and it go's to another point so on, but why you are not considering phase or delay? is there any special reason?

Thanks,
raj.

@Raj
In the link you provided.
Please take a closer look and there are controlled loops with three-inverters.

Quadrature delay is obtained by controlling the oscillations with three-inverter loop chain.

Correct me if i am worng.

hello despap,
thanks for your help, may be i have to read a lot about this.

Thanks,
raj.

Raj - I did some digging. Referring to here: http://www.ti.com/sc/docs/apps/msp/journal/aug2000/aug_07.pdf

Quote:


Now I believe that with an ring oscillator the circuit will start to oscillate where the phase is 180 degrees if the gain is more than one. However, as the amplitude of the signal grows the feedback will become MORE NEGATIVE (i.e. phase will decrease). Think about it: if the output is at one rail the feedback will cause it to move away from the rail so the feedback must be negative when the signal is large.

On the other hand, think about a signal whose frequency is at the 180 degree point of an even-numbers ring of inverters. The output will tend to grow, but the phase will become MORE POSITIVE feedback as the signal becomes larger. Thus the signal will keep increasing until it hits the rail. Again, think about the case where the output is at the rail; the feedback is positive at that point.

So positive feedback is needed for an oscillator, but the feedback must become less positive as the signal gets larger. This is not true with an even-numbered inverter chain. In fact, the feedback becomes more positive as the signal gets larger so it will grow without bounds.

That's my story at the moment anyway,  :)
rg