CAPAcITOR IN RING OSCILLATOR

[amriain]

I need a help about ring oscillator..Actually,what the waveform looks like if we put the capacitor in output of differential ring oscillator???

 

[wpchan05]

I need a help about ring oscillator..Actually,what the waveform looks like if we put the capacitor in output of differential ring oscillator???

there is already a cap at the output. Without capacitor, there is no delay, and the ring oscillator can't oscillate.

the larger the cap, the longer is the delay, and so the oscillation frequency decreases( or the period is larger)

 

[FvM]

Without capacitor, there is no delay, and the ring oscillator can't oscillate.

Only true with ideal transistors, I think. Real world devices have plenty have cpacitance and other kinds of delay without
adding a capcitor. By adding a capacitor to only one ring oscillator stage, you're able to stop oscillation with an unsuitable dimensioning.

Actually,what the waveform looks like if we put the capacitor in output of differential ring oscillator?

What's the waveform before? Why don't you try in a simulation or a real circuit?

 

[amriain]

I've already run the circuit and the waveform that i got is sinusoidal..this waveform without putting the capacitor...But after i put the capacitor and run the circuit, the waveform become linear...is that true???

 

[FvM]

the waveform become linear

What's a linear waveform? How many stages has your ring oscillator?

 

[amriain]

erm linear juz sumthing like straight line...i used 3 stage n the value capacitor that i used 150pf..are u hav any idea??

 

[FvM]

O.K., I think that's the same thing I mentioned earlier: You are able to stop oscillation by making the capacitance too large.
Adding a same value capacitor to at least two or all three stages should re-enable oscillation.

You can understand the reason when you plot the loop gain amplitude and phase of the ring oscillator. Normally, you have
a threefold pole that most likely satisfies the oscillator (Barkhausen) criterion. Adding one larger capacitor creates a
dominant pole, the gain dops below 1 at 180° phase shift.

 

[amriain]

oh really...thankz for your help..