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Ring VCO : Gain of amplifier stages

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nandu

nandu

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What happens to the frequency of oscillation of a ring oscillator if I increase the gain of each amplifier stage above the minimum required gain levels? Would the frequency increase or decrease? I would definitely prefer a time-domain explanation.
 

Dear Nandu,
For special stage such as 3 stage ring oscillator, oscillator frequency is decided by open loop 3db bandwidth of each amplifier.
If you don't change open loop 3db bandwidth then its frequency doesn't change.
But how you guarantee this when you change gain?
Is it right?
Regards,

nandu said:
What happens to the frequency of oscillation of a ring oscillator if I increase the gain of each amplifier stage above the minimum required gain levels? Would the frequency increase or decrease? I would definitely prefer a time-domain explanation.
Click to expand...
 

if u changed the gain by changing the w/l then the load capacitance 4 sure will be different and then the oscillation frequency
 

Well say I have some ideal amplifier whose gain I can change without varying its 3dB BW. In that case, as philipwang said, the frequency musn't vary. But
I read somewhere that the frequency is proportional to gain (assuming again BW is constant), and when I simulated this using the afore mentioned ideal amplifier, I found that frequency does increase proportionally with gain! This is something I'm unable to explain
 

I believe that the frequency will not be affected.

What happens is that if the frequency is increased beyond a certain limit, then some of the transistors might enter the non-linear region which by it self, reduces the gain once again. What should happen is that the output stage should have it's maximum swing to provide max. power to the next "block".
 

Yes I guess real ring oscillators do eventually hit some swing saturation. But Im simulating with ideal amplifier stages and I'm unable to explain the direct dependence of frequency on the amplifier gain.
 

actually by my opinion ,you should calculate is the delay＝TpLH（swing up time）＋ TpHL（swing down）,which instead of so－called bandwidth do control the frequency by a large signal manner。
 

I'm unable to explain the direct dependence of frequency on the amplifier gain
Click to expand...

I think they are not directly related
 

if assume you keep each stage's GBW the same,
higher gain means lower bw, therefore, larger
delay, hence lower frequency.
 

But in reality higher gain gives proportionally higher frequency.

This is because an amplifier with higher gain shows a steeper rise at its output than one with lower gain for the same input. This means that the succeeding inverting amplifier gets triggered much earlier than the amplifier with lower gain, and hence the toggling occurs faster.
 

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