why ptat current in ring oscillator?

[nannapaneni]

1) I had designed ring oscillator with 3-stage differential amplifier with resister load. when bias current for differential stage in constant, I observed frequency is changing with temperature. But with PTAT current frequency is constant with tempearture. I didn't undersatnd what was the reason. Can some body help me to understand why ptat current required for constant frequency with temperature, and how frequency of oscillations depends on bias current.

2) what all simulations to be done on rc oscillators for performance check? Till now I did loop gain and phase test in ac and transient analysis with intial node set to 0V. what all I need to check?

pls help me in this.

 

[dick_freebird]

Junction capacitance tends to rise with temperature
and drive strengths drop, so raising available current
can compensate this (with the -right- proportionailty).

 

[nannapaneni]

Yes ur true, temperature depency of mos caps is affecting