Transcript Transformer Based Oscillators Adesh Garg

Transformer Based
Oscillators
Adesh Garg
Outline

Why use a Transformer instead of an Inductor?
 What is a Monolithic Transformer?
 Physical Layout
 Equivalent Model
 Electrical Characteristics

Application of Transformers to Oscillator
 Ring Oscillator
 LC Tank Oscillator

References
Transformers Replacing Inductors

Inductors tend to have...
 Low quality factor (Q)
 Highly
Lossy
 Lack Tunability
 Large amounts of Chip area

Transformers have been used to correct the
above short coming [1-5]
What is a Monolithic Transformer?

Physical Layout
What is a Monolithic Transformer?

Equivalent Model [1]
What is a Monolithic Transformer?

Electrical Characteristics
 The coupling between stages for typical process
is usually between 0.75 < Km < 0.9 [1]
 Largely
dependant on width and spacing of the metal
traces
 Depends on layout and structure. Different
benefits for different architecture [1][2]
 Coupling,
Resonance, Loses, Symmetry
Background: Types of Oscillators

Ring Oscillator
 Uses the delay between stages to determine
Oscillator’s frequency (either transmission lines
or lumped delays).

LC Tank Oscillator
 Uses Inductor and Capacitor to determine
Oscillator’s frequency
Ring Oscillator [3]

Ring Oscillator
 Inductively Tuned
LC Tank VCO [4]


Transformer used to change impedance of
resonator tank.
Improves Q factor for equivalent inductor of
similar size
 Phase Noise improvement
LC Tank VCO [4] cont…
Conclusion


Transformers are relatively new and their
application to tuned circuits have only begin
to be explored
I expect to see continued research in this
field and the use of transformers for RF
circuit like the oscillator
References





[1] John Long, “Monolithic transformers for silicon RF IC design," IEEE J. of Solid State
Cir., vol. 35, no. 9, pp. 1368-1382, September 2000.
[2] Alireza Zolfaghari, Andrew Chan, and Behzad Razavi, “Stacked inductors and
transformers in cmos technology," IEEE J. of Solid State Cir., vol. 36, no. 4, pp. 620-628,
April 2001.
[3] John Long and Raymond Koo, “An inductively-tuned quadrature oscillator with
extended frequency control range," in Proceedings of the 2003 International Symposium
on Circuits and Systems, 2003, vol. 1, pp. 25-28.
[4] Matt Straayer, Jose Cabanillas, and Gabriel Rebeiz, “A low-noise transformer-based
1.7ghz cmos vco," in IEEE International Solid-State Circuits Conference Proceedings,
Feb 2002, 17.1.
[5] Bogdan Georgescu, Holly Pekau, James Haslett, and John McRory, \Tunable coupled
inductor q-enhancement for parallel resonant LC tanks," IEEE Transactions on Circuits
and Systems-II: Analog and Digital Signal Processing, vol. 50, no. 10, pp. 705-713,
October 2003.
References cont…


John Long and David Cassan, A 1-v transformer-feedback low-noise amplifier for 5-ghz
wireless LAN in 0.18um CMOS," IEEE J. of Solid State Cir., vol. 38, no. 3, pp. 427-435,
March 2003
John W. M. Rogers and Calvin Plett, “A 5-ghz radio front-end with automatically Q-tuned
notch filter and vco," IEEE J. of Solid State Cir., vol. 38, no. 9, pp. 1547-1554, September
2003.