Transformer Based Oscillators

1
Transformer Based Oscillators

• Adesh Garg

2
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

3
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

4
What is a Monolithic Transformer?

• Physical Layout

5
What is a Monolithic Transformer?

• Equivalent Model 1

6
What is a Monolithic Transformer?

• Electrical Characteristics
• The coupling between stages for typical process
  is usually between 0.75 lt Km lt 0.9 1
• Largely dependant on width and spacing of the
  metal traces
• Depends on layout and structure. Different
  benefits for different architecture 12
• Coupling, Resonance, Loses, Symmetry

7
Background Types of Oscillators

• Ring Oscillator
• Uses the delay between stages to determine
  Oscillators frequency (either transmission lines
  or lumped delays).
• LC Tank Oscillator
• Uses Inductor and Capacitor to determine
  Oscillators frequency

8
Ring Oscillator 3

• Ring Oscillator
• Inductively Tuned

9
LC Tank VCO 4

• Transformer used to change impedance of resonator
  tank.
• Improves Q factor for equivalent inductor of
  similar size
• Phase Noise improvement

10
LC Tank VCO 4 cont
11
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

12
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.

13
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.