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 < 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

7. 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

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 , 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 , 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
[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 , 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 , 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 , September 2003.