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CURRENT MIRRORS AND THIRD GENERATION CURRENT CONVEYORS

CURRENT MIRRORS AND THIRD GENERATION CURRENT CONVEYORS . MERİH YILDIZ. Introduction Current Mirrors Simple current mirror (SCM) Wilson current mirror (WSM) Improved Wilson current mirror (IMCM) Cascode current mirror (CCM) Improved active feedback cascode current mirror (IAFCCM)

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CURRENT MIRRORS AND THIRD GENERATION CURRENT CONVEYORS

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  1. CURRENT MIRRORS AND THIRD GENERATION CURRENT CONVEYORS MERİH YILDIZ

  2. Introduction • Current Mirrors • Simple current mirror(SCM) • Wilson current mirror(WSM) • Improved Wilson current mirror(IMCM) • Cascode current mirror(CCM) • Improved active feedback cascode current mirror(IAFCCM) • High Swing current mirror(HSCM) • Conclusion and Future work

  3. Introduction • One of the most widely used analog building blocks is the current mirror • Current mirrors are commonly used in • Signal processing • analog circuits ( our target applications are current conveyors use CMs as current references and mirroring devices) So in these applications CM impact both • Noise performance • Frequency response

  4. Understanding the frequency behavior of CMs is important for designing current-mode circuits • Analog signal processing circuits demand • Higher bandwidths • better noise performance • Current Mode techniques are adequate • Parasitic capacitances can be minimized • And their effect in system will be lower than in the case of voltage-mode circuits.[Sedra Smith 1989, Elec.Lett.] • Many references can be found considering CM’s accuracy and output impedance or low-voltage applications • Fewer papers consider the effect of noise and frequency in CMs • Due to increasing usage at higher frequency, six most used CMs configurations are analyzed

  5. Current Mirrors Simple CM Wilson CM Improved Wilson CM Cascode CM IAFCCM High Swing CM

  6. The CMs were chosen regarding several aspects, namely: accuracy, simplicity, low-voltage operation and widespread usage • The configuration used as reference is the simple current mirror (SCM) • When higher accuracy is demanded common circuits resort to Wilson and Cascode CMs. • WCM and IWCM exhibit large output impedance and high frequency response but are unsuitable for low-voltage operation • Cascode CM is more suitable for low-voltage operation and has high output impedance

  7. The frequency behavior of these Six CM’s configurations are compared • This comparison is based on the two-port representation of the CM and comprises the input impedance, the current gain, the output impedance and the reverse voltage gain Analytical Expression CM two port equivalent (with load)

  8. All transistors (except the mirroring devices-transistors M2 in all the CMs) are equal • Transistors M2 have an aspect ratio of a • Major influence over the CM frequency response are • the transconductance of the transistors (gm), • the gate to source capacitance (Cgs) • the output conductance (go) • the configured aspect ratio of transistors M2 (a)

  9. Frequency response of several current mirrors configuration

  10. Maximizing bandwidth in CM relies on three design strategies • Raising the quiescent current • decreasing both W and L • Decreasing a • Special care must be taken if current accuracy is also a design goal because • Decreasing L and raising ID also promotes on the output impedance, thus affecting current accuracy

  11. Load Effect • Connecting a load to the output of a CM affects the frequency response of the system, but not the maximum bandwidth [Luis Nero Alves, Rui L. Aguar,2002] • Typical load impedance consisting of admittance GLin parallel with capacitance CL, introduces in the frequency response one zero and one pole, as expressed

  12. Depending on the magnitude of the GLand CL this zero and pole can appear inside the system bandwidth causing a “kind of step” behavior • Besides this middle frequency, the upper cutoff frequency remains unchanged Frequency response Ai(s) (middle frequencies)

  13. Simulation Results • To test the six proposed CMs we used a reference input current source. • A capacitor CLin parallel with a resistance RLwas used as load • The CM’s output port also tied to a fixed voltage VDC sufficient to grant the saturation condition in all the output transistors • Default value for W/L ratio is 20mm/2mm, except transistors M2 that were designed with ratio a (a=1 as default) • The default current ID was 20mA

  14. Simulation Results are graphically summarized in figures Bandwidth versus W Bandwidth versus L Bandwidth versus ID Bandwidth versus a

  15. Improved Active Feedback Cascode Current Mirror (IAFCCM) • IAFCCM was proposed to • Increase the output voltage swing • Output impedance and matching accuracy gm: Transconductance of the transistorsrds: Output impedance of the transistors IAFCCM

  16. Simulation Results Output and Input Currents Output voltage Swing

  17. Conclusion • Theoretical demonstrations pointed that CMs frequency response is dominated by the relationship between gm, Cgs and go in their transistors • Simulation results showed good agreement with the analysis carried out • Future Work • High performance Third generation current conveyors will be investigated by using these six CMs

  18. THANK YOU Merih YILDIZ Doğuş University-İSTANBUL-TURKEY myildiz@dogus.edu.tr

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