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ECE 442 Solid-State Devices & Circuits 12. Frequency Response of CS Amplifiers

ECE 442 Solid-State Devices & Circuits 12. Frequency Response of CS Amplifiers. Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois jschutt@emlab.uiuc.edu. MOSFET - Gate Capacitance Effect. Triode region :. Saturation region :. Cutoff :.

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ECE 442 Solid-State Devices & Circuits 12. Frequency Response of CS Amplifiers

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  1. ECE 442 Solid-State Devices & Circuits 12. Frequency Response of CS Amplifiers Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois jschutt@emlab.uiuc.edu

  2. MOSFET - Gate Capacitance Effect Trioderegion: Saturation region: Cutoff:

  3. MOSFET – Junction Capacitances Overlap capacitance (gate-to-source):

  4. MOSFET High-Frequency Model

  5. CS - Three Frequency Bands

  6. Unity-Gain Frequency fT fT is defined as the frequency at which the short-circuit current gain of the common source configuration becomes unity Define: (neglect sCgdVgs since Cgdis small)

  7. Calculating fT For s=jw, magnitude of current gain becomes unity at fT~ 100 MHz for 5-mm CMOS, fT~ several GHz for 0.13mm CMOS

  8. CS - High-Frequency Response

  9. CS - High-Frequency Response

  10. CS - High-Frequency Response

  11. CS – Miller Effect Define Ceq such that

  12. CS – Miller Effect fo is the corner frequency of the STC circuit

  13. CS – Miller Effect

  14. Example Rsig = 100 kW, RG=4.7 MW, RD =15 kW, gm=1mA/V, rds=150 kW, RL=10 kW, Cgs=1 pF and Cgd=0.4 pF

  15. Example (cont’) Upper 3 dB frequency is at:

  16. BJT Capacitances Base: Diffusion Capacitance: Cde (small signal) where Qn is minority carrier charge in base where tF is the forward transit time (time spent crossing base)

  17. BJT Capacitances Base-emitter junction capacitance: Cjeo is Cje at 0 V. Voe is EBJ built in voltage ~ 0.9 V

  18. BJT Capacitances In hybrid pi model, Cde+Cje=Cp Collector-base junction capacitance Cmois Cm at 0 V. Voc is CBJ built in voltage ~ 0.9 V Cp is around a few tens of pF Cm is around a few pF

  19. High-Frequency Hybrid-p Model

  20. CE - Three Frequency Bands

  21. CS – Miller Effect – Exact Analysis

  22. CS – Miller Effect – Exact Analysis

  23. CS – Miller Effect – Exact Analysis We neglect the terms in s2since Miller If we multiply through by

  24. CS – Miller Effect – Exact Analysis From which we extract the 3-dB frequency point

  25. CS – Miller Effect – Exact Analysis If Gg is negligible IfRi =0

  26. Example For the discrete common-source MOSFET amplifier shown, the transistor has VT= 1V, mCox(W/L) = 0.25 mA/V2, l = 0, Cgs= 3 pF, Cgd= 2.7 pF and VA= 20 V. Assume that the coupling capacitors are short circuits at midband and high frequencies. • Find the 3dB bandwidth if Ri=0 • (b) Find the 3dB bandwidth if Ri= 10 kW

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