Lecture 17: Common Source/Gate/Drain Amplifiers

1. Lecture 17:Common Source/Gate/Drain Amplifiers Prof. Niknejad

2. Lecture Outline • MOS Common Source Amp • Current Source Active Load • Common Gate Amp • Common Drain Amp University of California, Berkeley

3. Common-Source Amplifier Isolate DC level University of California, Berkeley

4. Load-Line Analysis to find Q Q University of California, Berkeley

5. Small-Signal Analysis University of California, Berkeley

6. Two-Port Parameters: Generic Transconductance Amp Find Rin, Rout, Gm University of California, Berkeley

7. Two-Port CS Model Reattach source and load one-ports: University of California, Berkeley

8. Maximize Gain of CS Amp • Increase the gm (more current) • Increase RD (free? Don’t need to dissipate extra power) • Limit: Must keep the device in saturation • For a fixed current, the load resistor can only be chosen so large • To have good swing we’d also like to avoid getting to close to saturation University of California, Berkeley

9. Current Source Supply • Solution: Use a current source! • Current independent of voltage for ideal source University of California, Berkeley

10. CS Amp with Current Source Supply University of California, Berkeley

11. Load Line for DC Biasing Both the I-source and the transistor are idealized for DC bias analysis University of California, Berkeley

12. Two-Port Parameters From current source supply University of California, Berkeley

13. P-Channel CS Amplifier DC bias: VSG = VDD – VBIAS sets drain current –IDp = ISUP University of California, Berkeley

14. Two-Port Model Parameters Small-signal model for PMOS and for rest of circuit University of California, Berkeley

15. Common Gate Amplifier DC bias: University of California, Berkeley

16. CG as a Current Amplifier: Find Ai University of California, Berkeley

17. CG Input Resistance At input: Output voltage: University of California, Berkeley

18. Approximations… • We have this messy result • But we don’t need that much precision. Let’s start approximating: University of California, Berkeley

19. CG Output Resistance University of California, Berkeley

20. CG Output Resistance Substituting vs= itRS The output resistance is (vt / it)|| roc University of California, Berkeley

21. Approximating the CG Rout The exact result is complicated, so let’s try to make it simpler: Assuming the source resistance is less than ro, University of California, Berkeley

22. CG Two-Port Model Function: a current buffer • Low Input Impedance • High Output Impedance University of California, Berkeley

23. Common-Drain Amplifier Weak IDS dependence University of California, Berkeley

24. CD Voltage Gain Note vgs = vt – vout University of California, Berkeley

25. CD Voltage Gain (Cont.) KCL at source node: Voltage gain (for vSB not zero): University of California, Berkeley

26. CD Output Resistance Sum currents at output (source) node: University of California, Berkeley

27. CD Output Resistance (Cont.) ro || roc is much larger than the inverses of the transconductances  ignore Function: a voltage buffer • High Input Impedance • Low Output Impedance University of California, Berkeley

28. University of California, Berkeley