1. Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Lecture 17: Common Source/Gate/Drain Amplifiers Prof. Niknejad

2. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley Lecture Outline MOS Common Source Amp Current Source Active Load Common Gate Amp Common Drain Amp

3. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley Common-Source Amplifier Isolate DC level

4. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley Load-Line Analysis to find Q Q

5. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley Small-Signal Analysis

6. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley Two-Port Parameters: Find R in, R out, G m Generic Transconductance Amp

7. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley Two-Port CS Model Reattach source and load one-ports:

8. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley Maximize Gain of CS Amp Increase the g m (more current) Increase R D (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

9. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley Current Source Supply Solution: Use a current source! Current independent of voltage for ideal source

10. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley CS Amp with Current Source Supply

11. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley Load Line for DC Biasing Both the I-source and the transistor are idealized for DC bias analysis

12. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley Two-Port Parameters From current source supply

13. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley P-Channel CS Amplifier DC bias: V SG = V DD – V BIAS sets drain current –I Dp = I SUP

14. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley Two-Port Model Parameters Small-signal model for PMOS and for rest of circuit

15. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley Common Gate Amplifier DC bias:

16. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley CG as a Current Amplifier: Find A i

17. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley CG Input Resistance At input: Output voltage:

18. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley Approximations… We have this messy result But we don’t need that much precision. Let’s start approximating:

19. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley CG Output Resistance

20. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley CG Output Resistance Substituting v s = i t R S The output resistance is (v t / i t )|| r oc

21. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley Approximating the CG R out The exact result is complicated, so let’s try to make it simpler: Assuming the source resistance is less than r o,

22. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley CG Two-Port Model Function: a current buffer Low Input Impedance High Output Impedance

23. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley Common-Drain Amplifier Weak I DS dependence

24. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley CD Voltage Gain Note v gs = v t – v out

25. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley CD Voltage Gain (Cont.) KCL at source node: Voltage gain (for v SB not zero):

26. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley CD Output Resistance Sum currents at output (source) node:

27. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley CD Output Resistance (Cont.) r o || r oc is much larger than the inverses of the transconductances  ignore Function: a voltage buffer High Input Impedance Low Output Impedance

28. EECS 105 Fall 2003, Lecture 17Prof. A. Niknejad Department of EECS University of California, Berkeley