INTRODUCTION TO ELECTRONICS EHB 222E

Transcription

1 INTRODUCTION TO ELECTRONICS EHB 222E MOS Field Effect Transistors (MOSFETS II) MOSFETS 1/ INTRODUCTION TO ELECTRONICS 1

2 MOSFETS Amplifiers Cut off when v GS < V t v DS decreases starting point A, once transistor turns ON Initially v DS is high -> SATURATION: highest slope As v DS drops to < v GS V t -> TRIODE AT SATURATION: Coordinates of point B: (replace v DS with v GS - V t ) -> solve for v GS 2

3 Biasing the MOSFET Saturation AC signal is superimposed on DC: Small-signal voltage gain: Bias point (this is an inverting amplifier) 3

4 Biasing the MOSFET 4

5 Exercise 1 a) At saturation (preferred mode for amplification) 5

6 Exercise 1 continued b) Maximum allowable swing: v DS = 0.4 V (found in part 1) Should be > V t (0.2V) for SAT 1.8 V Max negative swing: = 0.2V Max symmetrical swing: ± 0.2 V V DS =0.4 V Indeed, v DS can go much higher in the positive side (up till 1.8 V) 6

7 Exercise 1 continued alternative way to find input swing: MOSFET is in SAT (at negative peak) when: When v DS is maximum -> v GS is minimum (inverting amplifier) avg v DS peak v DS > mean v GS + peak v GS - V t 7

8 Locating the bias (Q) point Q1: does not have sufficient room for positive swing (too close to V DD ) Q2: does not have sufficient room for negative swing (too close to triode/sat boundary) 8

9 Small-signal operation At saturation distortion I D (DC term) AC output: minimize distortion linearly proportional to input (v gs ) For negligible distortion: (neglect last term above) MOSFET transconductance 9

10 Small-signal operation 10

11 Small-signal Voltage gain: Small-signal assumption AC (signal) component 11

12 Small-signal Voltage gain: -> Negligible distortion at output v DSmax < V DD to avoid cut-off v DSmax > v GSmin - V t to avoid triode 12

13 Small-signal equivalent models With channel-length modulation MOSFET behaves like a voltage controlled current source gm v gs current (i d ) passes through R D 13

14 Small-signal equivalent models transconductance (saturation) Isolate V OV in 2 nd equation and replace into 1st (alternative expression for g m ) Transconductance is proportional to square root of drain current Transconductance is proportional to square root of W/L Isolate k n (alternative expression for g m ) 14

15 Exercise 2 15

16 Exercise 2 continued.. First determine DC operating point -> Capacitors act as open circuit -> No current passes through R G V DS = V GS -> SATURATION For simplicity, channel-modulation is neglected for DC operating point Use V GS that is found from 1 st equation 16

17 Exercise 2 continued.. Small-signal equivalent -> capacitors are short Small-signal parameters, found using DC values (formulas drived previously) 17

18 Exercise 2 continued.. Small-signal equivalent -> capacitors are short Voltage gain Insert i i equation -> into v o Since R G is very large (10 MΩ) 18

19 Exercise 2 continued.. Small-signal equivalent -> capacitors are short 1) Input resistance 2) Input swing: transistor has to stay in SATURATION 19

20 T-equivalent small-signal model(s) 20

21 Exercise 3 Find input resistance (R in ) and voltage gain (A v = v o /v i ) Small-signal equivalent (DC current source: open) Common-gate amplifier! 21

22 MOSFET Amplifiers COMMON-SOURCE COMMON-GATE COMMON-DRAIN 22

23 Common-source amplifier (most widely used gain stage) Small-signal When v i is set to 0 Open-circuit voltage gain (v o /v i = v o /v GS ) Neglecting channel-length modulation Overall voltage gain including R L 23

24 Common-source amplifier with source resistance Resistance connected to the source T model preferred (Voltage divider) G v = A v since R in = 24

25 Common-gate amplifier (voltage divider) (same as CS, but non-inverting) 25

26 Common-drain / source follower amplifier (voltage buffer) (v i = 0, exclude R L ) 26

27 Summary & Comparison 27

28 Summary & Comparison CS configuration is best suited as the gain stage One can tune CS parameters through replacing R s CG amplifier serves well as a high frequency amplifier Source follower acts as a voltage buffer -> connects high resistance source to low resistance load -> useful as the output stage of a multistage amplifier 28

29 Biasing MOS amplifiers Fixing gate voltage (V G ) For a fixed V G -> If I D increases (for any reason) -> V GS decrases (V G is fixed) -> decreases I D (remember I D depends on V GS ) -> Therefore, R s acts as a (negative) feedback resistance, maintains I D at a constant level. PRACTICAL IMPLEMENTATIONS 29

30 Biasing MOS amplifiers Using Drain-Gate feedback resistor Large R G -> no current through R G Identical to: from previous slide -> R D serves as a (negative) feedback resistance, keeping I D constant 30

31 Biasing MOS amplifiers Constant current source Current-source implementation Drain and Gate of Q1 are connected -> SAT Q 2 shares V GS with Q1 -> Q2 assume SAT (current mirror) 31