INTRODUCTION TO ELECTRONICS EHB 222E MOS Field Effect Transistors (MOSFETS II) MOSFETS 1/ INTRODUCTION TO ELECTRONICS 1
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
Biasing the MOSFET Saturation AC signal is superimposed on DC: Small-signal voltage gain: Bias point (this is an inverting amplifier) 3
Biasing the MOSFET 4
Exercise 1 a) At saturation (preferred mode for amplification) 5
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: 0.4-0.2 = 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
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
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
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
Small-signal operation 10
Small-signal Voltage gain: Small-signal assumption AC (signal) component 11
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
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
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
Exercise 2 15
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
Exercise 2 continued.. Small-signal equivalent -> capacitors are short Small-signal parameters, found using DC values (formulas drived previously) 17
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
Exercise 2 continued.. Small-signal equivalent -> capacitors are short 1) Input resistance 2) Input swing: transistor has to stay in SATURATION 19
T-equivalent small-signal model(s) 20
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
MOSFET Amplifiers COMMON-SOURCE COMMON-GATE COMMON-DRAIN 22
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
Common-source amplifier with source resistance Resistance connected to the source T model preferred (Voltage divider) G v = A v since R in = 24
Common-gate amplifier (voltage divider) (same as CS, but non-inverting) 25
Common-drain / source follower amplifier (voltage buffer) (v i = 0, exclude R L ) 26
Summary & Comparison 27
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
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
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
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