DMT 231 / 3 ELECTRONICS II

DMT 231 / 3 ELECTRONICS II Lecture II Basic Field Effect Transistors Amplifiers

MOSFET AMPLIFIER Graphical Analysis, Load Lines Small-signal Parameters

Graphical Analysis, Load Lines IDQ VDSQ Common-source transistor characteristic

MOSFET Parameters • Transconductance, gm gm0 is the gm value when VGS = 0

VDD vo id iD RD RD vO vgs + vi vDS + vGS - - vi VGSQ Small-signal Equivalent Circuit NMOS common source circuit with time-varying signal source in series with gate DC source AC equivalent circuit

vo id RD vgs vi Small-signal Equivalent Circuit • vO = vDS = VDD – iDRD • = VDD – (IDQ + id)RD • = (VDD – IDRD) - idRD • Small-signal parts: • vo = vds = -idRD • id = gmvgs • vi = vgs AC equivalent circuit

ids D + G + ids vgs gmvgs ro + vds vgs - - - S Small-signal Equivalent Circuit Common source NMOS with small signal parameters Simplified small signal equivalent circuit for NMOS Small-signal output resistance where

VDD VDD ID RD RD R1 R1 iD vO 0 + CC vG VG VDS - + VGS R2 - vi R2 DC Analysis – Saturation Region Voltage-divider circuit for MOSFET DC equivalent circuit

VDD ID RD R1 0 + VG VDS - + VGS R2 - DC Analysis– Saturation Region DC equivalent circuit

VDD 5 V 20 k 30 k RD R1 iD vO CC vG vi R2 20 k Example VTN = 1 V K = 0.1 mA/V2

VDD RD R1 ID + 0 VG VDS + - VGS R2 - DC equivalent circuit The transistor is biased in the saturation region because VDS > VDS(sat) = VGS – VTN = 2 – 1 = 1 V

MOSFET AMPLIFIER Common-Source Amplifier Common-Drain Amplifier Common-Gate Amplifier

COMMON-SOURCE AMPLIFIER- Basic Configuration Common-source circuit with voltage divider biasing & coupling capacitor

Small-signal equivalent circuit input resistance input voltage output voltage small-signal voltage gain

iD Transition point Q-point IDQ vDS VDSQ VDD DC Load Line DC load line & transition point

+VDD 12V RD 3 k C2 R1 vo 300 k C1 RSi RL 2 k 3 k R2 200 k vi RS 2 k COMMON-SOURCE AMPLIFIER - with Source Resistor CALCULATION EXAMPLE Given: VTN = 2V, K = 1 mA/V2,  = 0 Determine: i- Q-point values (ID , VDS) ii- small-signal voltage gain Stabilize the Q-point against variation of transistor parameters

VDD=12V RD=3 k R1=300 k VG R2=200 k RS=2 k Q-point values dc equivalent circuit

Q-point values (Cont) For VGS = 0.54 V, MOSFET  cutoff ‘coz VGS<VTN . Therefore, VGS = 2.96V So, the Q point values:

RSi Vo + Vi R1||R2 RD||RL Vgs gmVgs _ RS Small-signal analysis

+VDD RD C2 R1 C1 RSi RL vo R2 vi C3 RS COMMON-SOURCE AMPLIFIER - with Bypass Capacitor Minimize the lost in small-signal voltage gain while maintaining the Q-point stability

RSi Vo + Vi R1||R2 Vgs ro RD||RL gmVgs _ Ri Small signal equivalent circuit

+VDD R1 C1 RSi C2 vi R2 RS RL vo COMMON-DRAIN AMPLIFIER

RSi Vin _ + Vgs Vo Vi ro RS||RL R1||R2 gmVgs Small signal equivalent circuit input impedance Output impedance input voltage output voltage small-signal voltage gain

V+ RD RSi C1 C2 IQ vi RL vo RG C3 V - COMMON-GATE AMPLIFIER

Ri gmVgs RSi Vo _ Vi Ii Vgs RD RL + Small-signal equivalent circuit input impedance output voltage input voltage small-signal voltage gain input current

JFET AMPLIFIER Common-Source Amplifier Common-Drain Amplifier

VDD R1 RD C2 vo C1 RL vi R2 RS CS COMMON-SOURCE CIRCUIT

Vo + R1||R2 Vi Vgs ro RD||RL gmVgs - Small-signal equivalent circuit input voltage output voltage small-signal voltage gain

V+ C1 C2 vo vi RG RS RL V- COMMON-DRAIN CIRCUIT

gmVgs ro Vgs Vo - + Vi RG RS RL Small-signal equivalent circuit input voltage output voltage small-signal voltage gain

DMT 231 / 3 ELECTRONICS II