ANALOGUE ELECTRONICS I EKT 204 Basic FET Amplifier

Graphical Analysis, Load Lines Small-signal Parameters MOSFET AMPLIFIER Graphical Analysis, Load Lines Small-signal Parameters

NMOS Common Source circuit DC equivalent

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

gm0 is the gm value when VGS = 0 MOSFET Parameters Transconductance, gm gm0 is the gm value when VGS = 0 gm also known as gFS : forward transconductance

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

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 RD vi vo vgs id AC equivalent circuit

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

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

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

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

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

Common-Source Amplifier Common-Drain Amplifier Common-Gate Amplifier 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

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

COMMON-SOURCE AMPLIFIER - with Source Resistor vo RL RS RD R1 R2 C1 C2 vi +VDD 2 k 12V 3 k RSi 300 k 200 k 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

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

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

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

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

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

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

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

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

JFET AMPLIFIER JFET Parameters DC Analysis Common-Source Amplifier Common-Drain Amplifier

JFET Parameters Transconductance, gm Small-signal output resistor, ro Magnitude only i.e remove the –ve sign Vp= VGS(off)

JFET Amplifier: DC Analysis

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

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

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

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

Small-signal equivalent circuit Another form of small-signal equivalent circuit small-signal voltage gain