ANALOGUE ELECTRONICS I EKT 204 Basic FET Amplifier

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

NMOS Common Source circuit DC equivalent

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

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

Small-signal Equivalent Circuit VDD i. D +v GS vi RD + v. DS - v. O - vo id vi vgs RD VGSQ NMOS common source circuit with time-varying signal source in series with gate DC source AC equivalent circuit

Small-signal Equivalent Circuit vo id vi vgs RD v. O = v. DS = VDD – i. DRD = VDD – (IDQ + id)RD = (VDD – IDRD) - id. RD Small-signal parts: AC equivalent circuit • vo = vds = -id. RD • id = gmvgs • vi = vgs

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

DC Analysis – Saturation Region VDD R 1 CC VDD i. D RD v. G v. O ID R 1 0 + VG VDS + vi R 2 Voltage-divider circuit for MOSFET RD R 2 - VGS - DC equivalent circuit

DC Analysis – Saturation Region VDD ID R 1 RD 0 + VG VDS + R 2 - VGS - DC equivalent circuit

Example VDD 5 V VTN = 1 V K = 0. 1 30 k R 1 m. A/V 2 CC vi i. D v. G R 2 20 k RD v. O

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

MOSFET AMPLIFIER q q q 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 small-signal voltage gain output voltage

DC Load Line i. D DC load line & transition point Transition point Q-point IDQ VDSQ VDD v. DS

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

Q-point values VDD=12 V R 1=300 k dc equivalent circuit RD=3 k VG R 2=200 k RS=2 k

Q-point values (Cont) For VGS = 0. 54 V, MOSFET cutoff ‘coz VGS<VTN. Therefore, VGS = 2. 96 V So, the Q point values:

Small-signal analysis RSi Vo + Vi R 1||R 2 Vgs _ gm. Vgs RS RD||RL

COMMON-SOURCE AMPLIFIER - with Bypass Capacitor +VDD RD C 2 R 1 RSi vi C 1 RL vo R 2 RS C 3 Minimize the loss in small-signal voltage gain while maintaining the Q-point stability

Small signal equivalent circuit RSi Vo + Vi R 1||R 2 Vgs _ Ri ro gm. Vgs RD||RL

COMMON-DRAIN AMPLIFIER +VDD RSi vi C 1 R 1 C 2 RS RL vo

Small signal equivalent circuit RSi Vi Vin + Vgs _ G Vo S R 1||R 2 ro RS||RL gm. Vgs D input impedance Output impedance input voltage output voltage small-signal voltage gain

COMMON-GATE AMPLIFIER V+ RD RSi vi C 1 C 2 IQ RG V- RL C 3 vo

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