ANALOGUE ELECTRONICS CIRCUITS 1 EKT 204 Introduction to FET Amplifier: FET (Review)

FIELD EFFECT TRASISTOR (FET) ADVANTAGES OF FET TYPES OF FET & ITS OPERATION

FET Advantages Voltage-controlled amplifier: input impedance very high Low noise output: useful as preamplifiers when noise must be very low because of high gain in following stages Better linearity: distortion minimized Low inter-electrode capacitance: at high frequency, inter-electrode capacitance can make amplifier work poorly. FET desirable in RF stages (high frequency)

Types of FET FET JFET MOSFET MESFET Enhancement mode Depletion mode n channel Enhancement mode p channel Depletion mode

Junction FET (JFET) ohmic contact Structure n-channel p-channel Symbol

Metal-Oxide-Semiconductor MOS (MOSFET) DEPLETION p n p dielectric ENHANCEMENT metal p n p n-channel p-channel

JFET Operation depletion region VDD  VDD VGG Gate-source is reversed-biased  zero current at gate IDS flow through the channel and the value is determined by the width of depletion region and the width of the channel

electron inversion layer MOSFET Operation electron inversion layer G G S D S D SS SS No voltage applied to gate Current is zero +ve voltage applied to gate Electron inversion layer is created Current is generated between source and drain

FET BIASING JFET BIAS CIRCUITS MOSFET BIAS CIRCUITS Self-bias Voltage-divider bias MOSFET BIAS CIRCUITS Drain-feedback bias

Equivalence biasing of JFET & BJT <==> <==> <==>

JFET Bias Circuits - Self-Bias +VDD RD IG = 0 RG RS

JFET Bias Circuits - Voltage-divider Bias +VDD R1 RD ID VG R2 RS

MOSFET Bias Circuits - Voltage-divider Bias +VDD R1 RD R2

MOSFET Bias Circuits - Drain-Feedback Bias +VDD RD RG IG = 0

SELF-BIASED JFET VOLTAGE-DIVIDER BIAS JFET LOAD LINE SELF-BIASED JFET VOLTAGE-DIVIDER BIAS JFET

LOAD LINE - SELF-BIASED JFET +VDD 9V RD 2.2K RS 680 RG 10M Example Determine the Q-point for the JFET circuit. The transfer characteristic curve is given in the figure.

IDSS=4mA; so ID=IDSS=4mA VGS=-IDRS=-(4m)(680)=-2.72V For ID=0, VGS=-IDRS=(0)(680)=0V From the curve, IDSS=4mA; so ID=IDSS=4mA VGS=-IDRS=-(4m)(680)=-2.72V ID (mA) Q point is the intersection between the transfer characteristic curve and the load line 4 IDSS ID=2.25mA VGS=-1.5V Q 2.25 -VGS (V) -6 VGS(off) -2.72 -1.5

LOAD LINE - VOLTAGE-DIVIDER BIAS JFET +VDD 8V Example Determine the Q-point for the JFET circuit. The transfer characteristic curve is given in the figure. R1 2.2M RD 680 R2 2.2M RS 3.3K

For ID=0, For VGS=0, ID (mA) Q point is the intersection between -VGS (V) 12 IDSS -3 VGS(off) 1.8 1.2 -1.8 Q 4 VGS (V) Q point is the intersection between the transfer characteristic curve and the load line ID=1.8mA VGS=-1.8V

EXERCISES (Load Line JFET) 1. Determine the Q-point for the JFET circuit. The transfer characteristic curve is given in the figure. +VDD 6V RD 820 RS 330 RG 10M ID (mA) -VGS (V) IDSS = 5mA VGS(off)=-3.5

EXERCISES (Cont.) 2. Determine the Q-point for the JFET circuit. The transfer characteristic curve is given in the figure. +VDD 12V RD 1.8K R1 3.3M R2 2.2M RS 3.3K ID (mA) IDSS = 5mA -VGS (V) VGS(off)=-4V

FET CHARACTERISTICS JFET MOSFET

JFET CHARACTERISTICS DRAIN CHARACTERISTIC VP=|VGS (off)| Drain characteristics curve or sometimes known as current-voltage characteristics. VP : pinch-off voltage is the level of VDS where IDS becomes almost constant. IDSS : Value of drain-source current that is flowing in the JFET when VGS reaches pinch-off. Ohmic region/triode region : where the channel resistance is virtually constant i.e. ID and VDS follow the principle of Ohm’s law. Pinch-off region/saturation region: JFET acts as voltage-controlled constant-current source. Breakdown region: VDS value is too large - like avalanche in PN diode VP=|VGS (off)|

JFET CHARACTERISTICS TRANSFER CHARACTERISTIC Also known as drain current vs gate-to-source voltage characteristics.

JFET DATA SHEET For MMBF5459 VGS (off) = -8.0V (max) IDSS = 9.0 mA (typ.)

MOSFET CHARACTERISTICS TRANSFER CHARACTERISTIC (Depletion MOSFET) Depletion mode : means that a channel exists even at zero gate voltage. Negative gate voltage must be applied to the n channel depletion MOSFET to turn the device off.

MOSFET CHARACTERISTICS TRANSFER CHARACTERISTIC (Enhancement MOSFET) VTN : threshold voltage of the n-channel MOSFET. Define as the gate voltage required to turn on the transistor. For n-channel : VTN is positive value ‘coz positive gate voltage is required to create the inversion charge. If VG < VTN , the current in device is zero. K in formula can be calculated by substituting data sheet values ID(on) for ID and VGS at which ID(on) is specified for VGS

E-MOSFET DATA SHEET ID(on) = 75 mA (minimum) at VTN = 0.8 V and VGS = 4.5V VGS(th) = VTN