JFET and MOSFET Amplifiers UNIT III JFET and MOSFET Amplifiers

OUTLINE Small signal Analysis of JFET amplifiers Small signal Analysis of MOSFET and JFET BICMOS cascode Amplifier

FET ( Field Effect Transistor) Unipolar device i. e. operation depends on only one type of charge carriers. Voltage controlled Device (gate voltage controls drain current). Very high input impedance (109-1012 ). Source and drain are interchangeable in most Low-frequency applications. Low-power consumption. Less Noisy as Compared to BJT. Very small in size, occupies very small space in Ics.

Types of Field Effect Transistors (The Classification) JFET MOSFET (IGFET) n-Channel JFET p-Channel JFET FET Enhancement MOSFET Depletion MOSFET n-Channel EMOSFET n-Channel DMOSFET p-Channel DMOSFET p-Channel EMOSFET

The Junction Field Effect Transistor (JFET) Figure: n-Channel JFET.

JFET There are two basic configurations of junction field effect transistor, the N-channel JFET and the P-channel JFET. The N-channel JFET’s channel is doped with donor impurities meaning that the flow of current through the channel is negative (hence the term N-channel) in the form of electrons.

SYMBOLS Gate Drain Source Gate Drain Source Gate Drain Source n-channel JFET Offset-gate symbol n-channel JFET p-channel JFET

Small signal Analysis The coupling capacitors bypass capacitor are short circuitED Short the DC supply voltage Replace the FET with the hybrid-p model

Common source JFET

Small signal -Common source JFET-contd…

Small signal -Common source JFET-contd…

Source Follower JFET

Small signal equivalent-Source Follower JFET Av= gmRs/(1+gmRs)

Common Gate JFET

Small signal model-Common Gate JFET AV= -gmRd Rin=1/gm

Self bias of JFET

Small signal equivalent -Self bias of JFET-contd…

Enhancement MOSFET showing channel length L and channel width W.

MOSFET- SYMBOL

Small-signal equivalent circuit for FETs.

FET small-signal equivalent circuit that accounts for the dependence of iD on vDS.

MOSFET Characteristics

Common source Amplifier -MOSFET

Small signal -Common source Amplifier -MOSFET For drawing an a c equivalent circuit of Amp. Assume all Capacitors C1, C2, Cs as short circuit elements for ac signal Short circuit the d c supply Replace the FET by its small signal model

Analysis of CS Amplifier A C Equivalent Circuit Simplified A C Equivalent Circuit

Analysis of CS Amplifier-contd…

Small Signal ‘T’ Model : NMOSFET sedr42021_0439.jpg

Small Signal Models ‘T’ Model sedr42021_tb0402a.jpg

Analysis of CS Amplifier with Potential Divider Bias This is a CS amplifier configuration therefore the input is on the gate and the output is on the drain.

An Amplifier Circuit using MOSFET(CS Amp.)

A small signal equivalent circuit of CS Amp.

Common Source Amplifier (CS) Signal ground or an ac earth is at the source through a bypass capacitor Not to disturb dc bias current & voltages coupling capacitors are used to pass the signal voltages to the input terminal of the amplifier or to the Load Resistance CS circuit is unilateral Rin does not depend on RL and vice versa

Small Signal Hybrid “π” Model : (CS)

Small Signal Hybrid “π” Model : (CS)

Small-signal analysis performed directly on the amplifier circuit with the MOSFET model implicitly utilized sedr42021_0443a.jpg

Common-source amplifier with a resistance RS in the source lead sedr42021_0444a.jpg

The Common Source Amplifier with a Source Resistance The ‘T’ Model is preferred, whenever a resistance is connected to the source terminal. ro (output resistance due to Early Effect) is not included, as it would make the amplifier non unilateral

Small-signal equivalent circuit with ro neglected. sedr42021_0444a.jpg

Small-signal Analysis.

Voltage Gain : CS with RS

Small-signal Analysis. Source Resistance can be used to control the magnitude of the signal vgs& thus ensure that vgs does not become too large to cause non-linear distortion vgs << 2(VGS-Vt) << 2VOV

Common Source Configuration with Rs Rs causes a negative feedback thus improving the stability of drain current of the circuit but at the cost of voltage gain Rs reduces id by the factor (1+gmRs) = Amount of feedback Rs is called Source degeneration resistance as it reduces the gain

Small-signal equivalent circuit directly on Circuit sedr42021_0444a.jpg

BJT / MOSFET

Common Source Amplifier (CS) Summary Input Resistance is infinite (Ri=∞) Output Resistance = RD Voltage Gain is substantial

MOSFET -Source follower.

Small-signal ac equivalent circuit for the source follower.

Equivalent circuit used to find the output resistance of the source follower.

Common-gate amplifier.

Small signal-Common Gate

Small-signal ‘π’ models for the MOSFET sedr42021_0437a.jpg

Voltage swing limitation Up swing limited by transistor going in to cut off Vout (max)= VDD Lower swing limited by MOSFET entering in to linear region Vou(min)-VGG-VT

A common-gate amplifier based on the circuit sedr42021_0445a.jpg

Common Gate (CG) Amplifier The input signal is applied to the source Output is taken from the drain The gate is formed as a common input & output port. ‘T’ Model is more Convenient ro is neglected

A small-signal equivalent circuit sedr42021_0445a.jpg

A small-signal Analusis : CG sedr42021_0445a.jpg

A small-signal Analusis : CG sedr42021_0445a.jpg

Small signal analysis directly on circuit sedr42021_0445a.jpg

The common-gate amplifier fed with a current-signal input. sedr42021_0445a.jpg

Common Gate Rin in independent of RL & Rin = 1/gm & gm in order of mA/V. Input resistance of the CG Amplifier is relatively low (in order of 1kv) than CS Amplifier Loss of signal CG is acts as Unity gain current amplifier current buffer – useful for a Cascade circuitry

Summary-CG CG has much higher output Resistance CG is unity current Gain amplifier or a Current Buffer CG has superior High Frequency Response.

A common-drain or source-follower amplifier . A common-drain or source-follower amplifier sedr42021_0446a.jpg

Small-signal equivalent-circuit model sedr42021_0446a.jpg

Small-signal Analysis : CD sedr42021_0446a.jpg

A common-drain or source-follower amplifier :output resistance Rout of the source follower. sedr42021_0446a.jpg

A common-drain or source-follower amplifier A common-drain or source-follower amplifier. : Small-signal analysis performed directly on the circuit. sedr42021_0446a.jpg

Common Source Circuit (CS) sedr42021_tb0404a.jpg

Common Source Circuit (CS) With RS sedr42021_tb0404a.jpg

Common Gate Circuit (CG) Current Follower sedr42021_tb0404c.jpg

Small Signal Model MOSFET : CD sedr42021_p05144.jpg

Small Signal Analysis CD 1/gm gmvsg D sedr42021_p05144.jpg

Solution Small Signal Analysis : Input Resistance 1/gm gmvsg D Ig=0 Rin

Solution Small Signal Analysis : Output Resistance Itest 1/gm gmvsg D ID IRD 0 V Vtest IG=0 Rout

Solution Small Signal Analysis : Voltage Gain + 1/gm gmvsg D vsg - - + vo vi + -

Solution Small Signal Analysis : Voltage gain 1/gm gmvsg D + vsg - + vi -

Solution Small Signal Analysis : Voltage Gain 1/gm gmvsg D + vi -

Solution Small Signal Analysis : Voltage Gain Then

Solution Small Signal Analysis : Voltage Gain 1/gm gmvsg D - + vi + -

BICMOS sedr42021_0646a.jpg

BICMOS -contd… Combining the high gain of BJT and infinite impedance of MOSFET will lead to BiCMOS differential amplifier design. Rs = typical 100 KW BICMOS cascode amplifier has overall voltage gain of C-S, but with frequency response comparable to CB Amplifier.

BICMOS -contd… The basic idea is to combine the high Rin and large transconductance (g m) of a commonsource (common-emitter) amplifier with the current-buffering property and superior high-frequency response of the common-gate (common-base) circuit

BICMOS -contd… In response to input signal voltage vi, the CS transistor Q1 conducts a current signal gm1 vi in its drain terminal and feeds it to the source of the CG transistor Q 2 (cascode transistor). Q 2 passes signal to its drain and to the load RL. Q 2 acts as a buffer, presenting low Rin to the drain of Q1 and providing high Rout at output.