Small Signal Model MOS FieldEffect Transistors MOSFETs 1

Small Signal Model MOS Field-Effect Transistors (MOSFETs) 1

Quiz No 3 DE 27 (CE) 20 -03 -07 Rout. (a) (b) (c) Draw small signal model (4) Find expression for Rout (2) Prove vo/vsig = (β 1α 2 RC)/(Rsig+rπ) (4).

Figure 4. 2 The enhancement-type NMOS transistor with a positive voltage applied to the gate. An n channel is induced at the top of the substrate beneath the gate.

Enhancement-type NMOS transistor:

MOSFET Analysis i. D = i. S, i. G = 0

Large-signal equivalent-circuit model of an n-channel MOSFET : Operating in the saturation region.

Large-signal equivalent-circuit model of an p-channel MOSFET : Operating in the saturation region.

Large Signal Model : MOSFET

Transfer characteristic of an amplifier

Conceptual circuit utilized to study the operation of the MOSFET as a small-signal amplifier. The DC BIAS POINT To Ensure Saturation-region Operation

Signal Current in Drain Terminal

Total instantaneous voltages v. GS and v. D

Small-signal ‘π’ models for the MOSF

Common Source amplifier circuit Example 4 -10

Small Signal ‘T’ Model : NMOSFET

Small Signal Models ‘T’ Model

Single Stage MOS Amplifier

Amplifiers Configurations

Common Source Amplifier (CS) : Configuration

Common Source Amplifier (CS) • Most widely used • 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.

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

Common-source amplifier with a resistance RS in the source lead

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 & effect of using ro in model would be studied in Chapter ‘ 6’

Small-signal equivalent circuit with ro neglected.

Small-signal Analysis.

Voltage Gain : CS with RS

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+gm. Rs) = Amount of feedback • Rs is called Source degeneration resistance as it reduces the gain

Small-signal equivalent circuit directly on Circuit

A common-gate amplifier based on the circuit

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

A small-signal Analusis : CG

A small-signal Analusis : CG

Small signal analysis directly on circuit

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

Summary : CG 4. CG has much higher output Resistance 5. CG is unity current Gain amplifier or a Current Buffer 6. CG has superior High Frequency Response.

A common-drain or source-follower amplifier.

Small-signal equivalent-circuit model

Small-signal Analysis : CD

(a) A common-drain or source-follower amplifier : output resistance Rout of the source follower.

(a) A common-drain or source-follower amplifier. : Smallsignal analysis performed directly on the circuit.

Common Source Circuit (CS)

Common Source Circuit (CS) With RS

Common Gate Circuit (CG) Current Follower

Common Drain Circuit (CD) Source Follower

Summary & Comparison

Quiz No 4 • Draw/Write the Following: Types Symbols ‘π’ Model T Model gm Re/rs rπ/rg BJT npn pnp MOSFET n. MOS p. MOS 27 -03 -07

Problem 5 -44

SOLUTION : DC Analysis

SOLUTION : DC Analysis IE IB Check for Active Mode

Solution Small Signal Analysis

Solution Small Signal Analysis

Solution Small Signal Analysis : Input Resistance ib + vb - Rin

Solution Small Signal Analysis : Output Resistance Itest IE IRC IE/(1+ß) Rout

Solution Small Signal Analysis : Voltage Gain + veb - - + Vo vi + -

Solution Small Signal Analysis : Voltage gain + veb + vi -

Solution Small Signal Analysis : Voltage Gain + vi -

Solution Small Signal Analysis : Voltage Gain

Solution Small Signal Analysis : Voltage Gain + vi - Vo +

Problem

Small Signal Model MOSFET : CD

Solution Small Signal Analysis 1/gm D gmvsg

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

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

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

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

Solution Small Signal Analysis : Voltage Gain + vi -

Solution Small Signal Analysis : Voltage Gain

Solution Small Signal Analysis : Voltage Gain + vi - +

Solution Small Signal Analysis

Problem 6 -127(e)

DC Analysis 6 -127(e)

Small Signal Model

Small Signal Model

Small Signal Model Rin Rout

Problem 6 -127(f) Replacing BJT with MOSFET

Small Signal Model

Small Signal Model

Small Signal Model Rin Rout

Problem 6 -127(f)

Solution P 6 -127(f) + vbe 2 + veb 1 -

Solution P 6 -127(f) + vbe 2 + veb 1 + vi -

Problem 6 -127(f) with MOSFET

Solution P 6 -127(f) + vgs 2 + vsg 1 -

Solution P 6 -127(f) + vgs 2 + vsg 1 ig 1=0 + vi -

Comparison BJT/MOSFET Cct

Small Signal Model

Problem 6 -123 VBE=0. 7 V β =200 K’n(W/L)=2 m. A/V 2 Vt=1 V Figure P 6. 123

DC Analysis Figure P 6. 123

VBE=0. 7 V β =200 K’n(W/L)=2 m. A/V 2 Vt 1=1 V Vt 2=25 m. V DC Analysis 1 m. A 2 V IG 0. 7 V =0 I=0. 7/6. 8=0. 1 m. A

Small Signal Model

Small Signal Model

Small Signal Model : Voltage Gain + ig=0 vi - + vbe 2 -

Small Signal Model : Input Resistance ii + ig=0 vi Rin