# Transistor Circuit Notes

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Aero2 Signals & Systems (Part 2) Notes on BJT and transistor circuits

Bipolar Junction Transistors
• Physical Structure & Symbols
• NPN n-type Emitter region p-type Base region n-type Collector region

C Collector (C) B E (b)

Emitter (E)

Emitter-base junction (EBJ)

Base (B) (a)

Collector-base junction (CBJ)

• PNP - similar, but: • N- and P-type regions interchanged • Arrow on symbol reversed

• Operating Modes
Operating mode Cut-off Active Saturation Reverse-active EBJ Reverse Forward Forward Reverse CBJ Reverse Reverse Forward Forward

• Active Mode - voltage polarities for NPN

IC VCB > 0 B IB VBE > 0 E IE C

(Based on Dr Holmes’ notes for EE1/ISE1 course)

1

Aero2 Signals & Systems (Part 2) Notes on BJT and transistor circuits

BJT - Operation in Active Mode n E IE p electrons holes recombination

n C IC

{

IEn IEp

IB

B

• IEn , IEp both proportional to exp(VBE/VT) • IC ≈ IEn ⇒ IC ≈ IS exp(VBE/VT) • IB ≈ IEp 0 B

C

60 40

VBE 20 VBE (V) 0.0 0.2 0.4 0.6 0.8

E

• ACTIVE REGION: • IC ≈ 0 for VBE < ≈ 0.5 V • IC rises very steeply for VBE > ≈ 0.5 V • VBE ≈ 0.7 V over most of useful IC range

• IB vs VBE similar, but current reduced by factor β • CUT-OFF REGION: • IC ≈ 0 • Also IB , IE ≈ 0

(Based on Dr Holmes’ notes for EE1/ISE1 course)

3

Aero2 Signals & Systems (Part 2) Notes on BJT and transistor circuits

BJT Operating Curves - 2
• OUTPUT IC vs VCE (for β = 50)

IC (mA) 12 SAT 10 8 6 4 2 0 0 1

ACTIVE

IB = 200 µA IB = 160 µA IB = 120 µA IB = 80 µA IB = 40 µA

IC C B E VCE

IB

VCE (V) 2

• ACTIVE REGION (VCE > VBE): • IC = β IB , regardless of VCE i.e. CONTROLLED CURRENT SOURCE

• SATURATION REGION (VCE < VBE): • IC falls off as VCE → 0 • VCEsat ≈ 0.2 V on steep part of each curve • In both cases: • VBE ≈ 0.7 V if IB non-negligible

(Based on Dr Holmes’ notes for EE1/ISE1 course)

4

Aero2 Signals & Systems (Part 2) Notes on BJT and transistor circuits

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