Transistor Circuit Notes

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  • Topic: Electronic amplifier, Bipolar junction transistor, Transistor
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  • Published : March 29, 2013
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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)

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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)

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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)

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

Summary of BJT Characteristics
VCB > 0

CUT-OFF
• IC ≈ 0 • IB ≈ 0

ACTIVE
• IC = IS exp(VBE /VT) • IC = β IB • VBE ≈ 0.7 V if I C non-negligible

VBE < 0

VBE > 0

REVERSE-ACTIVE

SATURATION
• IC < β IB • VBE ≈ 0.7 V if I B non-negligible • VCE < VBE (by definition)

VCB < 0

• Also

IE = IB + IC (always)

• THIS TABLE IS IMPORTANT - GET TO KNOW IT ! • For PNP table: • Reverse order of suffices on all voltages in table i.e. VCB → VBC etc • Reverse arrows on currents in circuit i.e. arrows on IB, IC point out of PNP device, while arrow on IE points in.

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

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

Common-Emitter Amplifier Conceptual Circuit
RC IC V CC VOUT V IN

• Assume active mode: IC = IS exp(VIN/VT) • Apply Ohm’s Law and KVL to output side: VOUT = VCC - RCIC = VCC - RCIS exp(VIN/VT) (1.3)

NOTE: Called ‘common-emitter’ because emitter is connected to reference point for both input and output circuits. Common-Base and Common-Collector also important.

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

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

C-E Amplifier Input-Output Relationship
• e.g. VCC = 20 V, RC = 10 kΩ, IS = 10-14 A, VT = 25 mV.
VOUT (V) 20

ΔVIN ΔVOUT

15

10
Operating Point

5

0 0.50

VIN (V) 0.55 0.60 0.65 0.70

• Plenty of voltage gain i.e. ΔVOUT >> ΔVIN BUT: • Highly non-linear ⇒ Output distorted unless input signal very small ⇒ Need to BIAS transistor to operate in correct region of graph to get high gain without distortion

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

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

C-E Amplifier Small-Signal Response - 1
Aim: to get quantitative information about the small-signal voltage gain and the linearity of a C-E amplifier • Start with the large signal equations: VOUT = VCC - RCIC = VCC - RC IS...
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