Chapter 6. Bipolar Junction Transistors (BJTs)

Bipolar Junction Transistor Three terminal device Voltage between two terminals to control current flow in third terminal Invented in 1948 at Bell Telephone Laboratories Dominant until late 1980’s Reliable under harsh operating conditions –High frequency applications –High speed designs –High power applications

npn transistor n-type emitter (E) region, p-type base (B) region, n-type collector (C) region Two pn junctions (naming basis for bipolar junction transistor) Modes -Active: used for amplifier design -Cutoff -Saturation: used for logic design -Reverse active: limited operation

pnp transistor – dual of npn transistor

Active Mode of npn Transistor

Circuit Models for Active Mode npn Transistor

Practical Implementation E and C are not symmetrical. pnp transistors works dual to npn transistors much in the same way PMOSFET works dual to NMOSFET. (In this class, we will concentrate on npn transistors.)

Circuit Symbols for npn Transistors Biasing in active mode Directions of current flow

i C – v BE Characteristics Temperature Dependence

Common Base Characteristics Base voltage is fixed at zero. In active region, v CB ≥ V

Dependence of i C on Collector Voltage

Circuit Models with Output Resistance r o

Common Emitter Configuration

Common Emitter Saturation Model

Designing Linear Amplifiers (Active Region)

Amplifier Gain

Graphical Analysis

To determine i B, i C and v CE, you need to use both graphs.

Quiescent point must be selected to give a symmetric output swing.

Input part + _ V BB _ + 10 V R B1 R B2 10 VR B2 V BB R B1 + _ V BB R BB

Biasing BJT Determining a quiescent point for linearization Active mode operation Considerations –Stable with respect to manufacturing parameters (e.g., r o, β) –Desired gains –Acceptable output swing

Biasing with Single Power Supply Fix V BE or I B. Output directly depends on β Unstable with respect to temperature variation

Addition of Degeneration Resistor

Biasing with Two Power Supplies

Biasing with Feedback Resistor

Biasing with Current Source

Small Signal Analysis A quiescent point has been determined by biasing. Active mode operation –Forward biasing for base-emitter junction by V BE –Reverse biasing for collector-base junction by R C and V CC

The transistor performs as a voltage controlled current source with gain g m when input varies by 10 mV or less.

Hybrid π Model Short circuit voltage sources Open circuit current sources Short circuit capacitors

T Model

Hybrid π Model with Early Effect

Structure of Single Stage Amplifier

Common Emitter Amplifier

Common Emitter Amplifier with R E

Common Base Amplifier

Common Collector Amplifier

Digital Logic Inverter Transistor is in saturation mode. Logic 1: v I ≈ V CC → v O =V CEsat ≈ 0.2 V Logic 0: v I ≈ 0 → v O =V CC

v I – v O Transfer Function V CC = 5 V R B = 10 kΩ R C = 1 kΩ β = 50