Element 2 - Bipolar Junction Transistor (BJT)

Bipolar Junction Transistors (BJT)

LECTURE 6

Transistors Transistor is a 3 terminals semiconductor device that controls

current between two terminals based on the current (BJT) or voltage (FET) at the third terminal

Is used for the amplification or switching of electrical signals Two main categories of transistors:

Bipolar Junction Transistors (BJTs) and Field Effect Transistors (FETs)

The physics of "transistor action" is quite different for the BJT and FET

In analog circuits, transistors are used in amplifiers and linear regulated power supplies

In digital circuits, they function as electrical switches, including logic gates, Random Access Memory (RAM), and microprocessors.

Bipolar Junction Transistors (BJT) A bipolar transistor

essentially consists of a pair of PN Junction diodes that are joined back-to-back.

There are therefore two kinds of BJT, the NPN and PNP varieties.

The three layers of the sandwich are conventionally called the Collector, Base, and Emitter.

Modern Transistors

How the BJT works Figure shows the energy levels

in an NPN transistor under no externally applying voltages

In each of the N-type layers

conduction can take place by the free movement of electrons in the conduction band

In the P-type (filling) layer

conduction can take place by the movement of the free holes in the valence band

However, in the absence of any

externally applied electric field, we find that depletion zones form at both PN-Junctions, so no charge wants to move from one layer to another.

NPN Bipolar Transistor

How the BJT works What happens when we apply

a moderate voltage between the collector and base parts

The polarity of the applied

voltage is chosen to increase the force pulling the N-type electrons and P-type holes apart

This widens the depletion

zone between the collector and base and so no current will flow

In effect we have reverse-

biased the Base-Collector diode junction.

Apply a Collector-Base voltage

Charge Flow What happens when we apply a

relatively small Emitter-Base voltage whose polarity is designed to forward-bias the Emitter-Base junction.

This 'pushes' electrons from the Emitter into the Base region and sets up a current flow across the Emitter-Base boundary.

Once the electrons have managed to get into the Base region they can respond to the attractive force from the positively-biased Collector region.

As a result the electrons which get into the Base move swiftly towards the Collector and cross into the Collector region.

Hence a Emitter-Collector current magnitude is set by the chosen Emitter-Base voltage applied.

Hence an external current flowing in the circuit.

Apply an Emitter-Base voltage

Charge Flow Some of free electrons crossing the Base encounter a hole and 'drop into it'.

As a result, the Base region loses one of its positive charges (holes).

The Base potential would become more negative (because of the removal of the holes) until it was negative enough to repel any more electrons from crossing the Emitter-Base junction.

The current flow would then stop.Some electron fall into a hole

Charge Flow To prevent this happening we use the applied E-B voltage to remove the captured electrons from the base and maintain the number of holes.

The effect, some of the electrons which enter the transistor via the Emitter emerging again from the Base rather than the Collector.

For most practical BJT, only about 1% of the free electrons which try to cross Base region get caught in this way.

Hence a Base current, IB, which is typically around one hundred times smaller than the Emitter current, IE.

Some electron fall into a hole

Characteristics Maximum collector current (IC) – the maximum continuous current

that can flow in the collector leg of the transistor without damage to the transistor (50 mA to 50 A)

Maximum power dissipation (PD) – the maximum power the transistor can dissipate without being damaged (0.2 W to 250 W)

Small signal beta (β) or (hfe) – the signal current gain of the transistor in the common-emitter configuration (β = ic/ib)

DC beta (β) or (hFE) – the DC current gain of the transistor in the common-emitter configuration (10 to 1000). β = IC/IB

Maximum base current (IB) – the maximum current that can flow in the base leg of the transistor without damage to the transistor

Collector to base breakdown voltage (VCBO) – the maximum reverse-biased voltage that can be applied across the collector to base junction (20 V to 1500 V)

Characteristics Collector to emitter breakdown voltage (VCEO) – the maximum

voltage that can be applied across the transistor from collector to emitter (20 V to 800 V)

Emitter to base breakdown voltage (VEBO) – the maximum reverse-biased voltage that can be applied across the emitter to base junction (4 V to 20 V)

Gain-bandwidth product (fT) – the frequency at which the gain of the transistor drops to unity (1 MHz to 5000 MHz)

Output Characteristics Curve

VBE

IB

5 uA

10uA

15 uA

20 uA

0.7 V

Terminals & Operations

Three terminals: Base (B): very thin and lightly doped central region (little

recombination). Emitter (E) and collector (C) are two outer regions

sandwiching B. Normal operation (linear or active region):

B-E junction forward biased; B-C junction reverse biased. The emitter emits (injects) majority charge into base region

and because the base very thin, most will ultimately reach the collector.

The emitter is highly doped while the collector is lightly doped.

The collector is usually at higher voltage than the emitter.

Terminals & Operations

The NPN Transistor

Operation Forward bias of EBJ injects electrons from emitter into base

(small number of holes injected from base into emitter) Most electrons shoot through the base into the collector across

the reverse bias junction (think about band diagram) Some electrons recombine with majority carrier in (P-type) base

region

Current flow in a pnp transistor biased to operate in the active mode.

The PNP Transistor

Operation Mode

Operation Mode

Active: Most importance mode, e.g. for amplifier operation. The region where current curves are practically flat.

Saturation: Barrier potential of the junctions cancel each other out

causing a virtual short. Ideal transistor behaves like a closed switch.

Cutoff: Current reduced to zero Ideal transistor behaves like an open switch.

Operation Mode

Circuit Symbols

For a transistor to function properly as an amplifier, an external dc supply voltage (or voltages) must be applied to produce the desired collector current, Ic.

Several biasing techniques exist that include: Base Biasing Voltage Divider Emitter Biasing

Transistor Biasing Techniques

Base Bias, VBB

The simplest way to bias a transistor VBB is the base supply voltage – used to forward –bias the

base-emitter junction RB is used to provide the desired value of base current VCC is the collector supply voltage which provides the

reverse-bias voltage required for the collector-base junction of the transistor

RC provides the desired voltage in the collector circuit

Transistor Biasing Techniques

+vcc

+vBB

Dual supply Single supply

DC Load Line

Transistor Biasing Techniques

VCC = IC RC

IC(sat) = VCC / RC

VCE(off) = VCC

Q pointICQ

VCEQ

VCE

IC

IC(sat) = VCC / RC

Saturation region

Cut-off

VCE(off) = VCC

Active region

IC = β x IB

Example 1: If VBB = 5 V, VCC = 15 V, RB = 56 k Ω, RC = 1 kΩ, β = 100

Transistor Biasing Techniques

+vcc

+vBB

Vcc = IBRB + Vbe

IB = (VBB - Vbe)/ RB = (5 V- 0.7 V)/ 56 kΩ = 76.78 μA

IC = β x IB = 100 x 76.78 μA = 7.68 mA

VCE = VCC - ICRC = 15 V - 7.68 mA x 1 kΩ = 15 V – 7.68 V = 7.32 V

IC(sat) = 15 mA

Q pointICQ = 7.68 mA

VCEQ = 7.32 V

VCE

IC

IC(sat) = Vcc / RC = 15 V / 1 kΩ = 15 mA

VCE(off) = Vcc = 15 V

VCE(off) = 15 V

Example 2: If VCC = 12 V, RB = 390 k Ω, RC = 1.5 kΩ, β = 150

Transistor Biasing Techniques

Vcc = IBRB + Vbe

IB = (Vcc - Vbe)/ RB = (12 V – 0.7 V)/ 56 kΩ = 28.97 μA

IC = β x IB = 150 x 28.97 μA = 4.35 mA

VCE = VCC – ICRC = 12 V – 4.35 mA x 1.5 kΩ = 12 V – 6.52 V = 5.48 V

IC(sat) = Vcc / RC = 12 V / 1.5 kΩ = 8 mA

VCE(off) = Vcc = 12 V

IC(sat) = 8 mA

Q pointICQ = 4.35 mA

VCEQ = 5..48 V

VCE

IC

VCE(off) = 12 V

Voltage Divider Bias, VBB

The most popular way to bias a transistor The advantage lies in its stability As shown in the figure, two resistor R1 and R2 set up a

voltage divider on the base

Transistor Biasing Techniques

voltage across

provided

DC Load Line

Transistor Biasing Techniques

VCC = IC RC

IC(sat) = VCC / (RE + RC)

VCE(off) = VCC

Q pointICQ

VCEQ

VCE

IC

IC(sat) = VCC / (RE + RC)

Saturation region

Cut-off

VCE(off) = VCC

Active region

IC = β x IB

Emitter Bias, VEE

This type can only be used when dual (split) power supply is available

The advantage lies in its stability similar to voltage divider VEE forward-biases the emitter-base junction through the

emitter resistor, RE

Transistor Biasing Techniques

VB - VE = VBE

IE = (VEE - VBE)/RE

If RB is small enough, base voltage will be approximately zero. Therefore emitter current is,

BJT as a Switch Operation

In the circuits, transistor works as a switch, the biasing of the transistor, either NPN or PNP is arranged to operate it at the both sides of the I-V characteristics curves.

The areas of operation for a transistor switch are:

Cut-off region Saturation region

Cut-off RegionThe operating conditions of the transistor are zero input base current (IB), zero output collector current (IC) and maximum collector voltage (VCE) which results in a large depletion layer and current flowing through the device

BJT as a Switch Cut-off Region

In the circuits, transistor works as a switch, the biasing of the transistor, either NPN or PNP is arranged to operate it at the both sides of the I-V characteristics curves.

The input and Base are grounded (0v) Base-Emitter voltage VBE < 0.7v

Base-Emitter junction is reverse biased Base-Collector junction is reverse biased Transistor is "fully-OFF" (Cut-off region) No Collector current flows ( IC = 0 ) VOUT = VCE = VCC  Transistor operates as an "open switch”

“Cut-off Region” can be referred to as “OFF mode”.

BJT as a Switch Saturation Region

The transistor is biased so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage drop which results in the depletion layer being as small as possible and maximum current flowing through the transistor. Therefore the transistor is switched "Fully-ON“.

BJT as a Switch Saturation Region

The input and Base are connected to VCC

Base-Emitter voltage VBE > 0.7v

Base-Emitter junction is forward biased Base-Collector junction is forward biased Transistor is "fully-ON" (saturation region) Max Collector current flows (IC = Vcc/RL) VCE = 0 (ideal saturation) VOUT = VCE  Transistor operates as a "closed switch"

“Saturation Region” can be referred to as “ON mode”.

BJT as a Switch Basic NPN Transistor Switching Circuit

Digital Logic NPN Transistor Switching Circuit

PNP BJT as a Switch Digital Logic NPN Transistor Switching Circuit

Bipolar Transistor Configurations

There are three possible ways to connect BJT within an electronic circuit with one terminal being common to both the input and output. Common Base Configuration

has Voltage Gain but no Current Gain. Common Emitter Configuration

has both Current and Voltage Gain. Common Collector Configuration

has Current Gain but no Voltage Gain.

The Common Base Configuration

Schematic symbols

Dramatic symbol

The input is applied to the emitter The output is taken from the collector Low input impedance High output impedance Current gain less than unity Very high voltage gain Vin and Vout are in-phase.

The common base circuit is mainly used in single stage amplifier circuits such as microphone or radio frequency (RF) amplifiers due to its very good high frequency response

The Common Base Transistor Circuit

The Common Emitter Amplifier Circuit

Schematic symbols

Dramatic symbol

The Common Emitter Amplifier Circuit

The input is applied to the base The output is from the collector High voltage and current gain, Vout > Vin

High input impedance Low output impedance Phase shift between input and output is

180˚

The Common Collector (CC) Configuration

This type of configuration is known as a Voltage Follower or Emitter Follower

It is very useful for impedance matching applications because of its very high input impedance, hundreds to thousands of Ohms and a relatively low output impedance

The current gain approximately equal to the β value of the transistor itself

The load resistance is connected to the emitter so its current is equal to that of the emitter current

The Common Collector (CC) Configuration

Summary of Transistor ConfigurationsCharacteristic

sCommon

BaseCommon Emitter

Common Collector

Input Impedance

Low Medium High

Output Impedance

Very High High Low

Phase Angle 0o 180o 0o

Voltage Gain High Medium Low

Current Gain Low Medium High

Power Gain Low Very High Medium