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Bipolar Junction Transistors (BJT), Study notes of Basic Electronics

University of the Arts London (UAL)Basic Electronics

Transistor Basics. • A Bipolar Junction Transistor is a three layer (npn or pnp) ... The transistor operates in three modes depending on how.

Typology: Study notes

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Transistors
Bipolar Junction Transistors (BJT)
Transistor Basics
• A Bipolar Junction Transistor is a three layer (npn or pnp)
semiconductor device.
• There are two pn junctions in the transistor.
• The three layers are called the emitter, base and collector.
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Transistors

Bipolar Junction Transistors (BJT)

Transistor Basics

  • A Bipolar Junction Transistor is a three layer (npn or pnp)

semiconductor device.

  • There are two pn junctions in the transistor.
  • The three layers are called the emitter, base and collector.

Transistor Basics

  • The base is lightly doped and sandwiched between the collector and

the emitter. The collector is moderately doped and the emitter is

heavily doped.

  • The base region is much thinner than the either the collector or emitter

regions. Typical base widths are about 10-6^ m.

  • The collector region is usually thicker than the emitter as the largest

amount of heat is dissipated in the collector.

Transistor Basics

A Note About Labeling Voltages

  • Voltages with capital letter subscripts are DC voltages,

(VBE, VCC).

  • When a voltage has a single subscript it is measured from

the terminal to ground, (VE, VC).

  • When a voltage has a double subscript with different

letters it is the voltage measured between two terminals,

(VBE, VEC).

  • When a voltage has a double subscript with the same

letters it is a supply voltage, (VEE, VCC).

Cutoff

  • Both junctions are reverse biased.
  • No current flows. n p n VBE

VCE

VBE

VCE

E C

B

Saturation

  • Both junctions are forward biased.
  • Maximum current flows. n p n VBE

VCE

VBE

VCE

E C

B

VCE < VBE

VCE < VBE

Active Region

  • The BE junction is forward biased and the BC junction is reverse biased.
  • The base-emitter voltage is approximately 0.5 - 0.7 volts (the turn-on

voltage of the junction.

n p n VBE

VCE

VBE

VCE

E C

B

VCE > VBE ≈ 0.7 V

VCE > VBE ≈ 0.7 V

Active Operation

  • As there are few holes in the base for the electrons to combine with

most of the electrons diffuse in to the reverse biased collector-base

junction where the electric field in the depletion region sweeps them

across into the collector.

  • Remember electrons are the minority carriers in the p material so

they are swept across the depletion region.

n p n VEE

E C

B

VCC

IE IC

IB

Active Operation

  • A simplified electron energy diagram for the npn transistor.

Unbiased:

Active Operation

  • A simplified electron energy diagram for the npn transistor.

Biased:

Active Operation

  • A small percentage of the electrons injected in to the base from the

emitter do recombine with the holes in the base.

  • If left alone the base would slowly become more negative until the

flow of electrons across the base stopped. Electrons leave the base via

the wire contact maintaining the small amount of holes in the base.

  • This flow of electrons is the small base current. n p n VEE

E C

B

VCC

IE IC

IB

Active Operation

  • Kirchoff s rule gives us
  • The dc alpha is the ratio of collector to emitter current.
  • Typical transistors have values of α that range from 0.95 to

Active Operation

  • The ratio of the dc collector current to the dc base current

is the dc beta rating of the transistor. (Also called hFE.)

  • The dc beta can be related to the dc alpha.

Transistor Curves

• To understand why the transistor in the

active region can be used as an amplifier we

can look at collector characteristic curves.

• A version of the common-emitter circuit:

Transistor Curves

• The voltage difference between the collector and

emitter VCE can be adjusted by varying VCC.

• Likewise varying VBB adjusts the value of IB.

Example

VBB

VCC

10 V

IB = 100 μA R 400 CΩ β = 100

Cases

Transistor Amplifiers

Biasing

Biasing for the Active Region

  • In order for a transistor amplifier to work the transistor

must be in the active region.

  • One option is to bias the transistor by a using a number of

power supplies.

VEE

VCC

Voltage-Divider Biasing

  • Likewise the emitter current can

be found from

  • The collector current and DC

collector voltage are

. vout R 2 4.7 kΩ 2N R 1 10 kΩ +VCC 15 V RE 2.7 kΩ RC 3.9 kΩ vin

Q-Point

  • We still need to determine

the optimal values for the

DC biasing in order to

choose resistors, etc.

  • This bias point is called the

quiescent or Q-point as it

gives the values of the

voltages when no input

signal is applied.

  • To determine the Q-point

we need to look at the

range of values for which

the transistor is in the

active region.

Load Line

  • At saturation the resistance

offered by the transistor is

effectively zero so the

current is a maximum

determined by VCC and the

resistors RE and RC.

  • When the transistor is in

cutoff no current flows so

VCE = VCC.

  • If we connect these two

points with a straight line

we get all possible values

for IC and VCE for a given

amplifier.

Q-point

  • To determine the q-point we

overlay the load line on the

collector curves for the

transistor.

  • The Q-point is where the

load line intersects the

appropriate collector curve.

  • For example if the amplifier

is operated at IB = 20 μA the

Q-point is as shown on the

graph.

Transistor Amplifiers

Gain and Impedance

Gains

  • AC Gain is the ratio between the ac output and ac input

signal.

  • Voltage:
  • Current:
  • Power:

Decibels

• Gains are sometimes expressed in terms of

decibels.

• dB Power Gain:

• dB Voltage Gain

Ap ( dB ) = 10 log A p

Av ( dB ) = 20 log A v

Basic Amplifier Model

• The basic amplifier is characterized by its

gains, input impedance and output

impedance.