BJTs

TransistorThe transistor is the main building block “element” of electronics. A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power.It comes in two general types:

The Bipolar Junction Transistor (BJT).The Field Effect Transistor (FET).

Bipolar Junction Transistor (BJT)A BJT is a three-terminal device that uses the voltage of the two terminals to control the current flowing in the third terminal.It is, The basis for amplifier design. The basis for switch design. The basic element of high speed integrated digital

and analog circuits.

Applications of BJTDiscrete-circuit design.Analog circuits.High frequency application such as radio frequency analog circuit.Digital circuits.Bi-CMOS (Bipolar+CMOS) circuits that combines the advantages of MOSFET and bipolar transistors.

MOSFET: high-input impedance and low-power.Bipolar transistors: high-frequency-operation and high-current-driving capabilities.

BJT Types it comes in two different types,

1. NPN BJT.2. PNP BJT.

The simplified structure of a BJT consists of three semiconductors regions:→ Emitter (E).→ Base (B) which always refers to the center

region.→ Collector (C).

NPN BJT structure and symbol

Contd...

PNP BJT structure and symbol

Contd...

Contd...Both PNP and NPN structures are not symmetric. The n and p regions are different both geometrically and in terms of the doping concentration of the

For example, the doping concentrations in the collector(1015), base(1017) and emitter(1019).

Imp note: Therefore the behavior of the device is not electrically symmetric and the two ends cannot be interchanged.

OperationThe transistor consists of two PN junctions:• The Emitter-Base junction (E-B)• The Collector-Base junction (C-B)

Depending on the bias condition (forward or reverse) of each of these junctions, different modes of operation of the BJT are obtained:

Output characteristics

The characteristics of each region of operation

1. Cutoff region: Base-emitter junction is reverse biased. No current flow

2. Saturation region: Base-emitter junction forward biased,Collector-base junction is forward biased,IC reaches a maximum which is independent of IB and β.No control. Condition: VCE < VBE

3. Active region: Base-emitter junction forward biased. Collector-base junction is reverse biased. Control, IC=β IB (as can be seen from previous figure, there is a small slope of IC with VCE.) Condition: VBE < VCE < VCC

4. Breakdown region: IC and VCE exceed specifications.

Damage to the transistor.

Input characteristics

Amplifier

Output clipping due to fluctuations in input voltage

Q point

The Q-point represented on the IC versus VCE characteristic plot

Darlington PairThe Darlington Transistor named after its inventor Sidney Darlington, is a special arrangement of two standard NPN or PNP bipolar junction transistors (BJT) connected together to produce a

more sensitive transistor with a larger current gain.

Current gainTransistors have a characteristic called current gain. This is referred to as its hFE.

The amount of current that can pass through the load when connected to a transistor that is turned =the input current x the gain of the transistor (hFE).The current gain varies for different transistor and can be looked up in the data sheet for the device.

Typical values ranges from 25, 50,100 to 300

Need for Darlington pairIn some application the amount of input current available to switch on a transistor is very low. This may mean that a single transistor may not be able to pass sufficient current required by the load. As we know IO=II x hFE

If it is not be possible to increase the input current then we need to increase

the gain of the transistor.

This can be achieved by using a Darlington Pair.

Contd...A Darlington Pair acts as one transistor but with a current gain that equals:

Total current gain (hFE total) = current gain of transistor 1 (hFE t1) x current gain of transistor 2 (hFE t2)

Imp note: Normally to turn on a transistor the base input voltage of the transistor will need to be greater that 0.7V. As two resistors are used in a Darlington Pair this value is doubled. Therefore the base voltage will need to be greater than 0.7V x 2 = 1.4V.

Uni Junction Transistor (UJT)

The UJT as the name implies, is characterized by a single pn junction. It exhibits negative resistance characteristic that

makes it useful in oscillator circuits.

Note: A complementary UJT is formed by a P-type base and N-type emitter. Except for the polarity of voltage and current, the characteristic is similar to those of a conventional UJT.

Relationship between the emitter voltage and current.

We have 3 regions of operation1. Cut off region (From VE=0 to VE=VP)2. Negative resistance region (From VE=VP to VE=Vv i.e, till valley

point(IV,VV)).3. Saturation region (Beyond valley point). which exhibits a positive

resistance characteristic

Application of UJT as relaxation oscillator

Field Effect Transistor(FET)The field effect transistor is a semiconductor device, that uses electric field control of current.Two types,

JFET (Junction Field Effect Transistor) MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

Advantages of FETIn a conventional transistor, the operation depends upon the flow of majority and minority carriers. That is why it is called bipolar transistor. In FET the operation depends upon the flow of majority carriers only. It is called unipolar device. The input to conventional transistor amplifier involves a forward biased PN junction with its inherently low dynamic impedance. The input to FET involves a reverse biased PN junction hence the high input impedance of the order of M-ohm. It is less noisy than a bipolar transistor. It exhibits no offset voltage at zero drain current. It has thermal stability. It is relatively immune to radiation.

N type semiconductor bar

JFET

Drain current vs drain to source voltage

N-channel JFET with a reverse gate source

voltage

JFET used as a voltage controlled resistor (VCR)

Contd…

The circuit is a voltage divider attenuator. The output voltage is

VOUT = VIN rDS/(R + rDS)

Imp note: It is assumed that the output voltage is not so large as to push the VCR out of the linear resistance region.