Tipuri de tranzistori

8/11/2019 Tipuri de tranzistori

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Electrical Science1

The Bipolar Junction Transistor ( B.J.T.)


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Electrical Science2

Symbols


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Electrical Science3

How to remember the Symbols

The NPN transistor has an arrow that is:

N not P pointing

N in


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Electrical Science4

Transistor Biasing The base emitter junction must be forward

biased.

The base collector must be reversebiased.

Ad

Add the two batteries to the circuit on the right.Use the correct bias.

What type of transistor is this ?


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Electrical Science5

CBE III +=

This Rule must be memorised!

Transistor Currents


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Electrical Science6

Question

A transistor has a collector current of 0.1 Amps.

The base current is 100 micro Amps.

Find the emitter current, in milli amps.


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Electrical Science7

Solution:

First we convert all units to milli amps:

1) Collector current = 0.1 Amps. This is 100 milli Amps.

Base current is 100 micro Amps. This is 0.1 milli amps.

2) The formula:

Emitter current = 0 . 1 mA + 100 mA.

Emitter current = 100 . 1 mA

CBE III +=


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Electrical Science8

How transistors work

A small tap can control

the flow of water from

a pressure hose.

A small signal on the baseof a transistor can controlthe flow of current

from collector to emitter.

Tap = Base


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Electrical Science9

A Transistor amplifies signals.

The transistor makes the

signal louder.

A small signalis sent in.

A large signal comes out.

Unlike a resistor or capacitor.

a transistor must be poweredBy a battery.

We say it is an active device.

For example a radio receives a weak signal at the aerial Vin.It is amplified using a transistor and sent out to the speaker V out.

V in

The battery power supply is called Vcc,

because the voltage is connected indirectly to the collector.

V out


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Electrical Science10

Using an analogue meter to test Transistors


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Testing a Transistor in Circuit with a multimeter

o Vce Vc

Vbe

12 V

+

V

V

V

V

5 V

The Base Emitter PN junction uses 0.7 volts toturn on the transistor, which joins the collector to the emitter.Question: a)State the voltage reading on each meter.

b) Name these voltage readings. Vcc,Vce,Vbe, Vbb.

Vbb

Vcc


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Parameters

1) Current Gain:

B

C

fe

DCI

I

hcalledalsoisthis

currentbase

currentcollector==B

3) POWER:Transistor can only dissipate a limited amount of power.If the maximum power is exceeded the transistor willoverheat and be damaged.

2) CURRENTTransistor can only carry a limited current. If themaximum current is exceeded the transistor willoverheat and be damaged.


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QuestionA transistor has a maximum power rating of half

a watt. Its maximum voltage supply VCE is 20V

and maximum current IC is 50 mA.a) Fill in the following grid.

b) Will any of these situations damage the

transistor?


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Solutiona)

b) Yes. When the current is100mA the limit is exceeded,

and the transistor damaged.


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Heat Sinks

To cool down some transistors metal plates are attachedto them.

Without these heat sinks they would overheat.

These remove heat from the transistor just like the metalfins on a motor bike, or a radiator in a car.


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Transistor Cases

emitter

base

collector


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The Transistor as a Switch.

The LED will light because theBase has more than 0.7V whichturns the transistor on so the

collector is connected to theemitter.

Find the voltage at A.

+ V210V

D1

LED1

Q1

NPN

R450

R31k

R2100

R1100

ResistorA

500


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volts3.3V

005.00660V

R.IV

--------------------

volt66V

1000.00660V

R.IV

--------------------A.00660I

5001000

10I

RR

VI

4

4

4r44

3

3

3r33

r4andr3

r4andr3

43

r4andr3

r4andr3

=

=

=

=

=

=

=

+

=

+

=

r

r

r

r

r

r


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The Transistor as a Switch.

This LED will NOT light becausethe Base has less than 0.7V whichturns off the transistor and so the

collector is disconnected from theemitter.

Find the voltage at point A.

ResistorA

50

1K


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volts48.0V

05.009520V

R.IV

--------------------

volt52.9V

1000.009520V

R.IV

--------------------A.009520I

501000

10I

RR

VI

4

4

4r44

3

3

3r33

r4andr3

r4andr3

43

r4andr3

r4andr3

=

=

=

=

=

=

=

+

=

+

=

r

r

r

r

r

r


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22/39

Electrical Science22volts9V

9000.1000

1V

R.IV

--------------------

volt1V

1000.1000

1

V

R.IV

--------------------

mA.1I

10000

10I

RR

VI

4

4

4r44

3

3

3r33

r4andr3

r4andr3

43

r4andr3r4andr3

=

=

=

=

=

=

=

=

+

=

r

r

r

r

r

r


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State the voltage bias on the base.VCC+30V

R122k

RC1.0k

R210k

RE1.0k

Solution : The voltage at the base is 9.38 Volts.

A

Resistor


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Question

If the current gain BDC is 50 finda) IBb) ICc) IEd) VBe) VC

+ VCC20V

+ VBB3V

RC

1.0kRB10k


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+ VCC20V

+ VBB3V

RC1.0k

RB10k

IBIc

IE

VB

VC


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Solution a) First we find IB

IB is the current in the base. Current is V/R V is 3 - 0.7 dropped by the base emitter PN

junction.

The base current = 2.3/10,000

IB = 0.000230 Amps.

IB = 0.230 milli Amps. IB = 230 micro Amps.


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Solution b) To find IC

We know: Gain means output compared toinput.

Gain = IC / IB

Multiply both sides by IB

Gain IB = IC

IC = 50 . 230 u AmpsIC = 11500 u Amps

IC = 11. 5 mA.


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Electrical Science 28

Solution c)

To find IE. IE = IC + IB =11.7 mA

Solution d) VB is always 0.7 volts higher than the voltage at the emitter. In this case the emitter is grounded so VB is 0.7 volts

Solution e) Vc is Vcc the voltage dropped across Rc. Vc = 20 (11.5 mA)(1 k) Vc = 8.5 volts.


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a) You measure the voltage across

R1 as 4.5 volts.

Find Vce.

b) R5 uses 0.1 m Volts.

Find the current through it.

c) Find Ic.

d) Find the current Gain of the

circuit.

e) Find Ie.

+ V210V

D1LED1

Q1NPN

R4

50

R31k

R2100

R1100

R5 = 120 ohms

Question:The transistor is not fully on.

We say it is not saturated.


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Answer A) The supply is 10 volts

R1 uses 4.5 volts.

R2 uses 4.5 volts. Same resistance and current.

The diode used 0.7 volts.

These add up to 9.7 volts. So the transistor uses 0.3 volts between collector and

emitter and this is called Vce.

B) Using Ohms Law :

uA0.83I

A0.00000083I

120

Volts10000I

120

Voltsm10I

resistorTHAT

resistorTHATacrossvoltageTheresistorarouthcurrent thThe

R5

R5

R5

R5

=

=

=

=

=


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c) Solution( Use Ohms Law)

Ampsm45I

amps045.0I

100volts4.5I

resistorTHAT

resistorTHATacrossvoltageTheresistorarouthcurrent thThe

R1

R1

R1

=

=

=

=


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Solution part d

54216Gain

ma000830

ma45Gain

Ib

IcGain

=

=

=


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ma45.00083Ie

mA.00083ma45IeIbIcIe

d)

=

+=

+=


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Q15 March 2005

Redraw the transistors and show the direction of conventional current


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Q15 March 2005

Solution:1) Fill in the P and N terminals.2) Fill in the battery + and -.

The Base emitter must be forward biased. The base collector reverse biased.

N

N

N

P

P

P

+

+

+

-

-

-


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Q15 March 2005

Solution:

Conventional current flows from the + of the battery to theof the battery.

N

N

N

P

P

P

+

+

+

-

-

-


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Q15 May 2005

The hfe of an NPN transistor is 250.

If the transistor is operating under non-saturated conditions in a circuit andthe collector current found to be 25mA determine the base current.


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Solution100 micro amps.

Saturation: means the transistor is turned on fully.Increasing the voltage on the base will have no effect.


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Question

A transistor circuit has the follow measurements.

Vcc 30 VIB = 0.25 mA

Ic = 50 mA

Find IE.

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