tipuri de tranzistori
TRANSCRIPT
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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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Electrical Science11
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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Electrical Science12
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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Electrical Science13
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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Electrical Science14
Solutiona)
b) Yes. When the current is100mA the limit is exceeded,
and the transistor damaged.
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Electrical Science15
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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Electrical Science16
Transistor Cases
emitter
base
collector
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Electrical Science17
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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Electrical Science18
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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Electrical Science19
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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Electrical Science20
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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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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Electrical Science23
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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Electrical Science24
Question
If the current gain BDC is 50 finda) IBb) ICc) IEd) VBe) VC
+ VCC20V
+ VBB3V
RC
1.0kRB10k
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Electrical Science25
+ VCC20V
+ VBB3V
RC1.0k
RB10k
IBIc
IE
VB
VC
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Electrical Science26
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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Electrical Science27
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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Electrical Science 29
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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Electrical Science 30
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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Electrical Science 31
c) Solution( Use Ohms Law)
Ampsm45I
amps045.0I
100volts4.5I
resistorTHAT
resistorTHATacrossvoltageTheresistorarouthcurrent thThe
R1
R1
R1
=
=
=
=
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Electrical Science 32
Solution part d
54216Gain
ma000830
ma45Gain
Ib
IcGain
=
=
=
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Electrical Science 33
ma45.00083Ie
mA.00083ma45IeIbIcIe
d)
=
+=
+=
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Electrical Science 34
Q15 March 2005
Redraw the transistors and show the direction of conventional current
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Electrical Science 35
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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Electrical Science 36
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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Electrical Science 37
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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Electrical Science 38
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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Electrical Science 39
Question
A transistor circuit has the follow measurements.
Vcc 30 VIB = 0.25 mA
Ic = 50 mA
Find IE.