Chapter 37 Transistors I. Transistor - Is a current control device A. Are made of P and N material. B. Have three leads. C. A small current or voltage at one lead can control a much larger current at the other two leads. D. Can be used as a current switch turning on and off current. E. Act as variable resistors to control current flow.

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Chapter 37 Transistors

I.
Transistor
-

Is a current control device

A. Are made of P and N material.

B. Have three leads.

C. A
small

current or voltage at one lead can control a much larger current at the other two
leads.

D. Can be used as a current swi
tch turning on and off current.

E. Act as variable resistors to control current flow.

F. Act as amplifiers to increase voltage or current.


II.
Bipolar
-
Junction Transistors (BJTs)

A. Output is controlled by a small input current.

B. Constriction

1. A
crystal containing 3 separate regions.

a.
Can be a PNP









b.
Can be NPN.









*Note: Will concentrate on the NPN (electrically similar to the PNP but the I and V are opposite)



2.
Base/Gate (middle)
-

Thinner than the ends and has fewer d
oping atoms.

a.
Controls current flow through the transistor.

3. Emitter (end)
-

Heavily doped and acts as the main source of electron current.

4. Collector (end)
-

Moderately doped and receives most of the electrons from the
emitter.







C. Biasing
-

The

2 junctions (emitter
-
base and collector
-
base) must be biased correctly for
the transistor to work.

1. Emitter
-
Base is always connected forward biased with voltage and resistance
values selected to produce a small current flow ( a very small base current
will control
the very large collector current).

2. Collector
-
Base always connected in reverse bias.

a. Voltage must be higher than the emitter
-
base voltage.

D. Operation
-

Key to good operation i
s

correct biasing.

1. 2 voltage sources are used to bias
the junctions.

2. The higher the base current (Ib) the higher the collector current (Ic).

a. A small increase in Ib results in a large increase in Ic.

3. Power up

a. Ib flows so Ic flows(emitter
-
base junction allows electrons to jump the
barrier).

b.
Most of the current flows to the collector(95%) attracted by the positive
polarity with the remainder flowing through the base.

1. Collector current
-

mA to Amps.

2. Base currents
-

uA to mA.

E. Current Gain
-

Measure of how well the circuit can ampli
fy small base currents.

1. Also know as Beta. B = Ic/Ib.

a. Range from 10 to several hundred.

F. Packages and Ratings

1. Small
-
Signal Transistors

a. In metal, molded epoxy, or plastic case

b. Voltage, current, and power

ratings

2. Large
-
Power Trans
istors

a. Same as above but on a larger scale.

b. Some are mounted on heat sinks to dissipate heat

3. Handling Precautions

a. Keep away from excessive heat

b. Don
=
t exceed capacity ratings.

c. Avoid voltage spikes(transient voltage).

G. Testing

1.

Use an ohmmeter
-

procedure is the same as testing a diode.

2. Multimeter
-

diode function (Si should read .
6
V

to .7V
).


III. Field Effect Transistors (FETs)

A. Uses very little current.

B. Controls output current
by
a
varying
A
electric field
@

(contr
olling
input voltage)




(BPJ use input current to control output
current).






C. Construction











1. Source

2. Gate (input)

3. Drain (output)

4. FETs are unipolar
-

Current flows through only 1 type of material.

D. Are faster and need l
ess drive power than BJTs.


IV. Junction FET (figure 37
-
1
5
)

A. Come in both N and P channel (polarity is opposite the channel material).

**Will only cover the N
-
channel (P channel is just the opposite).

B. Operation (figure 37
-
17)

1. N
-
Channel acts as
a resistor and conducts a current between drain and source.

2. Biasing

a. Positive fixed voltage on drain
-

Causes current flow.

(1). JFETs are normally on (full current flowing with no gate circuit).

b. Negative voltage on gate (reverse biased)
-

Cau
ses depletion region to

constrict cutting off current
(pinching
-
off)

(current is controlled when a gate
circuit is connected).

c. Never forward bias the JFET
-

causes excessive current flow.


V. MOSFET
-

Metal Oxide FET

A. Most popular transistor.

(Has a

glass
-
like insulation)

B. Works like a FET except the gate has no electric contact with the source or drain.







C. Come in both N
and P channel.

1. Action is
the same, but polarities are
reversed.

D. Made in 2 ways

1. Depletion mode

(D
-
type)

-

Normally on (figure 37
-
20
)

(Has a channel).

2. Enhanced mode
(E
-
type)
-

Normally off (figure 37
-
2
1
)

(has a no channel).

E. Operation

(D
-
type)

1. No voltage applied to the gate
-

normal current flow.

2. Positive
voltage

is applied to the gate more cur
rent flow.

3. Negative voltage applied to the gate drain current slows.


F. Operation (E
-
type)


gate to source voltage must exceed the threshold voltage.

G
. Testing

1. Cannot use a ohm meter

2. Must use a special MOSFET tester.


VI. Unijunction Tran
sistors

(UJT)









A. Works as a voltage controlled switch (Base 1 to emitter switch).

B. Does not amplify.

C. Leads

1. Base 1
-

Lightly doped



2. Base 2
-

Lightly doped

3. Emitter
-

heavily doped

D. Operation

1. Resistance between base 1 and
base 2 is high when emitter is open.

2. Voltage is applied to the bases (B2 + and B1 is
-
) then a signal (voltage)is applied
between emitter and base 1.

a. Zero emitter voltage
-

small current between B1 and B2
-

no current
through B1 and emitter.

b. Sm
all positive emitter voltage
-

threshold voltage is reached UJT switches
on
-

High current between emitter and B1.


VII. Phototransistors
-

Usually a NPN BJT with a light sensitive, collector
-
base junction (PN).

A. When the collector
-
base junction is exp
osed to light it creates a control current that
switches the transistor on.

1. The brighter the light the more current.

B. Construction
-

could have
two

or
three

leads.