Electronics notes

1. Transistor
 A three lead semiconductor device that acts as:
 electrically controlled switch
 current amplifier
 digital logic design.
 Transistor is analogous to a faucet.
– Turning faucet’s control knob alters the flow
rate of water coming out from the faucet.
– A small voltage/current applied at transistor’s
control lead controls a larger current flow through its
other two leads.

3. Transistor Types
Bipolar Junction Transistor (BJT)
– NPN and PNP
Field Effect Transistor (JFET)
– N-channel and P-channel
Metal Oxide Semiconductor FET (MOSFET)
– Depletion type (n- and p-channel)
– Enhancement type (n- and p-channel)

4. BJT Structure
The BJT has three regions called the emitter, base, and
collector. Between the regions are junctions as indicated.
B
(base)
C(collector)
n
p
n
Base-Collector
junction
Base-Emitter
junction
E (emitter)
B
C
p
n
E
p
npn pnp
The base is a thin
lightly doped region
compared to the
heavily doped emitter
and moderately doped
collector regions.
The BJT is constructed with three doped semiconductors
region separated by two PN junctions.

5.  NPN
 PNP
 NPN: a small input current and a
positive voltage applied @ its base
(with VB>VE) allows a large current to
flow from collector to emitter
 PNP: a small output current and a
negative voltage @ its base (with
VB<VE) allows a much larger current
to flow from emitter to collector.
BJT Types

7. BJT Water Analogy
NPN (VB > VE)

8. BJT Water Analogy
PNP (VB < VE)

9. BJT Operation
In normal operation, the base-emitter is forward-biased
and the base-collector is reverse-biased.
npn
For the pnp type, the voltages
are reversed to maintain the
forward-reverse bias. –
+ –
+
–
+
+
BCreverse-
biased
–
BE forward-
biased
–
+
+
–
–
BCreverse-
biased
+
BE forward-
biased
–
+
pnp
For the npn type shown, the
collector is more positive
than the base, which is more
positive than the emitter.

11. BJT Currents
IE IE
IC
IB
IC
IB
n
p
n
p
n
p
+
– +
–
–
+
IE
IC
IB
+
–
+
IE
IC
IB
+
–
–
npn pnp
The direction of conventional current is in the direction of the arrow
on the emitter terminal. The emitter current is the sum of the
collector current and the small base current.
That is, IE = IC + IB.

12. Beta
 The common-emitter current gain is defined as a beta and it is
the ratio of collector current to base current.
 It is represented by βDC . It is the ratio of a dc collector current
(IC) to the corresponding dc base current (IB) and it is represented
by
 βDC =IC / IB

13.  The ratio of the dc collector current (IC) to the dc emitter
current (IE) is the dc alpha (α DC )
 (αDC )= IC / IE

14. BJT Regions of Operation
Bipolar Junction transistors have the ability to operate within
three different regions:
1. Active Region - the transistor operates as an amplifier and Ic =
β.Ib
2. Saturation - the transistor is "fully-ON" operating as a switch
and Ic = I(saturation)
3. Cut-off - the transistor is "fully-OFF" operating as a switch
and Ic = 0

15. BJT Regions of Operation
The first region is called cutoff. This is the case where the transistor is
essentially inactive. In cutoff, the following behavior is noted:
 IB = 0 (no base current)
 Ic = 0 (no collector current)
 VBE < 0:7V (emitter-base junction is not forward biased)
.
Whenever we observe the
terminals of a BJT and see that
the emitter-base junction is not
at least 0.6-0.7 volts or base
lead open, the transistor is in
the cutoff region.
In cutoff, the transistor appears
as an open circuit between the
collector and emitter terminals.
IB =0 –
+
–
+
ICEO
RC
VCC
VCE ≅VCC
RB

16. Cutoff
 In a BJT, cutoff is the condition in which there is no base
current, which results in only an extremely small leakage
current (ICEO) in the collector circuit.
 For practical work, this current is assumed to be zero.
IB =0 –
+
–
+
ICEO
RC
VCC
VCE ≅VCC
RB
 In cutoff, neither the base-
emitter junction, nor the base-
collector junction are forward-
biased.
 Both base-emitter junction and
base-collector junction are
reverse -biased

17. Saturation
 The second region is called saturation. In saturation, the
following behavior is noted:
 VCE 0<0.2V ; In this case, VCE assumes the value
VCE(sat)
 IB > 0; and IC > 0
 VBE 0 >0.7V
 Saturation is where the base current has increased well beyond
the point that the emitter-base junction is forward biased.
 In fact, the base current has increased beyond the point where
it can cause the collector current flow to increase.
 In saturation, the transistor appears as a near short circuit
between the collector and emitter terminals.

18. Saturation
 In a BJT, saturation is the condition in which there is
maximum collector current.
 The saturation current is determined by the external circuit
(VCC and RC in this case) because the collector-emitter
voltage is minimum (≈ 0.2 V)
 In saturation, an increase of
base current has no effect on
the collector circuit and the
relation IC = bDCIB is no
longer valid.
–
+ –
+
VCC
VBB
VCE = VCC – IC RC
RB
RC
IB
IC
–
+
– +

19. Active Region
 The final region of operation of the BJT is the forward active
region.
 It is in this region that the transistor can act as a fairly linear
amplifier. In this region:
 0:2 < VCE < Vcc ; where Vcc is the supply voltage
 IB > 0, and Ic > 0
 VBE 0:7V

20. The collector characteristic curves show the relationship
of the three transistor currents.
The curve shown is for a fixed
based current. The first region is
the saturation region.
BJT Characteristics
IC
B
C
A
0 0.7 V VCE(max)
VCE
Saturation
region
Active region
Breakdown
region
As VCE is increased, IC increases
until B. Then it flattens in region
between points B and C, which
is the active region.
After C, is the breakdown
region.

21. Active Region
• Transistor is on and the collector to emitter voltage is
somewhere between the cutoff and saturated states.
• In this state, the transistor is able to amplify small variations in
the voltage present on the base. The output is extracted at the
collector.
• In the forward active state, the collector current is proportional
to the base current by a constant multiplier called beta, denoted
by the symbol β
Thus, in the forward active region we will also observe
that:
Ic = β*Ib

22. The collector characteristic curves illustrate the relationship of the
three transistor currents.
0
IC
VCE
IB6
IB5
IB4
IB3
IB2
IB1
IB = 0
Cutoff region
By setting up other values of
base current, a family of
collector curves is developed.
bDC is the ratio of collector
current to base current.
BJT Characteristics
It can be read from the curves.
The value of bDC is nearly the
same wherever it is read.
C
DC
B
I
I
b 

23. What is the bDC for the transistor shown?
Choose a base current near the
center of the range – in this
case IB3 which is 30 mA.
IC
VCE
IB6
IB5
IB4
B3
I
IB2
B1
I
IB = 0
= 10 A
m
= 20 A
m
= 30 A
m
= 0
= 60 A
m
= 40 A
m
= 50 A
m
10.0
8.0
6.0
4.0
2.0
0
(mA)
Read the corresponding
collector current – in this case,
5.0 mA. Calculate the ratio:
C
DC
B
5.0 mA
30 A
I
I
b
m
   167
BJT Characteristics

24. DC Load Line
the transistor. It is drawn by
connecting the saturation
and cutoff points.
The transistor characteristic
curves are shown superimposed
on the load line. The region
between the saturation and
cutoff points is called the
active region.
The DC load line represents the circuit that is external to
0
IC
VCE
IB =0 Cutoff
VCE(sat) VCC
IC(sat)
Saturation

25. DC and AC Quantities
The text uses capital letters for both AC and DC currents and voltages
with rms values assumed unless stated otherwise.
DC Quantities use upper case roman subscripts. Example: VCE.
(The second letter in the subscript indicates the reference point.)
AC Quantities and time varying signals use lower case italic
subscripts. Example: Vce.
Internal transistor resistances are indicated as lower case
quantities with a prime and an appropriate subscript. Example: re
’.
External resistances are indicated as capital R with either a
capital or lower case subscript depending on if it is a DC or ac
resistance. Examples: RC and Rc.