Transcript Slide 1

Analog Electronics
Lecture 4:Transistors
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Semiconductor material and pn-junction diode
P-type semiconductor
N-type semiconductor
PN- Junction
Diode
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Diode and its resistive behavior
R=0
R = inf
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Transformative Resistor
Controlled Resistor
Signal controlled
transformative resistor
R
Control signal
Vsup
Transformative Resistor
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
How does it switch?
It leads to an electronic switch
Controlled Resistor
Signal controlled
transformative resistor
R
Control signal
Vsup
Transformative Resistor
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
How does it amplify?
It leads to signal amplification
Controlled Resistor
Signal controlled
transformative resistor
R
Control signal
Vsup
Transformative Resistor
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
How does it amplify?
Constructing amplifying device
A semiconductor is doped with
penta and tri valent impurities to
divide it into three regions.
Three regions make 2 PN junctions.
A region called ‘Emitter’ is heavily doped as an n-type material. It has an excess
of electrons in conduction band.
Next to Emitter a lightly doped p-region with fewer holes is called ‘Base’
The 3rd region is n-type and is called ‘Collector’
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Constructing amplifying device
The heavily doped ntype emitter region
has a very high
density of
conduction-band
(free) electrons.
These free electrons
easily diffuse through the
forward biased BE
junction into the lightly
doped and very thin ptype base region.
The base has a low density of holes, which are the
majority carriers.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
How does it amplify?
Constructing amplifying device
Note the junction biases for
this discussion.
A very little free electrons recombine with holes
in base and move as valence electrons through
the base region and into the emitter region as
hole current.
The valence electrons leave the crystalline
structure of the base and become free electrons in
the metallic base lead and produce the external
base current.
Majority of free electrons move toward the reverse-biased BC junction and swept
across into the collector region by the attraction of the positive collector supply
voltage.
The free electrons move through the collector region, into the external circuit, and
then return into the emitter region along with the base current.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
How does it amplify?
Transistor Currents
–
+
–
+
IC
IC
IC
IC
IB
+
n
IB
IB
–
+
p
n
p
IB
–
n
p
IE
IE
IE
npn
–
IE
–
pnp
+
+
The conventional current flows in the direction of the arrow on the
emitter terminal. The emitter current IE is the sum of the collector
current IC and the small base current IB.
That is, IE = IC + IB.
The emitter current is slightly more that collector current.
The voltage drop between base and emitter is VBE whereas the voltage drop
between collector and base is called VCE.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
How does it amplify?
Current Relationships
The collector current is directly proportional to the base
current.
IC ∝ IB
βDC as constant of proportionality.
IC = βDC IB
This eq explains amplification of current.
Ratio of DC collector current and DC base current.
βDC == IC/IB
Ratio of DC collector current to the DC emitter current.
Collector and emitter currents are ca
IC = αDC IE,
αDC is always less than 1
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Bipolar Junction Transistors
This is Bipolar Junction Transistor
This semiconductor device with three doped regions:
C (collector)
i. Emitter
ii. Base
iii. Collector
And two pn junctions:
i.
ii.
Base-emitter junction.
Base-collector junction
C
npn
n
B
(base)
pnp
Base-Collector
junction
p
n
p
B
Base-Emitter
junction
E (emitter)
n
p
E
The device is named as ‘Bipolar Junction
Transistor’ or BJT
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
More about BJTs
BJT Biasing
In order for a BJT to operate properly , the two pn junctions
must be correctly biased with external dc voltages.
For the npn type shown, the
collector is more positive
than the base, which is more
positive than the emitter.
For the pnp type, the voltages
are reversed to maintain the
forward-reverse bias.
BC reversebiased
BC reversebiased +
–
–
–
++
– –
BE forward- +
+
biased
BE forwardbiased
+
–
+
–
–
+
npn
pnp
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
BJT Circuit Analysis
Currents and voltages in BJT
IB : dc base current
IE: dc emitter current
IC: dc source current
VBE: dc voltage at base wrt. emitter
VCE: dc voltage at collector wrt.
emitter.
VCB: dc voltage at collector wrt. Base.
VBE = 0.7V
VCE = VCC – IC RC
IB = (VBB – VBE ) / RB
VCB = VCE – VBE
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
BJT Collector Characteristics Curve
The collector characteristic curves shows three mode
of operations of transistor with the variation of
collector current IC w.r.t VCE for a specified value of
base current IB.
VBB is set to produce a certain
value of IB and VCC is zero and
VCE is zero.
IC
C
B
As VCE is increased, IC increases
until B.
Both BE and BC junctions are
forward biased and the
transistor is in Saturation region.
Electronic Devices, 9th edition
Thomas L. Floyd
Saturation
region
A
0
0.7 V
VCE(max)
VCE
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
BJT Collector Characteristics Curve - Saturation
In saturation, an increase of base current has no effect on the
collector circuit and the relation IC = bDCIB is no longer valid.
IC(SAT) =VCC –VCE(SAT) /RC
–
RC
+
IC
RB
+
VCE = VCC – IC RC
+
At this point, the transistor
current is maximum and voltage
across collector is minimum, for a
given load.
Electronic Devices, 9th edition
Thomas L. Floyd
VBB
IB
–
+
–
VCC
–
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
BJT Collector Characteristics Curve - Linear
As VCE is increased furthers and exceeds 0.7V the base-collector
junction becomes reverse-biased and the transistor goes into the
active, or linear, region of its operation.
IC levels off and remains
essentially constant for a
given value of IB as VCE continues
to increase.
IC
B
Active region
C
the value of IC is determined
only by the relationship expressed
as
A
0
Electronic Devices, 9th edition
Thomas L. Floyd
0.7 V
VCE(max)
VCE
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
BJT Collector Characteristics Curve family
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.
bDC 
I B6
I B5
IC
IB
It can be read from the curves.
The value of bDC is nearly the
same wherever it is read in
active region.
Electronic Devices, 9th edition
Thomas L. Floyd
IC
I B4
I B3
I B2
I B1
Cutoff region
0
IB = 0
VCE
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
BJT Collector Characteristics Curve –Cut off
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.
RC
In cutoff, neither the base-emitter
junction, nor the base-collector
junction are forward-biased.
Electronic Devices, 9th edition
Thomas L. Floyd
RB
+
ICEO
VCE ≅ VCC
IB = 0
–
+
–
VCC
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
BJT Switches
A BJT can be used as a switching device in logic circuits to turn on or
off current to a load. As a switch, the transistor is normally in either
cutoff (load is OFF) or saturation (load is ON).
+ VCC
RC
RB
IC = 0
RC
RC
RB
C
+VBB
0V
IB = 0
E
In cutoff, the transistor
looks like an open switch.
Electronic Devices, 9th edition
Thomas L. Floyd
+VCC
+VCC
IB
+VCC
IC(sat)
IC(sat)
RC
C
+
–
E
In saturation, the transistor
looks like a closed switch.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
DC Load Line
IC
Saturation
IC(sat)
Here VCE = 0 and
IC = IC-Sat = VCC -VCE(Sat) / RC
Cutoff
IB = 0
0 VCE(sat)
V CE
VCC
Here IB = 0 and
VCE = VCC
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
The DC Operating Point
Bias establishes the operating point (Q-point) of a transistor amplifier;
the ac signal moves above and below this point.
Improper biasing can cause distortion in the output signal as the
transistor may go into the saturation and cutoff region.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
The DC Operating Point
The point at which the load line intersects a characteristic
curve represents the Q-point for that particular value of IB.
Point A,Q,B represents the Qpoint for IB 400mA. 300 mA
and 200 mA respectively.
Assume a sinusoidal Ib is
superimposed on VBB
varying between 200uA to
400uA. It makes the collector
current varies between 20 mA
and 40 mA.
IC (mA)
Load line
Ic
40
ICQ
Ib
A
400 µ A
Q
300 µ A = IBQ
30
B
20
0
1.2
3.4
VCEQ
200 µ A
VCE (V)
5.6
Vce
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
The DC Operating Point
A signal that swings
outside the active
region will be clipped.
For example, the
bias has established
a low Q- point. As a
result, the signal is
will be clipped
because it is too
close to cutoff.
Electronic Devices, 9th edition
Thomas L. Floyd
IB
Q
IC
ICQ
Input
signal
Q
Cutoff
0
VCC
VCE
Cutoff
Vce
VCEQ
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Summary
Voltage-Divider Bias
A practical way to establish a Q-point is to form a voltagedivider from VCC.
+V
+V V
+15
R1 and R2 are selected to establish VB. If
the divider is stiff, IB is small compared
R
R
 R2 
R
R
to I2. Then,
27
kW
VB  
V
1.2
kW
 CC
CC
CC
1
1
 R1  R2 
IB
Determine the base voltage for the
 R2 
circuit.
V 
V
B
Electronic Devices, 9th edition
Thomas L. Floyd

 CC
 R1  R2 
12 kW



  15 V  
 27 kW  12 kW 
C
C
βDC = 200
I2
R
R22
12 kW
R
REE
680 W
4.62 V
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Voltage-Divider Bias
A practical biasing technique that utilize single biasing sources instead
of separate VCC and VBB.
A dc bias voltage at the base of the transistor can be developed by a
resistive voltage divider that consists of R1 and R2
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
DC Load Line
RC
What is the saturation current and
the cutoff voltage for the circuit?
Assume VCE = 0.2 V in saturation.
ISAT 
βDC = 200
+
V BB
3V –
220 kW
–
VCC
15 V
VCO  VCC  15 V
3.0 V  0.7 V
 10.45 m A
220 kW
IC = b IB = 200 (10.45 mA) = 2.09 mA
Electronic Devices, 9th edition
Thomas L. Floyd
+
RB
VCC  0.2 V 15 V  0.2 V

 4.48 mA
RC
3.3 kW
Is the transistor saturated? I B 
3.3 kW
Since IC < ISAT, it is not saturated.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
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.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
The FET
The idea for a field-effect transistor (FET) was first
proposed by Julius Lilienthal, a physicist and inventor. In
1930 he was granted a U.S. patent for the device.
His ideas were later refined and
developed into the FET. Materials
were not available at the time to
build his device. A practical FET
was not constructed until the
1950’s. Today FETs are the most
widely used components in
integrated circuits.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
The JFET
The JFET (or Junction Field Effect Transistor) is a normally ON device.
The n-channel is connected with two leads i.e. Drain and Source
Two p-type regions are diffused in n-type material and both connected to
gate.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
The JFET Construction
For the n-channel device, when the drain is positive with
respect to the source and there is no gate-source voltage,
there is current in the channel.
When a negative gate voltage is
applied to the FET, the electric
field causes the channel to
narrow, which in turn causes
current to decrease.
Electronic Devices, 9th edition
Thomas L. Floyd
RD
D
n
G
VGG
–
+
+
p
p
–
VDD
n
S
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
The JFET-Symbol and biasing
The symbol for an n-channel JFET is
shown, along with the proper polarities of
the applied dc voltages. For an n-channel
device, the gate is always operated with a
negative (or zero) voltage with respect to
the source.
RD
Drain
Gate
–
VGG
Electronic Devices, 9th edition
Thomas L. Floyd
+
VDD
–
Source
+
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
JFET Biasing
Self-bias is simple and effective, so it is the most common
biasing method for JFETs. With self bias, the gate is
+V = +12 V
essentially at 0 V.
DD
An n-channel JFET is illustrated. The current
in RS develops the necessary reverse bias that
forces the gate to be less than the source.
RD
1.5 kW
VG = 0 V
Assume the resistors are as shown and the
drain current is 3.0 mA. What is VGS?
+
RG
VG = 0 V; VS = (3.0 mA)(330 W) = 0.99 V
VGS = 0 – 0.99 V =  0.99 V
Electronic Devices, 9th edition
Thomas L. Floyd
1.0 MW
RS
–
IS
330 W
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
The E-MOSFET
The metal oxide semiconductor FET uses an insulated gate
to isolate the gate from the channel. Two types are the
enhancement mode (E-MOSFET) and the depletion mode
(D-MOSFET).
E-MOSFET
An E-MOSFET has no
channel until it is induced by
a voltage applied to the gate,
so it operates only in
enhancement mode. An nchannel type is illustrated
here; a positive gate voltage
induces the channel.
Electronic Devices, 9th edition
Thomas L. Floyd
RD
Drain
ID
SiO2
Gate
Induced
channel
n
+
+
+
+
p substrate
n
+
VGG
n
–
–
–
–
+
–
VDD
n
–
Source
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
The D-MOSFET
The D-MOSFET has a channel that can is controlled by the
gate voltage. For an n-channel type, a negative voltage
depletes the channel; and a positive voltage enhances the
channel.
D-MOSFET
RD
A D-MOSFET can
operate in either
mode, depending on
the gate voltage.
RD
n
–
VGG
+
–
–
–
–
–
–
+
+
+
+
+
+
n
p
+
–
VDD
–
–
–
–
–
–
p
+
–
VDD
+
n
operating in D-mode
Electronic Devices, 9th edition
Thomas L. Floyd
+
+
+
+
+
+
VGG
–
n
operating in E-mode
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
The MOSFET
MOSFET symbols are shown. Notice the broken line
representing the E-MOSFET that has an induced channel.
The n channel has an inward pointing arrow.
E-MOSFETs
D
D
G
n channel
Electronic Devices, 9th edition
Thomas L. Floyd
G
G
S
D-MOSFETs
D
D
S
p channel
G
S
S
n channel
p channel
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Thank You
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.