know about Transistor

1. Chapter 17
Electronics Fundamentals
Circuits, Devices and Applications - Floyd
© Copyright 2007 Prentice-Hall
Summary
Bipolar junction transistors (BJTs)
The BJT is a transistor with three regions and two pn
junctions. The regions are named the emitter, the base, and
the collector and each is connected to a lead.
There are two types of
BJTs – npn and pnp.
n
n
p
p
p
n
E (Emitter)
B (Base)
C (Collector)
E
B
C
Separating the regions
are two junctions.
Base-Collector
junction
Base-Emitter
junction

2. Chapter 17
Electronics Fundamentals
Circuits, Devices and Applications - Floyd
© Copyright 2007 Prentice-Hall
Summary
BJT biasing
For normal operation, the base-emitter junction is forward-
biased and the base collector junction is reverse-biased.
npn
BE forward-
biased
BC reverse-
biased
For the npn transistor, this
condition requires that the base
is more positive than the emitter
and the collector is more
positive than the base.
+
+
For the pnp transistor, this
condition requires that the base
is more negative than the
emitter and the collector is more
negative than the base.
pnp
+
+

3. Chapter 17
Electronics Fundamentals
Circuits, Devices and Applications - Floyd
© Copyright 2007 Prentice-Hall
Summary
BJT currents
A small base current (IB) is able to control a larger collector
current (IC). Some important current relationships for a BJT
are:
E C B
I I I
 
C DC E
α
I I

C DC B
β
I I

I
I
I
IB
IE
IC

4. Chapter 17
Electronics Fundamentals
Circuits, Devices and Applications - Floyd
© Copyright 2007 Prentice-Hall
Summary
Voltage-divider bias
Because the base current is small, the approximation
2
B CC
1 2
R
V V
R R
 
  

 
is useful for calculating the base voltage.
R1
R2
RC
RE
VB
VE
After calculating VB, you can find VE
by subtracting 0.7 V for VBE.
Next, calculate IE by applying Ohm’s
law to RE:
C E
I I

Then apply the approximation
Finally, you can find the collector voltage
from C CC C C
V V I R
 
VC
E
E
E
V
I
R


5. Chapter 17
Electronics Fundamentals
Circuits, Devices and Applications - Floyd
© Copyright 2007 Prentice-Hall
Summary
Voltage-divider bias
Calculate VB, VE, and VC for the circuit.
2
B CC
1 2
6.8 k
15 V =
27 k + 6.8 k
R
V V
R R
  
 
 
   
  
 
 
R1
R2
RC
RE
VE = VB  0.7 V =
C E 2.32 mA
I I
 
  
C CC C C 15 V 2.32 mA 2.2 k
V V I R
     
E
E
E
2.32 V
2.32 mA
1.0 k
V
I
R
  

27 k
6.8 k 1.0 k
2.2 k
+15 V
2N3904
3.02 V
2.32 V
9.90 V

6. Chapter 17
Electronics Fundamentals
Circuits, Devices and Applications - Floyd
© Copyright 2007 Prentice-Hall
Summary
CE amplifier
In a common-emitter amplifier, the input signal is applied
to the base and the output is taken from the collector. The
signal is larger but inverted at the output.
R1
R2
RC
RE
Input coupling
capacitor
Output coupling
capacitor
Bypass
capacitor
C3
C2
C1
VCC

7. Chapter 17
Electronics Fundamentals
Circuits, Devices and Applications - Floyd
© Copyright 2007 Prentice-Hall
Summary
CC amplifier
In a common-collector amplifier, the input signal is
applied to the base and the output is taken from the
emitter. There is no voltage gain, but there is power gain.
R1
R2
RE
C1
VCC
The output voltage is nearly
the same as the input; there is
no phase reversal as in the
CE amplifier.
The input resistance is larger
than in the equivalent CE
amplifier because the emitter
resistor is not bypassed.

8. Chapter 17
Electronics Fundamentals
Circuits, Devices and Applications - Floyd
© Copyright 2007 Prentice-Hall
Summary
The BJT as a switch
BJTs are used in switching applications when it is
necessary to provide current drive to a load.
In cutoff, the input voltage is too
small to forward-bias the
transistor. The output (collector)
voltage will be equal to VCC.
In switching applications, the transistor
is either in cutoff or in saturation.
RC
VCC VCC
RC
VOUT
IIN = 0 = VCC
IIN > IC(sat)/bDC
= 0 V
When IIN is sufficient to saturate
the transistor, the transistor acts
like a closed switch. The output is
near 0 V.

9. Chapter 17
Electronics Fundamentals
Circuits, Devices and Applications - Floyd
© Copyright 2007 Prentice-Hall
Bipolar junction
transistor (BJT)
Class A
amplifier
Saturation
Selected Key Terms
An amplifier that conducts for the entire input
cycle and produces an output signal that is a
replica of the input signal in terms of its
waveshape.
A transistor with three doped semiconductor
regions separated by two pn junctions.
The state of a transistor in which the output
current is maximum and further increases of
the input variable have no effect on the output.

10. Chapter 17
Electronics Fundamentals
Circuits, Devices and Applications - Floyd
© Copyright 2007 Prentice-Hall
Cutoff
Q-point
Amplification
Common-
emitter (CE)
Class B
amplifier
The dc operating (bias) point of an amplifier.
Selected Key Terms
A BJT amplifier configuration in which the
emitter is the common terminal.
The non-conducting state of a transistor.
An amplifier that conducts for half the input
cycle.
The process of producing a larger voltage,
current or power using a smaller input signal
as a pattern.

11. Chapter 17
Electronics Fundamentals
Circuits, Devices and Applications - Floyd
© Copyright 2007 Prentice-Hall
Junction field-
effect transistor
(JFET)
MOSFET
Depletion mode
Enhancement
mode
Metal-oxide semiconductor field-effect
transistor.
A type of FET that operates with a reverse-
biased junction to control current in a channel.
Selected Key Terms
The condition in a FET when the channel is
depleted of majority carriers.
The condition in a FET when the channel has
an abundance of majority carriers.

12. Chapter 17
Electronics Fundamentals
Circuits, Devices and Applications - Floyd
© Copyright 2007 Prentice-Hall
Common-source
Oscillator
Feedback
A circuit that produces a repetitive waveform
on its output with only a dc supply voltage as
an input.
An FET amplifier configuration in which the
source is the common terminal.
Selected Key Terms
The process of returning a portion of a
circuit’s output signal to the input in such a
way as to create certain specified operating
conditions.