ESE 372 / Spring 2013 / Midterm 1 solution
ESE 372 / Spring 2013 / Midterm 1 solution
ESE 372 / Spring 2013 / Midterm 1 solution
ESE 372 / Spring 2013 / Midterm 1 solution
ESE 372 / Spring 2013 / Lecture 10
Equivalent models of amplifiers
Signal source
Load
Amplifier
Needs power supplies and might need bias
Model of output from amplifier Model of input to amplifier Output is defined by VIN
Thevenin
or Norton
1
ESE 372 / Spring 2013 / Lecture 10
Voltage–current amplifier.
It is required to deliver current to load in response to voltage at input.
q
p
g
p
It i
It is good when:
d h
then
and
d
then
2
ESE 372 / Spring 2013 / Lecture 10
What about pn‐junction under reverse bias?
Current under reverse bias before breakdown does not depend on voltage, i.e. this part of diode IV is characterized by infinite differential resistance.
This current is very small. What determines the value of reverse saturation current?
3
ESE 372 / Spring 2013 / Lecture 10
pn‐junction under reverse bias.
define
Deletion region
Reverse bias makes depletion region wider but it has no influence on
influence on Thus current does not depend on reverse bias voltage, i.e. differential resistance is infinite
differential resistance is infinite.
Need to control this current.
How?
4
ESE 372 / Spring 2013 / Lecture 10
How the current under reverse bias can be changed?
depends on
Option 1: by light.
. Can we change them?
See solar cells and photodetectors
Option 2: by injection.
Current increases but depends only on injection intensity.
It still does not depend on reverse bias voltage.
H i j ti
How injection can be achieved?
b
hi d?
5
ESE 372 / Spring 2013 / Lecture 10
Pn‐junction under forward bias.
EExcess electrons are being injected into p‐
l t
b i i j t di t
side of pn‐junction under forward bias.
6
ESE 372 / Spring 2013 / Lecture 10
Let’s put together FB and RB pn‐junctions.
n‐type
p‐type
FB
n‐type
RB
IIN
IOUT
qVFB
BASE
qVRB
Emits extra electrons into
electrons into
base and defines IIN
Collects these electrons
7
ESE 372 / Spring 2013 / Lecture 10
npn Bipolar Junction Transistor (BJT)
Emitter
Base
Collector
8
ESE 372 / Spring 2013 / Lecture 10
Base current
Emitter
Base
Collector
“Useful” currents:
“Useless”:
Emitter efficiency:
Base transport factor:
Common base current gain:
9
ESE 372 / Spring 2013 / Lecture 10
npn‐BJT ‐ Common Base
Modes of operation
Forward active
Forward active
Saturation
Cutoff
10
ESE 372 / Spring 2013 / Lecture 10
Example.
Find maximum value of I
Fi
d
i
l
f I0 for BJT to f BJT t
operate in forward active (FA) mode.
Saturation:
Forward active:
11
ESE 372 / Spring 2013 / Lecture 10
Example – cont.
If assume
This is impossible, hence:
12
ESE 372 / Spring 2013 / Lecture 10
npn‐BJT ‐ Common Emitter
In forward active or in saturation:
In forward active:
Input IV
Output IVs
Output IVs
saturation
forward active
13
ESE 372 / Spring 2013 / Lecture 10
Example
Find maximum value of I
Fi
d
i
l
f I0 for BJT to f BJT t
operate in forward active (FA) mode.
14
ESE 372 / Spring 2013 / Lecture 10
BJT in Saturation
15
ESE 372 / Spring 2013 / Lecture 10
BJT Models
Forward active
Reverse active
16
ESE 372 / Spring 2013 / Lecture 10
Ebers–Moll Model 17