Transcript Power Point Slides

An introduction to
Junction Transistors
BITX20 bidirectional SSB transceiver
A BITX20 single stage
A simplified single stage
+12 V
R1
R2
0V
A potential divider
+12 V
+12 V
R1
R1
V
R2
0V
R2
0V
An NPN Transistor
+12 V
R1
Collector
Base
R2
0V
Emitter
The transfer resistor (transistor)
Emitter (-)
Collector (+)
Electrons
N
P
Base
(Original patent used point contact)
N
Electrons are
negative 
A silicon atom (Si)
?
?
Si
?
Has 4 outer electrons
The outer electron shell
needs 8 to be “full”
(standing wave pattern)
?
Silicon will try to lend
or borrow 4
Silicon (group 4) bonds
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
A pure silicon crystal lattice
An Arsenic atom (As)
Has 5 outer electrons
?
One surplus for fitting
in to the lattice
As
?
?
Arsenic doping (group 5) – N type
Si
Si
As
Si
Si
Si
Si
Si
Si
Si
Si
As
A Gallium atom (Ga)
?
?
Has 3 outer electrons
?
Ga
?
?
One short for fitting in
to the lattice
Gallium doping (group 3) – P type
Si
Ga
Si
Si
Si
Si
Si
+
Si
+
Si
Si
Ga
Si
Holes are
positive
A P-N Junction (N on left)
N Type
Si
Si
Depleted
Si
Si
As
Si
Si
P Type
Si
Si
-
Si
Si
Si
+
Si
As
Si
Si
Si
Si
Ga
Si
Ga
Si
Si
Si
Si
Si
As
Si
Si
Si
Si
Ga
Si
Si
Si
Ga
Si
Si
Si
+
Si
As
Si
Si
Si
Si
-
Si
Si
Si
+
Si
As
Si
Si
Si
Si
Si
Si
Ga
Si
Si
Si
Si
Si
As
Si
Si
As
Si
Ga
Si
Si
Si
Ga
Si
Si
Si
+
Si
As
Si
Si
Si
Si
Si
As
Si
Si
Si
Si
Si
Si
+
Si
Si
Si
Si
Ga
Si
Ga
What causes the depletion?
• Electrons move from left to right to fill the + holes
• Where electrons and holes combine the area is “depleted”
of current carriers
• This leaves the left (N Type) positive so eventually this
prevents the depletion spreading any more.
• Applying negative to N type replaces the depleted carriers
and the current resumes (Forward biased diode)
• Applying positive to the N type removes more electrons
and increases the depletion. Almost no current flows.
(Reverse biased diode)
Diode junction
(BC107 base-emitter)
Milliamps
0.5
0.4
0.3
0.2
0.1
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Volts
The transfer resistor (transistor)
Emitter (-)
Collector (+)
Electrons
N
P
Base
(Original patent used point contact)
N
Electrons are
negative 
An alloy NPN Transistor (powered up)
P
Emitter (-)
Base
N
Depletion
N
Collector (+)
Most alloy transistors (e.g. OC71) were germanium PNP
Characteristics of transistors
•
•
•
•
Geometry
Carrier movement
Collector “collection” efficiency (Alpha)
Asymmetry: Efficiency / Breakdown voltages
•
NPN transistors are normally better than PNP
since electron mobility is better than hole
mobility
Current gain of transistors
For the original “common base” circuit the ratio of collected
current to emitted current was measured. This is called Alpha.
Values have improved to well over 0.99 (always less than 1).
However normally we quote the current gain, called Beta.
Beta = Collector current / Base current
Beta values of over 200 are common.
NPN Transistor circuits
• Common base
• Emitter follower (common collector)
• Common emitter
Collector
Base
Emitter
The first transistor circuit: Common base
+12V
R
Output
Collector
Current
Base
0V
Variable
R
-1 V
Input
Emitter
Current
Common Emitter
+12 V
R
Input
Base
Voltage
(Positive)
0V
Emitter
Grounded
Diode junction
(BC107 base-emitter)
Milliamps
0.5
0.4
0.3
0.2
0.1
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Volts
Emitter follower
Collector
to supply
line
+12 V
Input
Base
Voltage
R
0V
Output
Emitter
Voltage
Our original circuit
•Potential divider bias to
linear region
+12 V
R1
Collector
Base
•Partly common Emitter
Emitter
R2
0V
•Partly an Emitter follower
•Voltage gain set by
Collector / Emitter resistor
ratios
•More in a later talk
Questions?
(Summary follows)
Common Emitter
+12 V
R
Input
Base
Voltage
(Positive)
0V
Emitter
Grounded
Features of Common Emitter
•
•
•
•
High voltage gain
High current gain
Medium input impedance due to high current gain
High output impedance. For HF capacitive loading
will need to be resonated reducing bandwidth.
• Bad HF & bandwidth as falling beta with
frequency reduces gain.
Emitter follower
Collector
to supply
line
+12 V
Input
Base
Voltage
R
0V
Output
Emitter
Voltage
Features of Emitter followers
•
•
•
•
•
Voltage gain of almost exactly 1
High current gain
High input impedance (due to high current gain)
Low output impedance (Good for unknown loads)
Good HF & bandwidth as falling beta with
frequency matters less.
Common base
+12 V
R
0V
Input
Emitter
Voltage
(Negative)
Output
Collector
Voltage
Features of Common Base
•
•
•
•
•
•
Current gain of approximately 1 (alpha)
Low input impedance (due to low current gain)
High output impedance (Base screens collector)
High voltage gain (if input impedance matched)
Works with a low gain transistor (beta)
Good HF & bandwidth as falling beta with
frequency matters less.
Appendix
A Planar NPN Transistor
Collector
N
Base
P
Emitter
N
A Planar PNP Transistor on an N substrate
Collector
Base
P N
N substrate
Emitter
P