an introduction to junction transistors. bitx20 bidirectional ssb transceiver
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An introduction toJunction Transistors
BITX20 bidirectional SSB transceiver
A BITX20 single stage
A simplified single stage
R2
R1
+12 V
0 V
R2
R1
+12 V
0 V
R2
R1
+12 V
0 V
V
A potential divider
R2
R1
+12 V
0 V
An NPN Transistor
Base
Collector
Emitter
The transfer resistor (transistor)
Emitter (-) Collector (+)
Base
(Original patent used point contact)
Electrons
NNP
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
As
Si
Si
Si
Si
-
-
A Gallium atom (Ga)
Has 3 outer electrons
One short for fitting in to the lattice
Ga
? ?
??
?
Gallium doping (group 3) – P type
Si Ga Si Si
Si
Si
Si
Si
Si
Ga
Si
Si
+
+
Holes are positive
A P-N Junction (N on left)
Si Si As Si
Si
Si
Si
As
Si
Si
Si
Si
-
-
Si Si As Si
Si
Si
Si
As
Si
Si
Si
Si
-
-
As Si
Si
Si
Si
As
As Si
Si Si
Si Si Si Ga Si Si
Si
Si
Si
Si
Si
Ga
Si
Si
+
Si Ga Si Si
Si Si
Si
Si
Ga
Si
Si
+
Si Ga
Si
Si
Si
Si
+
Si Ga
Si
Si
Si
Si
+
Ga
Si
Si
Si
As
Si
Si
Si
Si
-As Si
Si Si
Si Si
Si
Si
Ga
Si
Si
+
Si
Si
Si
SiGa
N Type P TypeDepleted
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)
0.1 0.2 0.3 0.4 0.5 0.6 0.7Volts
0.1
0.2
0.3
0.4
0.5Milliamps
The transfer resistor (transistor)
Emitter (-) Collector (+)
Base
(Original patent used point contact)
Electrons
NNP
Electrons are negative
An alloy NPN Transistor (powered up)
Emitter (-) Collector (+)
Base
Depletion
N N
P
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
Base
Collector
Emitter
The first transistor circuit: Common base
R
+12 V
-1 V
OutputCollectorCurrent
0 V
VariableR
InputEmitterCurrent
Base
Common Emitter
R
+12 V
0 V
InputBase
Voltage(Positive) Emitter
Grounded
Diode junction (BC107 base-emitter)
0.1 0.2 0.3 0.4 0.5 0.6 0.7Volts
0.1
0.2
0.3
0.4
0.5Milliamps
Emitter follower
R
+12 V
0 V
OutputEmitterVoltage
InputBase
Voltage
Collectorto supply
line
Our original circuit
BaseCollector
EmitterR2
R1
+12 V
0 V
•Potential divider bias to linear region
•Partly an Emitter follower
•Partly common Emitter
•Voltage gain set by Collector / Emitter resistor ratios
•More in a later talk
Questions?
(Summary follows)
Common Emitter
R
+12 V
0 V
InputBase
Voltage(Positive) 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
R
+12 V
0 V
OutputEmitterVoltage
InputBase
Voltage
Collectorto supply
line
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
R
+12 V
0 V
OutputCollectorVoltage
InputEmitterVoltage(Negative)
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
N NP
Collector Base Emitter
A Planar PNP Transistor on an N substrate
Collector Base Emitter
N substrate
NP P
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