An introduction to Junction Transistors

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An introduction toJunction Transistors
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BITX20 bidirectional SSB transceiver
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A BITX20 single stage
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A simplified single stage
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A potential divider
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An NPN Transistor
Collector
Base
Emitter
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The transfer resistor (transistor)
Emitter (-)
Collector ()
Electrons
Base
Electrons are negative ?
(Original patent used point contact)
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A silicon atom (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
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Silicon (group 4) bonds
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A pure silicon crystal lattice
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An Arsenic atom (As)
Has 5 outer electrons One surplus for fitting in
to the lattice
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Arsenic doping (group 5) N type
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A Gallium atom (Ga)
Has 3 outer electrons One short for fitting in to
the lattice
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Gallium doping (group 3) P type
Holes are positive
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A P-N Junction (N on left)
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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)

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Diode junction (BC107 base-emitter)
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The transfer resistor (transistor)
Emitter (-)
Collector ()
Electrons
Base
Electrons are negative ?
(Original patent used point contact)
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An alloy NPN Transistor (powered up)
Depletion
Emitter (-)
Collector ()
Base
Most alloy transistors (e.g. OC71) were germanium
PNP
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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

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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.
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NPN Transistor circuits

• Common base
• Emitter follower (common collector)
• Common emitter

Collector
Base
Emitter
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The first transistor circuit Common base
Base
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Common Emitter
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Diode junction (BC107 base-emitter)
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Emitter follower
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Our original circuit

• 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

Collector
Base
Emitter
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Questions?(Summary follows)
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Common Emitter
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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.

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Emitter follower
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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.

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Common base
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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.

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Appendix
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A Planar NPN Transistor
Collector
Base
Emitter
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A Planar PNP Transistor on an N substrate
Collector
Base
Emitter