THE MIGHTY TRANSISTOR
Kristen A. McIntyre
K6WX
ORIGINS
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I love using transistors
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used to be mysterious
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wonderfully versatile
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visual thinking
Bay Area Radio Builders
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at the TechShop
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chalk-talk
WHAT IS A TRANSISTOR?
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A small tube?
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A three legged bug?
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A demonstration of
quantum mechanics?
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A little slice of heaven?
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A voltage or current
controlled current source
WHY BOTHER?
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There’s a chip for that
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appliance ready to go
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3 wires is hard
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Calculations are needed
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So many choices
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what’s right?
LET’S BOTHER
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Learn some fundamentals
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Extremely useful for small circuits
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Basic building block
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Just what the doctor ordered
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easier than chips in many cases
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control more power
Sorry for random schematic
styles
SOME ASSUMPTIONS
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You know what Current and Voltage
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You know how Ohm’s Law works
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You know what resistors do
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interacts with Ohm’s Law
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Understand Kirchhoff ’s Current Law
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You have a rough idea of how capacitors work
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Currents are ‘conventional’ in direction
SEMICONDUCTOR PHYSICS:
THE DIODE
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N-type, P-type
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doping = flaws
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quantum interference effects
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Create junction - PN touch
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Charge carriers
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electrons
Reverse voltage
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depletion layer - insulator
EXTENDING THE DIODE
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NPN & PNP
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C-B is reverse biased
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B-E is forward biased
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Base injects ‘holes’
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emitter has excess electrons
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e move toward collector
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electrons pulled by E-field
NPN & PNP
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Order of doped material
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Voltage / Current reversal
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NPN is usually faster
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NPN is usually convenient
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Unlike tubes, you have
choice!
ANOTHER APPROACH:
ELECTRIC FIELDS
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Field Effect Transistor
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E-field changes depletion region
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Gate current very low
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Symmetric / reversible (in principle)
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Types
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enhancement mode
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depletion mode
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junction
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MOS
CIRCUIT MODEL: DC
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Ebers-Moll
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current source
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base current control
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ignore reverse current
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NPN easier to visualize
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Consistent config is easier
CIRCUIT MODEL EXTENDED:
EBERS-MOLL FOR AC
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Add in capacitance
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Base-Emitter
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Collector-Base (Miller)
Add in resistors
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non-ideal junctions
A SIMPLIFYING ASSUMPTION
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Replace the B-E diode
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assume linearity
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use resistor
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only in ‘linear region’
Hybrid Pi model
ISSUES AT AC
= Voltage
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Miller capacitance
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same as for tubes
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B-C feedback for AC
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reduces gain
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causes oscillation (phase
delay)
= Current
MORE SIMPLIFYING
ASSUMPTIONS
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Vbe is 0.6V always
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Base current zero
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collector current == emitter current
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Collector can supply infinite current
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Transistor ‘works’ to make these assumptions hold
3 CIRCUITS FOR TRANSISTORS
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Common Emitter
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Common Base
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Emitter Follower
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a.k.a. Common Collector
What does “Common”
mean?
COMMON EMITTER
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+V
Simplest amplifier
voltage amplification
Output
Prone to gain problems
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emitter ‘degeneration’
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easier to model
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little gain dependency
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Medium impedance out, medium in
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Miller capacitance
Input
0.6V
-V
COMMON BASE
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Voltage shifter
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Can have gain
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Miller capacitance non-issue
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fast
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Impedance depends on
resistors
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PNP easier to visualize
EMITTER FOLLOWER
(COMMON COLLECTOR)
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Emitter follows Base
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Thermal sensitivity
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Current amplifier
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high impedance in
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low impedance out
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Asymmetry in current
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Some Miller effect
EQUIVALENCE WITH TUBES
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Each of the 3 circuits can be
realized with a tube
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C, D
cathode = emitter, source
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grid = base, gate
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plate = collector, drain
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Similar to FETs
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Voltages higher, currents lower
B, G
E, S
WHAT DEVICE TO USE?
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2N3904, 2N3906
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garden variety
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2N2222
TIP-22, TIP-120
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medium power
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TIP-22 Darlington PNP
2N3055
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high power, slow
KEY SPECS
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Max collector current, voltage
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Max power dissipation
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Secondary breakdown
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Beta β (current gain) - careful
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• ft
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don’t depend
- careful
can run faster, oscillation
MORE CIRCUITS
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Variants of the 3 basic
circuits
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usually combined for
interesting results
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special properties used
Some of my favorites
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a random collection
CURRENT SOURCE
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Use two properties
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Vbe is constant 0.6V (sort of)
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collector is naturally a current
source
Diode voltage is constant (sort of)
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Zener diode
Control current with emitter
resistor
DIFFERENTIAL PAIR
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Vout = g(V+ - V-)
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Needs current source
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Emitter degeneration
preferred
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2 outputs, mutually inverted
DARLINGTON
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Extreme current
amplification
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Base current multiplied by
first transistor’s β
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Sometimes hard to turn off
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Slow out of saturation
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Miller effect can be extreme
VBE MULTIPLIER
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Very clever circuit!
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Multiplies Vbe at Vce by g
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who cares?
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inverse with temperature
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negative thermal feedback
Common in audio amps
VOLTAGE BUFFER
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Most typical emitter follower
circuit
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Gain of 1 in voltage
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current gain, but
asymmetric
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can fix, but there are
complications
Watch out for Vbe variations
LEVEL SHIFTER
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Common base
configuration
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No appreciable Miller effect
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Replicate AC signals with or
without gain
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could use a capacitor, but
no DC
Vin
Common
Base
V+
Emitter
Follower
Vout
V-
COMPLEMENTARY PAIR
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Fix the asymmetric emitter
follower problem
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Replace that with crossover
problem
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fix that with bias
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thermal problems
Use Vbe multiplier for
thermal compensation
BASIC CLASS-A AMPLIFIER
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Typical voltage gain circuit
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Sensitive to Miller effect
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Current asymmetry
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Bias must be managed
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DC points may not be
optimal
CASCODE
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Not “Cascade”!!
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common emitter + common base
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Wonderfully clever
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current control, not voltage
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cancel Miller effect
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keep upper base still
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keeps lower collector still
Stable!
OSCILLATOR (COLPITTS)
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Very simple
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Classic ham oscillator
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output -> input via LC
circuit
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Must be biased correctly
OSCILLATOR (PHASE SHIFT)
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Idea: delay, delay, delay
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Create a phase inversion
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use R-C circuits, or equiv.
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with gain = oscillation
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works with any gain
element
If R’s and C’s same, calculation
is easy
2 POLE FILTER (SALLEN-KEY)
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2-pole low-pass, high-pass,
bandpass filter
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Just needs a gain element
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op-amp or transistor
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Unity gain is easy
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Can cascade
ADD AN OP-AMP
PASS REGULATOR
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Use emitter follower
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Include the transistor in the
feedback loop
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loads vary, so …
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compensate for β, Vbe, and
thermal drift
Feedback can also produce
over unity gain, if needed
HIGH CURRENT AMPLIFIER
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Some designs use common
emitter
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high impedance - hard to
stabilize
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not recommended
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Emitter follower is most stable
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Use Vbe multiplier to bias with
thermal compensation
Vbe
Multiplier
Complementary
Pair
SATURATION
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Take the device well out of
the linear region - full-on
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Vce at a minimum
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Saturated with charge carriers
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slow to drain
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transistor is briefly ‘stuck’
Acts like a switch
INVERTING SWITCH
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Use saturation property
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Use V/I inversion
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Use current multiplication
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Simple version has current
asymmetry
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Might be slow to turn off
V+
Out
In
V0
TIMER
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Exploit capacitor V/I
relationship
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I = C dv/dt, I/C = dv/dt
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current source
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linear voltage rise
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Reset with transistor
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Use transistor as comparator
OR GATE
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Use saturated inverters
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‘Wire OR’ the collectors
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Buffer and invert again to
get the right output
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Extend with more inverters
for more inputs
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RTL - resistor-transistor logic
AND GATE
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More saturated inverters
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This time, invert twice
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A ‘zero’ brings down the
common connection
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Can buffer and invert if
needed
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More RTL
TOUCH SWITCH
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Use body as capacitor
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Slows down input transistor
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oscillator makes dV/dt
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Amplify pulses from slowed base
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Pump charge into capacitor
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resistor drains charge
When at threshold, turn on
MORE CAPABLE AMPLIFIER
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Uses all three basic
topologies
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Complementary pair
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Includes Vbe multiplier
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Can replace op-amp with
differential pair
HOMEOSTATIC FAN
CONTROLLER
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Many interesting sub-circuits
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Uses Vbe to measure
temperature
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Transistor comparator &
current source
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Used every Friday for 9am
talknet
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Designed board
RF USAGE
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Frequency dependent calculations
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tuned circuits common
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not exclusively tuned, though
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input & output impedance
Use Ls, Cs for coupling
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AC coupled
Transformers allow device
symmetry for class A/B
CONCLUSIONS
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Transistors are cool!
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Transistors are hot!
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3 circuits form the basis of most everything
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Quantum physics in a small box
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Can understand more about our radios
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they’re on the license exam
QUESTIONS?
THANK YOU!