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Electronic DevicesEighth Edition
Floyd
Chapter 11
Phototransistors
- Photodiode with amplifier
- B/E exposed to radiation.
- Collector current is a linear function of irradiance. (assuming a constant beta).
- Linearity is over a much narrower range than a photodiode or photoconductor.
- IC versus VCE is plotted based on steps of irradiance.
- Responstivity of a phototransistor (RE) is for a specified black body radiation (usually at a colour temp. of 2870 K)
Phototransistor
- Responsivity is the ratio of electrical output to the applied radiation. It is a figure of merit.
- Dark current is the main factor in limiting detection sensitivity.
- Dark current is a function of a.) ambient temperature (20
deg. C causes a decade increase in dark current) and 2.) VCE.
- Rise time is poor due to a.) the combined capacitance of the B/E and C/E junctions and b.) the lifetimes of the
carriers in the depletion region of the junction.
VCC
Vout
VCC C
E
Q1Q2
n-type
p-typen
EmitterBase
Collector
Window
Optocouplers
Optotisolator.
=>Electrically Insulated (insulation resistance ohms).
=>Capacitance < 1 pf.
=>Insulation voltage strength of several KV.
Eliminates ground effects from 2 systems with different ground potentials.
1210
Optocouplers
•Specifications:
1.) Operating speed.
a.) BW – typically refers to NRZ ( )
b.) Baud rate.
2.) Propagation delay.
3.) CTR (forward current transfer ratio)
4.) Isolation: ability to separate desired and stray input signals. Differential modes (desired) versus Common
mode (undesired).
BITB tf 1=
100×=
F
O
I
ICTR
Optocouplers
5.) Insulation.
6.) Limiting parameters: Pmax, current, voltage
ratings,same as typical semiconductor data.
7.) Aging characteristics: Ex. CTR deterioration due to
LED source of optocoupler.
Signal Sources:
a.) D.C.
b.) pulse
c.) A.C.
U1
A C
K E
U2
Q1
Selected Key TermsSelected Key TermsSelected Key Terms
4-layer diode
Thyristor
SCR
The type of 2-terminal thyristor that conducts
current when the anode-to-cathode voltage reaches
a specified “breakover” value.
A class of four-layer (pnpn) semiconductor
devices.
Silicon-controlled rectifier; a type of three
terminal thyristor that conducts current when
triggered by a voltage at the single gate terminal
and remains on until anode current falls below a
specified value.
Selected Key TermsSelected Key TermsSelected Key Terms
LASCR
Diac
Triac
Light-activated silicon-controlled rectifier; a four
layer semiconductor device (thyristor) that
conducts current in one direction when activated
by a sufficient amount of light and continues to
conduct until the current falls below a specified
value.
A two-terminal four-layer semiconductor device
(thyristor) that can conduct current in either
direction when properly activated.
A three-terminal thyristor that can conduct current
in either direction when properly activated.
Selected Key TermsSelected Key TermsSelected Key Terms
SCS
UJT
PUT
Silicon-controlled switch; a type of four-terminal
thyristor that has two gate terminals that are used
to trigger the device on and off.
Unijunction transistor; a three terminal single pn
junction device that exhibits a negative resistance
characteristic.
Programmable unijunction transistor; a type of
three terminal thyristor (physically more like an
SCR than a unijunction) that is triggered into
conduction when the voltage at the anode exceeds
the voltage at the gate.
Excercise
Basic 4-layer device: Exercise 1
part 2
A certain 4-layer diode is biased in the forward-blocking region with an anode-to-cathode voltage of 20V. Under this bias condition, the anode-current is 1uA. Determine the resistance of the diode in the forward-blocking region.
Basic 4-layer device: Exercise 1
part 2
If the anode current is 2uA and VAK=20V, what is the 4-layer diode’s resistance in the forward-blocking region?
Basic 4-layer device: Exercise 2
part 1Determine the value of anode current in the diagram when the device is on, VBR(F)=100V. Assume VBE=0.7 and VCE(sat)=0.1V for the internal transistor structure.
Basic 4-layer device: Exercise 2
(contd…)
What is the resistance in the forward-conduction region of the 4-layer diode in the figure?