Transcript Computer Netwroks

Electronic Circuits
10CS32
Venugopala Rao
Dept of CSE
SSE, Mukka
Optoelectronic devices
 Photodiode types:
 There are various types of photodiodes depending on the
construction.
 PN Photodiodes
 PIN Photodiodes
 Schottky type Photodiodes
 Avalanche Photodiodes.
 PN Photodiodes
 Comprises of PN junction as shown
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Optoelectronic devices
 When light with sufficient energy is made to fall on diode,
photoinduced carriers are generated.
 These include electrons in the conduction band of P - type
material and holes in the valence band of the N-type
material.
 When photodiode is reverse biased, photoinduced electrons
move downwards from P-type material to N-type material.
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Optoelectronic devices
 Similar action will take place w.r.t. holes and they move
from N-side to P-side
 Shorter wavelength signals are absorbed at the surface itself
an longer wavelength signals penetrate into the diode.
 Applications:
 Used in medical instrumentation, analytical instrumentation etc
 PIN diode:
 Here additional high resistance layer is added between P and N
layers as shown
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Optoelectronic devices
 This coat reduces the diffusion time and hence increases the
response time.
 Low capacitance being major feature of PIN photodiodes, it
offers large bandwidth. Hence it is useful for high speed
photometry and optical communication applications.
 Schottky diode:
 In these, there is a thin gold coating on the N-type material to ensure
the Schottky effect.
 This diode has better performance under UV radiation.
 Avalanche Photodiode:
 These are high speed high sensitivity photodiodes which makes use
of internal gain mechanism by applying higher reverse voltages as
compared to PIN diodes.
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Venugopala Rao A S, SSE Mukka
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Optoelectronic devices
 APDs are constructed with uniform junction which exhibits avalanche
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effect at about 30V to 200V.
Electron-hole pair generated due to incident light are accelerated
applied electric field. This ensures that, new electrons are moved to
conduction band.
Thereby current increases rapidly.
APD is also fast responsive similar to PIN photodiode.
Offer higher SNR and hence used in sensitive applications like long
distance OFC communication
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Venugopala Rao A S, SSE Mukka
Optoelectronic devices
 Circuit symbol and V-I Characteristics:
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Venugopala Rao A S, SSE Mukka
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Optoelectronic devices
 Here curve 1 shows the graph of dark state. This is similar to
conventional diode.
 When light falls on diode, curve shifts downwards as shown.
 If the terminals of the photodiode are shorted, a photocurrent
proportional to light intensity will flow from anode to
cathode.
 If the circuit is open, open circuit +ve voltage is seen at the
anode.
 From the graph it is clear that, ISC is linear w.r.t light
intensity and VOC is logarithmically varying w.r.t light
intensity.
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Venugopala Rao A S, SSE Mukka
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Optoelectronic devices
 Two operating modes:
 Photovoltaic mode
 Photoconductive mode
 In photovoltaic mode, no bias voltage is applied, due to the
incident light a forward voltage is produced across the diode.
 In photoconductive mode, reverse bias voltage is applied across
the diode.
 This widens the depletion region and hence speeds up the
response time.
 For applications with bandwidth less than 10kHz, photovoltaic
mode is used and for all other applications photoconductive
mode is used.
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Venugopala Rao A S, SSE Mukka
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Optoelectronic devices
 Application circuit:
 In photovoltaic mode, photodiodes are used for low speed
applications or for detecting low light intensities.
 Following are the two application circuits for this operation
 The output voltages of these are given by Idet X R and Idet X R
where Idet is the current through diode.
 The second circuit offers better linearity than circuit (a). This is due
to the fact that, for better linearity, is to be as small as possible
across photodioe. (Rf/ A)
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Venugopala Rao A S, SSE Mukka
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Optoelectronic devices
 Following are the circuits for PD working in
photoconductive mode.
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 Output voltage is obtained in the same manner as discussed
earlier.
 APDs are also connected in the same manner except that
they need higher reverse voltage.
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Optoelectronic devices
C
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C
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C
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