Photoreceptors II
Photoreceptors II
• Logarithmic photoreceptor, passive and active
• Adaptive photoreceptor
• Circuit techniques – Capacitive feedback, resistive
“adaptive” elements, cascode, Miller effect,
Speedup in transimpedance amplifiers, secondorder system analysis
• Imaging arrays
• Passive pixel sensor
• Active pixel sensor (APS)‫‏‬
• Charge-coupled device (CCD)‫‏‬
Delbruck, CNS course 28.1.06
T. Delbruck, S.C. Liu
Visual Pathway in Brain (Side View)
Visual Pathway in Brain (Bottom View)
Cross-Section through Human Retina
Cross-Section through Human Eye
Cones and Rods
Outer nuclear layer
Outer plexiform layer
Horizontal cell
Bipolar cell
Inner nuclear layer
Inner plexiform layer
Ganglion cell
1
Two types of Photoreceptors:
Rods & Cones
Rods (~126 million)
•
•
•
•
Spectral Sensitivity
S
R
Sensitive in very dim light – lowest 3 decades
Saturate at high illumination
One light-sensitive pigment in blue-green
Not present in fovea
L
M
Cones (~6 million)
• Only sensitive to brighter light – upper 9 decades
• Three types with pigments with different wavelength
sensitivities  color vision
3 types of cones: L cones (sensitive to long wavelengths),
M cones (sensitive to medium wavelengths), S cones
(sensitive to short wavelengths) are only 10 % of the total
number of cones, and absent from the center of the fovea.
Rods (R) are sensitive to intermediate wavelengths.
Rod/Cone Distribution
X1000
Rodieck
Photoreceptor Response
• Outer segments are modified cilia
with disks filled with opsin, the
molecule that absorbs photons,
as well as voltage-gated Na
channels.
• The protein opsin contains a
pigment molecule called retinal.
In rods, the opsin+retinal together
are called rhodopsin. In cones,
there are diferent types of opsins
combined with retinal to form
pigments called photopsins.
Rodieck
Photoreceptor Response
• When a photon absorbed by a
rod/cone visual pigment, the
retinal undergoes a
photoisomerization to all-trans
retinal which changes
conformation of opsin.
• This conformation change
triggers a biochemical cascade
that causes hyperpolarization of
the cell and a decrease of
glutamate release.
• Bipolar cells respond to the
subsequent decrease in
glutamate.
Response to flash of light,
(Baylor, 1987)
2
Biological phototransduction uses a chain of amplifiers
Biological photoreceptors amplify changes
more than DC
Turtle cone, intracellular recording
Mahowald,
Mahowald,1992
1992
Normann & Perlman, 1979
Logarithmic Photosensors
M1
V out
How the source follower
receptor works
M1
M2
M1
Vb
Vout
Vout
Iph
Iph
Iph
E
Diode-connected
MOSFET
E
E
D1
D1
D1
Double diode-connected
MOSFET
Source-follower
MOSFET
Characteristics of Logarithmic Photosensors
Diode-connected MOSFET:
Vout
Double diode-connected MOSFET:
Vout
Source-follower MOSFET:
Vout
Contrast encoding:
Logarithmic Photosensor with Feedback Loop


 Vdd

  1  I ph 
 Vdd  U T
log 

2
 I0 
 I ph 

  V g  U T log 
 I0 
dV out  U T
dI ph
• Photodiode has large
capacitance
Ib
 I ph

log 

 I0
UT
M3
Vb
M1
Vout
Vs
M2
Iph
E
– Small photocurrent takes
a long time to change VS
• High-gain feedback loop
clamps VS and speeds
up response
• Output voltage is read at
the gate of M1
D1
I ph
3
Adaptive Photoreceptor
Current-Voltage Characteristics of Logarithmic
Photosensor with Feedback Loop
• Logarithmic photosensor covers large range of
illumination, but contrast-response is small (~100mV for a
b/w transition (one decade of illumination))‫‏‬
• Imaging arrays suffer from fixed-pattern noise (FPN), i.e.
different photosensors have different output signals for
same illumination, due to spatial variation of fabrication
parameters (different I0, 
1.85

 Iph 


Vout 1


VS UT log

 I0 

A dIph UT dIph
dVout UT

A1 Iph  Iph
1.8
1.7
1.65
1.6
dVS 
1.55
0.01
0.1
1
10
100
– Low-FPN amplification of transient signals required to detect lowcontrast features
– Use capacitive divider for amplification and resistive element for
adaptation
UT dIph UT dIph

A1 Iph A Iph
1000
E(mW/m2)
Amplification of Response by 
Speedup of response by A
Adaptive Photoreceptor
Adaptive photoreceptor from a general feedback
perspective
Ib
C1
M3
R1
Vfb
b(s)
x(s)
+
C2
y(s)
a(s)
Vs
M2
Iph
+
• Transient amplification of
logarithmic signal Vfb onto
Vout by capacitive gain stage
(C1, C2)‫‏‬
Vout • Slow adaptation of Vout to
Vfb and change of
adaptation state via current
through resistive element R1
• State of adaptation stored
as charge Qfb on Vfb node
Vb
M1
E
D1
Comparing source-follower and adaptive photoreceptor
Adaptive Photoreceptor Response Curves
Square wave input over 7 decades illumination
Adaptive receptor
200mV
Vout (V)
1.75
Time
5s
Decade changes in
background
illumination
Delbruck and Mead 1993
Source follower
receptor
Constrast ratio 2:1
Brightest 3W/m2
4
Characteristics of Adaptive Photosensor
Resistive Elements

 I ph 


Vfb  1
VS UT log I 
 0 

C C2
Transient amplification: AC  1
C2
M2
Adapted signal:
M3
Ifb
Qfb C1Vdd
Vb
Iout
Vfb
Vout
Vfb
Vout  ACVfb 
Small-signal response:
A dIph
U dIph
dVout  ACUT
 AC T
A1 I ph
 I ph
C2
Vfb
Expansive element
with low
common-mode
sensitivity
Delbruck 1993
Vout
Q1
M1
Transient signal:
Vout
M1
Expansive element
with more
symmetric
characteristics
Compressive element
Delbruck and Mead 1993
Liu 1998
Expansive Resistive Element
Vout > Vfb : diode-connected MOSFET
D
S
Vfb
Vout
Ifb
p
n
D
Iout
p+
p+
n+
S
ISD
B
Vfb > Vout : 2-collector bipolar transistor
E
Vfb
Vout
Ifb
p
C2
n E
C1
C2
Iout
p+
IEC2
n+
p+ C1
IEC1
B
IEB
Delbruck and Mead 1993
Current-Voltage Characteristics of Expansive Resistive
Element
Current-Voltage Characteristics of Expansive Resistive
Element
-2
10
-4
10
-I (V >V )
fb
fb
out
-I
(V >V )
out
fb
out
-6
I (A)
10
Ifb(Vout>Vfb)
-8
10
-10
10
Iout(Vout>Vfb)
-12
10
0
0.5
1
1.5
2
|V -V | (V)
out
fb
Delbruck and Mead 1993
5
Step-Response of Adaptive Photosensor
Adaptive Photosensor with Cascode
E
• Miller effect: Parasitic gateto-drain capacitance of M2 is
driven by Vout node and thus
effectively amplified by large
gain of inverter
 Shield drain of M2 from
large output voltage swing
with cascode M4 to speed
up response
Ib
2.2
C1
2.1
M3
Vb
R1
M1
Vfb
Vout
Vout (V)
2
C2
1.9
M4
Vc
1.8
Vs
1.7
M2
Iph
1.6
E
D1
1.5
0
50
100
150
200
Tim
e(s)
Delbruck 1993
Small signal analysis of active log photoreceptor
Measured effect of cascode on adaptive photoreceptor







 


 















 


 


















 









   

 







 
 







6


 










 
Root locus plot of poles of H(s) of adaptive photoreceptor
 



 




 
 






 

 
 






Delbruck and Mead 1993
Imaging arrays
Integrating Photodiode Pixels
• 1D or 2D array of photosensors can record optical images
projected onto it by lens system
• Individual photosensor in an imaging array is called pixel
(picture element)‫‏‬
• High image resolution requires small pixels
• Outputs of a set of pixels have to be multiplexed onto a
single signal line  small read-out duty cycle
• Two strategies for pixel operation:
• Continuous-time mode: Conversion of photogenerated
charge into steady-state photocurrent
• Integration mode: Collection of photogenerated charge
on capacitor until readout, followed by reset
Vref
Vr
M1
Vrs
M1
Vph
Vph
E
E
D1
D1
Photogates for Charge-Coupled Devices (CCDs)‫‏‬
depletion
p- substrate
p- substrate
E
E
Surface charge storage
 Surface-channel CCD
Active pixel
Acquisition period: charge packets collected under photogates
Readout period: charge packets shifted underneath photogates
by clocking gate potentials between two values
n diffusion
depletion
Vro
Charge-Packet Transport in CCDs
Vg
inversion
M3
Vout
Passive pixel
Vg
M2
Bulk charge storage
 Buried-channel CCD
F3
F2
F1
F2
F1
t1
t2
t3
t4
t5
t6
t1
t2
t3
t4
3-phase clock
2-phase clock with stepped oxide
7
In biological photoreceptors, the rise time is not
proportional to the gain
CCD Registers
imaging
array
vertical
readout
register
transfer
gate
vertical
readout
register
imaging
array
Time to peak
response (s)
output
horizontal readout register
output
Gain (arbitrary units)
horizontal readout register
Frame-transfer CCD
Interline-transfer CCD
Rise time ~ Gain0.25
The gain is distributed
Light
a
a
Signal
a
log(gain)
log(gain)
log(bandwidth)
log(Illumination)
log(frequency)
8