Biomedical Engineering Reference
In-Depth Information
by applying the rate equations of the photochemical cycle of bR. The concentrations of dif-
ferent intermediate states can be described as:
dC
()
t
(17.3)
k
Ct
()
k
Ct
()
i
dt
(
i
1)
i
(
i
1)
i
(
i
1)
i
where the parameters
k
(
i
1
)
i
and
k
i
(
i
1
)
(
i
L
,
M,
and
M
1
) represent the rate constants of
1, respec-
tively. The parameter
k
BK
represents the rate constant of the photochemical reactions from
the ground state
B
to state
K
initiated by incident light. This can be written as
thermal relaxation from intermediate (
i
1) to
i
and from intermediate
i
to
i
ktI
() 1 )
,
FtI
()
,
(17.4a)
BK
B
BK
where
It
()
It
()
FtI
()
,
i
hv
hc
(17.4b)
Herein,
is the refractive index of the substrate;
B
is the absorption cross-section of
B
state;
K
; and
F
(
t,I
) is the photon density flux
of the incident light. The frequency of the incident light and the speed of light in vacuum
are represented as
BK
is the quantum yields of photoreaction
B
and
c
, respectively, where
h
is Planck's constant.
Modeling a bR-based photoreceptor is an essential step in integrating it with signal pro-
cessing circuit. An equivalent circuit model of a bR photoreceptor is shown in Figure 17.6a.
Each component in the equivalent circuit model represents an analog found in the bR pho-
toreceptor's physical model (Figure 17.6b). The overall behavior of the bR molecule is that
of a current generator, which is given by the dipole current,
I
(
t
). Considering the finite
resistance of protein medium,
R
s
, the dipole current source can also be described by an
equivalent electric potential source,
E
ph
(
t
). Since the lipids within the PM exhibit a high
dielectric constant, they can be modeled as the combination of a series capacitor
C
l
and a
series resistor
R
l
. The parallel combination of resistor
R
m
and capacitor
C
m
, where the cur-
rent source describes the behavior in nonilluminated bR molecules in conjunction with
lipids that are perpendicular to the PM orientation direction. The series resistance
R
c
orig-
inates from ohmic contacts between film surfaces and electrodes. Capacitor
C
j
is the junc-
tion capacitance connecting the external circuit and the bR film. Load resistance
R
f
is the
measuring instrument's input impedance. Contact resistance
R
c
and junction capacitance
C
j
can be ignored for analysis purpose because their values are insignificant in compari-
son to the other components.
17.3.2
Front-End Circuit Design
17.3.2.1 Three Common Front-End Circuit Designs
Photoreceptor performance depends primarily on its front-end circuit design.
Generally, front-end circuits should demonstrate low noise and provide sufficiently
wide bandwidth to ensure accurate representation of input light signals. They should
also provide any necessary amplification and buffering. The choice of front-end circuits
is determined by the photoreceptor's intrinsic characteristics and the intended applica-
tion. Conventional circuit designs may not be appropriate for extremely small-current