Biomedical Engineering Reference
In-Depth Information
Brief pulse stimulation
Long square pulse stimulation
E
p
(
t
)
E
p
(
t
) =
E
o
⋅
(
t
)
E
o
p
R
e
= 0
m
=
0
p
p
s
l
R
e
> 0
s
mc
>
m
> 0
mc
exists
s
l
Special case in which no
l
mc
(e.g.,
R
s
=
∞
)
l
s
∞ ≥
R
e
0
s
s
l
s
l
s
R
m
C
m
≥
m
>
mc
l
s
l
l
FIGURE 15.4
Relaxation-time course of AC photoelectric responses under different measurement conditions and in response to
different kinds of light sources. The parameter
m
is the characteristic
RC
relaxation time as defined by Eq. (15.7).
mc
is the critical value at which the time course is changed from monotonic to biphasic and
vice
versa
. See text for further explanation. (From Hong, F. T. (1980). Displacements photocurrents in pigment-con-
taining biomembranes: artificial and natural systems. In: Blank, M. (Ed.).
Bioelectrochemistry: Ions, Surfaces,
Membranes
(Advances in Chemistry Ser. No 188). Washington, DC: American Chemical Society, pp. 211-237.)
The parameter
When the access impedance is neither very large nor very small compared with the
source impedance (e.g., when matched to the source impedance), the photocurrent splits
into two branches, one branch to charge
C
m
and the other to reach the external measuring
device. The photosignal relaxation then reflects the interaction of the two
RC
networks
and decays with two exponential time constants, which are neither
R
p
C
p
nor
R
m
C
m
. In such
a brief time interval, the access impedance
R
e
remains constant, and an exact expression
for the photocurrent can be obtained [34]:
1
1
RC
Eu
R
()
exp
ut
-
t
s
s
p
p
∫
It
()
d
u
RC
1
1
0
em
s
l
p
s
(15.4)
1
1
RC
Eu
R
()
exp
ut
t
l
s
p
p
∫
d
u
RC
1
1
0
em
s
l
p
l
where
s
and
l
are defined by the following equations:
2
11
2
1
1
1
1
1
1
1
1
(15.5)
4
RC
RC
RC RC
s
p
mp
m
p
mp
m
pmsp
pm
2
11
2
1
1
1
1
1
1
1
1
(15.6)
4
RC
RC
RC RC
l
p
mp
m
p
mp
m
pmsp
pm
