Biomedical Engineering Reference
In-Depth Information
R
c
C
l
R
l
C
m
R
m
C
j
R
f
R
s
+
E
ph
(
t
)
−
(a)
Output
+
C
I
R
I
−
C
m
R
m
+
−
+
R
s
−
I
ph
(
t
)
(b)
FIGURE 17.6
(a) Equivalent RC circuit of one pixel of bR-based photoreceptor array. The bR molecules are modeled as a cur-
rent generator and represented by an equivalent electric potential source
E
ph
(
t
) in series with the source resist-
ance
R
s
. (b) Physical model of one bR pixel. The dipole moment of each bR molecule crosses the membrane in
the same direction. The bR molecules are surrounded by the lipid bilayers.
signals that are generated by detectors with virtually infinite resistance, as is demon-
strated by a bR-based photoreceptor. Therefore, highly optimized circuit designs are
required, where high-input impedance and low-noise amplifiers should be applied.
Figure 17.7 illustrates the simplified equivalent bR circuit model in combination with
three common front-end designs: a voltage follower, a transimpedance amplifier, and a
switched integrator.
A voltage follower is simply an operational amplifier with a voltage gain of unity where
the high-input impedance is limited by its input bias current (Figure 17.7a). This is com-
monly referred to as a voltage buffer and is used here to isolate the high photoreceptor
load from the remaining processing circuitry. A dried bR photoreceptor exhibits particu-
larly high resistance, which significantly limits the bandwidth of this amplifier design.
Rather than measuring photovoltage, photocurrent generated by the bR photoreceptor
can be amplified directly by using a transimpedance amplifier (Figure 17.7b). In this con-
figuration, the load resistor
R
f
is connected in the feedback loop of the amplifier. The
inverting input of the amplifier is connected to the photoreceptor output, where the
noninverting input is grounded. This technique effectively shorts out the membrane
