Biomedical Engineering Reference
In-Depth Information
The signal current
I
ph
produces shot noise when it flows through the feedback imped-
ance
Z
f
. Its output voltage is given by
V
2
qI
f Z
(17.9)
iph
ph
f
where the feedback impedance
Z
f
is
RZ
RZ
fCf
Z
(17.10)
f
f
Cf
and
Z
Cf
1/
s
C
f
. Eq. (17.9) indicates that higher photocurrent will increase shot noise.
However, higher photocurrent also produces a much larger signal. The resulting SNR
improves proportionally with the square root of the photocurrent increase.
Photoreceptor thermal noise caused by thermal isomerization is apparent inside the bR
film at any temperature above absolute zero. Photoreceptor thermal noise output voltage
is given as
4
KT
R
V
fZ
(17.11)
Rm
f
m
Normally, front-end amplifiers introduce two frequency-dependent noise sources: a cur-
rent noise
V
in
and a voltage noise
V
en
. The current noise is actually shot noise produced by
the bias current in the amplifier input stages; its output voltage is in the form of
Vi
Z
(17.12)
in
n
f
Amplifier voltage noise is attributed to thermal noise in resistive components of input
stage transistors and is represented as
Z
Z
f
Ve
1
(17.13)
en
n
m
where the photoreceptor impedance
Z
m
is given as
RZ
RZ
mCm
Z
(17.14)
m
m m
and
Z
Cm
1/
s
C
m
is the equivalent impedance of the bR film capacitance. Eq. (17.13) indi-
cates that the high bR film resistance can help reduce amplifier voltage noise.
The feedback resistor also causes thermal current noise; however, this noise is avoided
by employing a switched integrator design, where only capacitors are located within the
feedback loop.
To obtain the output signal, the transfer function for the overall photoreceptor is
required and it can be described as
1
(
s
)
Z
Z
1
f
3
Ts
()
(17.15)
1
1
C
(
s
)(
s
)
l
s
f
1
2
