Page 254 - Organic Electronics in Sensors and Biotechnology

An Intr oduction to Or ganic Photodetectors 231

both small; state-of-the-art op-amps have noise values in the
nV/ Hz and fA/ Hz range.
• Use as large a feedback resistor as possible; although this
increases the thermal noise from the feedback resistance, this
drawback is outweighed by the improved signal gain.
• Minimize the measurement bandwidth B, ideally matching it
to the signal bandwidth to eliminate extraneous noise at other
frequencies; this can be done by adding a bandpass filter after
the op-amp to reject unwanted frequencies.
• Maximize the shunt resistance of the photodiode, preferably
ensuring it is at least as large as the feedback resistor, in order
to minimize the effect of the amplifier voltage noise; fortui-
tously, maximizing the shunt resistance also minimizes ther-
mal noise from the photodiode.
• Minimize the capacitance of the photodiode (again to
minimize the effects of amplifier voltage noise).
To determine what can feasibly be detected using an organic pho-
todiode, we estimate the output noise voltage based on typical per-
formance characteristics for a high-precision low-noise op-amp and
2
for an organic PV device of area 1 mm . We assume a noise voltage of
8 nV/ Hz and a noise current 0.6 fA/ Hz—the data sheet specifi-
cations for the state-of-the-art AD795 op-amp from Analog Devices.
We also assume a feedback resistance of 1 GΩ, which is about the
highest that can be realistically used, and a shunt resistance of 1 GΩ
and a capacitance of 400 pF for the photodiode. Inserting these values
into Eq. (6.57) and assuming a modest measurement bandwidth of
1 kHz, we obtain a root mean squared noise voltage at the output of
0.4 mV. This is equivalent to a root mean squared current noise at the
input of 0.4 pA, implying, on the basis of the amplifier performance
alone, it should be possible to measure currents of ~1 pA and above.
The photodiode has a thermal noise of 0.1 pA and shot noise of 0.02 pA
(assuming a photocurrent of 1 pA). The amplifier characteristics are
therefore the main determiner of sensitivity. The measurement noise is
lowered to approximately 0.1 pA if the photodiode capacitance is
reduced by a factor of 10. Increasing the shunt resistance by a factor of
10 leads to only a marginal reduction in the measurement noise.

Amplifier Stability
A final issue to note when using transimpedance amplifiers is stability.
The photodiode capacitance and the feedback resistance together act as
a low-pass filter that introduces a phase lag into the feedback loop,
which approaches 90° at high frequencies. The op-amp itself introduces
an extra phase lag that also approaches 90° at high frequencies, leading
to a combined “lag” of almost 180°. Since the signal is fed back into the