circuit for measuring small changes in the intensity of infrared
radiation at a wavelength of 1 μm. Values characterising the
photodiode and operational amplifier are given below.
a) Given an incident infrared signal power of 1 nW, and
assuming the Johnson noise in the photodiode is small enough to be
neglected, draw the noise equivalent circuit for the photodiode and
amplifier and calculate the noise referred to the amplifier input
in a bandwidth from 0.1 to 10 Hz. Include a full explanation of all
the steps in your calculation.
(11 marks)
b) Fully explaining all the steps in your calculation,
determine the detectivity when the incident infrared signal has a
power of 1 nW. (5 marks)
c) Explaining your reasoning, suggest how the detection
system could be modified to improve its detectivity.
Photodiode:
Operational amplifier:
Responsivity 0.2 A W-1 at 1 μm
rms voltage noise 5 nV Hz-1/2 above 1/f corner frequency of
60 Hz
rms current noise 2 pA Hz-1/2 above 1/f corner frequency of
120 Hz
Boltzmann’s constant, k = 1.38x10^-23 J
K-1 Charge on the electron, e =
1.6x10^-19 C
- 2 Figure Q2 1 On The Following Page Shows A Photodiode Circuit For Measuring Small Changes In The Intensity Of Infrar 1 (20.76 KiB) Viewed 27 times