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Simply an amplifier and a +/- 4 volt power supply.
Starting with the power supply. An input voltage of 12 to 16 volts DC
is regulated to + 8 volts. R1 assures that a minimum of 10 milliamps
flows through the 1N5227, 3.6 volt Zener diode. All together, this
makes approximately 8.6 volts. This would have been simpler with a 7808
regulator, and a low power version of the regulator would have been
plenty sufficient, but one works with what one has on hand.
The 8 volts is split by R2 and R3 to produce a ground reference voltage
at half the regulated supply voltage. This results in power to the
opamp of +4 volts and -4 volts. Very little current flows between the
power positive and negative rails, so U1A, which provides a low
impedance ground reference, is not really needed, but the opamps came
two to a package, and not having to pay much attention to the effects
power supply current returning through the ground made life a little
simpler, so I went ahead and used U1A in this role.
The photocell is connected to the amplifier through approximately two
meters of Belden RG-174/U miniature coax cable.
All I want to measure is the average current so the
current from the photocell is filtered by C3 to reduce the amount of
hum and other modulation of the current. The capacitor, along with R4
form a 20 Hz low single pole low pass filter which after
examination of the output on a scope, is adequate for my needs.
The actual amplification is done by the marvelous TLC27l2 opamp.
Feedback
from the output to the inverting input through 3.3k R5 and the 500k pot
allows transimpedances from very low 3.3 k ohms to 500 k ohms. R4 is
to limit noise gain while R5 limits the closed loop gain, thereby
assuring
that some loop gain remains and that the amplifier remains stable.
My
experience with the amplifier
I have used this amplifier along with the photodiode it was
intended to be used with a few times now for comparative measurements
(measure a reference, then measure the device under test) and am very
happy with its performance. I did not expect it to be as good as
it is, and I give all of the credit to the TLC27L2 opamp. Short term
drift was not apparent, and the offset with no current input only
varies about 1.5 millivolts as the transconductance is varied from 3.3k
to 500k.
If you see this
posted on a web site other than www.cappels.org, please let me know at
the email address below.