Cappels' project pages
Return to HOME
This is a low
frequency amplifier with
an adjustable transimpedance that is
intended to be used to take relative measurements of a wide range
of photo currents.
Not having many parts, this amplifier can be put together in
a short amount of time.
Find updates at www.projects.cappels.org
I needed to make some relative optical measurements. I had some small
but relatively large area photo diodes, that when operated as
photovoltaic cells generated between 20 nanoamps and 200 microamps with
the range of optical power available from a light source I needed to
test. That is why this amplifier came into being.
The output of the amplifier is to be measured on a digital voltmeter.
Besides being able to accept 20 na to 200 micro amps and output
that range form 100 mv to 2 or 3 volts, the amplifier needed to have a
low input resistance so the diode could be run at or near zero bias so
that its output would be a linear function of light on the photocell.
It also has to have plenty of connectors. Along the bottom of the board
in the image above, from left to right, are a power input connector
because it would use an isolated unregulated DC power supply that is
sometimes used for other things, a voltage output connector so that its
output could be plugged into a digital voltmeter, and an input
connector so that it can accept current from a variety of photodiodes.
The objective of the design was an amplifier that could be built
quickly and use "ordinary" parts, except for the op amp, which needed
to have low input offset current and a decently low input offset
voltage. I also did not want to worry about low noise design, hence the
22 uf capacitor on the input and the use of a digital voltmeter to read
A few keywords to help people find this via search
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.
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
that some loop gain remains and that the amplifier remains stable.
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
If you see this
posted on a web site other than www.cappels.org, please let me know at
the email address below.
Photocell, solar cell, DIY light meter, DIY LED measurement tool.
Contents ©2010 Richard Cappels All Rights Reserved. Find updates
First posted in October, 2010
You can send email to me at
"(at)" with "@" before mailing.
presented on this page is for personal, nonprofit educational and
use only. This material (including object files) is copyrighted by
Cappels and may not be republished or used directly for commercial
For commercial license, click
and intellectual property notice
(Summary: No warranties, use these pages at your
own risk. You may use the information provided here for personal and
educational purposes but you may not republish or use this information
for any commercial purpose without explicit permission.) I neither
express nor imply any warranty for the quality, fitness for any
particular purpose or user, or freedom from patents or other
restrictions on the rights of use of any
software, firmware, hardware, design, service,information, or advice
mentioned,or made reference to in these pages. By utilizing or relying
on software, firmware, hardware, design, service,information, or advice
provided, mentioned, or made reference to in these pages, the user
takes responsibility to assume all risk and associated with said
activity and hold Richard Cappels harmless in the event of any loss or
expense associated with said activity. The contents of this web site,
unless otherwise noted, is copyrighted by Richard
Cappels. Use of information presented on this site for personal,
educational and noncommercial use is encouraged, but unless explicitly
with respect to particular material, the material itself may not be
or used directly for commercial purposes. For the purposes of this
copying binary data resulting from program files, including assembly
code and object (hex) files into semiconductor memories for personal,
educational or other noncommercial use is not considered republishing.
desiring to use any material published in this pages for commercial
should contact the respective copyright holder(s).