Minimization of common mode effects in differential amplifier arrangement which processes signals from two high impedance photosensors is achieved by connecting one photosensor in feedback loop of amplifier and using field effect transistors in the input circuit

Fujimoto, H.

English

NASA Technical Reports Server

1972	 B72-10100 
NASA TECH BRIEF 
NASA Pasadena Office	
-0 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Stable Photosensor Amplifiers 
The problem: 
To minimize common mode effects in a differential 
amplifier arrangement which processes signals from 
two high-impedance photosensors. 
-V	 -v	 -V	 -V	 -V 
The solution: 
Connect one photosensor in the feedback loop of 
the amplifier and use field-effect transistors in the 
input circuit. 
How it's done: 
The operational amplifier arrangement was de-
veloped for use in the JPL "Biosatellite Urinalysis 
Instrument;" it is part of the system designed for the 
remote determination of creatinine in the urine of 
small animals. The amplifier processes data generated 
by cadmium sulfide photocells as they view light
transmitted by solutions; appropriate filters are used 
to enhance the sensitivity of analysis as is commonly 
done in analytical procedures based on photometry. 
As reference, a cuvette containing only the reagent 
solution used for the analysis is illuminated by the 
common light source (a lamp); light transmitted by 
the cuvette is sensed by the reference photocell. The 
reference photocell serves as one input source to the 
positive terminal of the operational amplifier that is 
set up to operate in a differential mode. The test 
photocell, which receives light through a cuvette con-
taining a colored solution formed by reaction of 
creatinine with the reagent, is connected in the feed-
back loop of the amplifier. 
The overall arrangement is thus seen to be the 
familiar filter photometer in wide use in chemical 
laboratories. However, since the impedance of the 
resistive photosensors used in the flight instrument is 
fairly high (from 100 to over 800 megohms), two 
field-effect transistors (FET) operating as a high 
impedance differential input stage are used to couple 
the photosensors to the relatively low input imped-
ance of the operational amplifier. The input FET's 
are of primary concern since they can be major con-
tributors to output voltage drift caused by local and 
ambient temperature changes. Because the input 
stages are not preceded by any voltage gain, bias 
changes due to temperature fluctuation will appear 
at the output just as though there had been a change 
in reference voltages. The matched FErs operating 
in differential mode minimize initial offset of the 
gate-source voltage, and the effects of changes in drain 
current values are also held at a minimum by biasing 
each FET at its zero-temperature-coefficient voltage; 
since the devices are matched, the bias point is very 
(continued overleaf) 
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https://ntrs.nasa.gov/search.jsp?R=19720000100 2020-03-17T03:24:58+00:00Z
nearly the same for both transistors (about +1.5 V 
at 150 microamperes). The two FETs are biased in 
a manner which places their drains close to ground 
and provides optimum coupling to the integrated-
circuit operational amplifier. Dual NPN transistors 
are used to shift the drain voltage to zero, that is, 
the optimum value for the operational amplifier. 
Stability of the light source is of great importance; 
hence, a power supply with a regulation of better 
than 0.01% was used. The photocells are screened for 
matched characteristics with respect to temperature 
and resistance excursions; matching was effected to 
better than 1% for both parameters. Moreover, the 
entire circuit was operated at the Q-point in the 
voltage-transfer curve, that is, the point where changes 
in test cell resistance cause no change in output 
voltage.
Notes: 
1. A variation of this basic circuit has been developed 
for the fluorometric determination of calcium in 
spacecraft operations. 
2. Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: B72-10100 
Patent status: 
No patent action is contemplated by NASA. 
Source: Hisashi Fujimoto of 
Caltech/JPL
under contract to 
NASA Pasadena Office 
(NPO-11561)
Category 01
S 


Stable photosensor amplifiers

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