Photoactivated bridge facilitates equal performance of analog multipliers over a wide frequency range. The multiplier operates from direct current to an upper frequency limited by either the light source or the closed-loop amplifier

Hartenstein, R. G.

English

NASA Technical Reports Server

September 1965
	
Brief 65-10287 
NASA TECH BRIEF H 
NASA Tech Briefs are issued by the Technology Utilization Division to summarize specific 
technical innovations derived from the space program. Copies are available to the public from 
the Clearinghouse for Federal Scientific and Technical information, Springfield, Virginia, 22151. 
Photoresistance Analog Multiplier Has Wide Range 
E2 
The problem: Analog multiplication involving two 
or more variables of either polarity performed over a 
wide frequency range. To achieve equal performance 
with previous multipliers would require extremely 
complex, expensive electronics requiring major adjust-
ments. 
The solution: Solid-state photodiodes are placed 
in the arms of a simple Wheatstone bridge and ex-
posed to a light source whose intensity is proportional
to the input signal voltage. The voltage potential 
across the bridge is proportional to the product of the 
voltage applied to the light source and the voltage 
applied to the bridge. 
How it's done: The photoresistance analog multi-
plier has two basic units, the light source and the 
Wheatstone bridge. If E2 is held constant, the resist-
ances of the photodiodes Rd vary inversely with the 
potential of El. If E1 increases, Rd decreases, causing 
(continued overleaf) 
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Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
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Ea to fall and Eb to rise, in turn causing a linear in-
crease in Eout, which is Eb - Ea. If El decreases, then, 
by the inverse process, a linear decrease in Eout occurs. 
If E l is held constant, then an increase in E2 causes 
an increase in E0 t and a decrease in E2 causes a de-
crease in Eout . Thus the voltage Eout is a linear func-
tion of the light source and the voltage applied to the 
bridge, or Eout, is proportional to El times E2. 
The differential amplifier in the closed loop of the 
light source compares the output of a photodiode, 
identical to those employed in the bridge circuit, to the 
input Et. The output of the photodiode and potential 
of El are made essentially equal (discounting the bias 
offset). 
Notes: 
1. A series of multiplications can be carried out or 
variables may be taken to any desired power by the 
addition of other photoactivated bridges.
2. The multiplier operates from direct current to an 
upper frequency limited by either the light source 
or the closed-loop difference amplifier. 
3. Inquiries concerning this invention may be directed 
to:
Technology Utilization Officer 
Goddard Space Flight Center 
Greenbelt, Maryland, 20771 
Reference: B65-10287 
Patent status: NASA encourages the immediate 
commercial use of this invention. Inquiries about ob-
taining rights for its commercial use may be made to 
NASA, Code AGP, Washington, D.C., 20546. 
Source: Raymond G. Hartenstein 
(GSFC-360) 
Brief 65-10287	 Category 01


Photoresistance analog multiplier has wide range

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