Four-stage transistorized electrometer eliminates the need for a logarithmic compression network. It measures very low currents and produces a digital output directly indicative of the input current magnitude

Doong, H.

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July 1965
	
Brief 65-10206 
NASA TECH BRIEF	 All 
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. 
Sensitive Electrometer Features Digital Output
- -	
Chang Circuit -
 -	 Level 	 Gate j	 Astable Oecdlarnr] 
The problem: To design an electrometer that will 
measure very low currents (10- 6 to 10- 12 amperes) 
and produce a digital output linearly related to the 
magnitude of the input. 
The solution: A four-stage, transistorized electrom-
eter consisting of a charging circuit, a level detector, 
a gate, and an astable oscillator, plus a feedback loop 
to reset the charging circuit. 
How it's done: The gate transistor Qi is normally 
not 'ondiitino and nreventc the noitive supp ly from
voltage is applied to Q2, which generates an output 
pulse. The trailing edge of this pulse is fed back to Q, 
which turns on and discharges C, returning it to zero 
charge level. The level detector Q3 is no longer ener-
gized and the gate Qi turns off. This process is re-a 
peated, producing a series of pulses from the oscillator 
Q2 . Because the time required for C to charge de-
pends on the magnitude of the input current, the fre-
quency of the output pulses from Q2 is a direct indica-
tion of input current magnitude. 
Notes: 
being applied to the astable oscillator Q;. The astable	 I. This circuit eliminates the need for a logarithmic 
oscillator, therefore, generates no output. When an 	 compression network, a temperamental feature of 
input current is applied to the circuit, capacitor C	 prior circuits. 
starts to charge. When the charge on C reaches some 	 2. This electrometer will permit advantage to be 
predetermined level, the level detector Q3 is triggered,	 taken of the capabilities of state-of-the-art sensors 
The level detector turns the gate Q  on and a positive	 in very low current ranges (10- 6 to 10- 12 amperes). 
(continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States Govern-
ment, nor NASA, nor any person acting on behalf of NASA: A. Makes 
any warranty or representation, express or implied, with respect to the 
accuracy, completeness, or usefulness of the information contained in
this document, or that the use of any information, apparatus, method, 
or process disclosed in this document may not infringe privately-owned 
rights; or B. Assumes any liabilities with respect to the use of, or for 
damages resulting from the use of, any information, apparatus, method, 
or process disclosed in this document.
https://ntrs.nasa.gov/search.jsp?R=19650000204 2020-03-11T17:23:52+00:00Z
Previously, data at levels 10- 10 amperes or lower 
have been lost. 
3. Inquiries concerning this invention may be directed 
to:
Technology Utilization Officer 
Goddard Space Flight Center 
Greenbelt, Maryland, 20771 
Reference: B65-10206
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: Henry Doong 
(GSFC-288) 
S
Brief 65-10206	 Category No. 01


Sensitive electrometer features digital output

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