An electrometer amplifier uses a field effect transistor to measure currents of low amperage. The circuit, developed as an ac amplifier, is used with an external filter which limits bandwidth to achieve optimum noise performance

Munoz, R.

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November 1964	 Brief 64-10143 
NASA TECH BRIEF	 I& 
This NASA Tech Brief is issued by the Technology Utilization Division to acquaint 
industry with the technical content of an innovation derived from the space program. 
Field-Effect Transistor Improves Electrometer Amplifier 
The problem: Accurate measurement in a rigorous 
environment of small currents approaching the theo-
retical noise limit. Vacuum-tube electrometers employ 
a filament at high temperature thus producing an 
undesirable level of thermal noise and drawing 
appreciable power. The vacuum tube is also vulner-
able in a harsh environment. 
The solution: An electrometer amplifier using a 
field-effect transistor (FET) to measure currents as 
low as 10 3 ampere at room temperature with a 
1-cycle bandwidth. 
How it's done: The circuit illustrated was devel-
oped as an ac amplifier to be used with an external 
filter which limits bandwidth to achieve optimum 
noise performance. A close-coupled feedback pair, 
Q 1 and Q 2, is used in the input stage to achieve 
in-phase operation and cancellation of source-to-gate 
capacitance in the FET Q 3. By feedback, Q 2 improves 
the gain of the input source-follower circuit from a
value of approximately +0.6 to very nearly + 1.0. DC 
bias in the FET is accomplished through the feedback 
resistor R 1, thus eliminating the noise-producing 
biasing resistors ordinarily used. The amplifier draws 
only 4.5 miliwatts and is useful in the frequency 
range from 100 cps to several kilocycles. 
Notes: 
I. Capability of this circuit depends on the FET 
characteristics of low noise and high leakage 
resistance. When FET's having higher leakage 
resistance become available, sensitivity will be 
increased. 
2. Inquiries concerning this invention may be directed 
to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California, 94035 
Reference: B64-10143
(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=19640000043 2020-03-11T17:04:53+00:00Z
Patent status: NASA encourages the immediate 
commercial use of this invention. Inquiries about 
obtaining rights for its commercial use may be made 
to NASA Headquarters, Washington, D.C., 20546. 
Source: Robert Munoz 
(ARC-36) 
Brief 64-10143	 Category No. 01


Field-effect transistor improves electrometer amplifier

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