Input impedances of several hundred megohms and input capacitances of less than one picofarad are achieved with a conventional unity gain are buffer amplifier modified by addition of positive feedback. The circuit is stable over a temperature range of 25 to 70 deg C

Birchenough, A. G.

English

NASA Technical Reports Server

e0 
December 1970
	
M. 1313
	 Brief 70-10651 
KSC HQS. 
—1-
 NA	 Dicrrcr 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage commercial 
application. Tech Briefs are available on a subscription basis from the National Technical Information Service, Springfield, 
Virginia 22151. Requests for individual copies or questions relating to the Tech Brief progam may be directed to the Tech-
nology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
AC-Coupled Ultrahigh Input Impedance Amplifier 
High input impedance and low input capacitance 
have been achieved with a conventional unity gain 
buffer amplifier modified by the addition of positive 
feedback. The circuit, which is stable over a tempera-
ture range of 25° to 70°C, achieves input impedances 
of several hundred megohms and input capacitances 
of less than 1 picofarad (pf).
CF = 68 Picofarads ClN 2 Picofarads 
=0 .1.Microfarad 
R
.A = 5009 R8=1OK 
The high input impedance is obtained by positive 
feedback through capacitor CA to the plus (+) input 
of the amplifier (see fig.). The input capacitance plus 
the capacitance to ground is C, which can be can-
celled by adding the feedback capacitor CF and ad-
justing resistor RA. The following relationship applies:

RA CIN 
Low frequency response is determined primarily by 
capacitor CA, for which an electrolytic capacitor can 
be used. High frequency response is limited by the 
operational amplifier. The value of RA isdetermined 
by applying a square wave to the input and adjusting 
RA to produce an acceptable square wave output, 
in much the same manner that a lox scope probe 
is adjusted. 
The circuit was used to amplify a 5 i.tsec wide pulse 
coupled through a capacitor of less than 1 pf. Slew rate 
for the circuit was approximately 0.5 V/isec. 
Gains other than unity can be obtained, but this 
would probably be simpler if another separate ampli -
fier is used. 
Note: 
No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B70-10651 
Patent status: 
No patent action is contemplated by NASA. 
Source: Arthur G. Birchenough 
Lewis Research Center 
(LEW-1 1154) 
Category 01 
This document was prepared under the sponsorship of the National 
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 ownerights.
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Ac-coupled ultrahigh input impedance amplifier

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