Preamplifier chops and amplifies very low level output of thermopile infrared detectors that will be used to measure thermal energy flux of Jupiter and its departure from a blackbody spectrum. Output signal has negligible phase shift. AC signal is RC-coupled to input stage of other amplifiers

Callaway, P. W.

English

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1R9
C9 P14 
r1­-
-1:4 
MVA 
1972	 Brief 72-10165 
NASA TECH BRIEF 
Ames Research Center 
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 program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Differential Input Preamplifier 
The problem: 
To chop and amplify the very low level output of 
thermopile infrared detectors that will be used to 
measure the thermal energy flux of Jupiter and its 
departure from a blackbody spectrum. For subse-
quent synchronous demodulation, the output signal 
must have a negligible phase shift and very low input 
noise at the AC carrier frequency. 
The solution: 
An amplifier which chops input signals (because 
the input signal levels are much lower than the input 
drift of available solid state devices); the resulting 
AC signal is RC-coupled to the input stage of other 
amplifiers. 
How it's done: 
The input DC signal is modulated full wave by 
two pairs of matched-junction field-effect transistors 
Qi, Q2. A two-phase chopper driver signal alter-
nately turns off one pair of transistors with a reverse 
bias voltage and allows the other pair to conduct at 
zero bias. The signal input is reversed every half 
cycle, producing a full wave differential signal to 
the input RC coupling network (C2, R3, C5, R9). 
The differential input signal is applied to the gates 
of a matched dual junction FET (Q3), which has 
excellent noise characteristics at low current. The 
amplified signal is applied to the input of a mono-
lithic AC differential amplifier: closed-loop feedback 
is achieved by a feedback resistor between the out-
put and the source connection of Q3. Closed loop 
gain is set by the ratio of the feedback resistor R14 
to the Q3(2) source resistor R8. A matched network
(R4, 116) is used in the source connections to Q8 
for maximum common mode rejection. It is possible 
to increase common mode rejection about 8 dB by 
trimming R4, but adequate rejection is achieved 
Chopper Drive 
# 1 In 
p	 II-
C5j 
__ ___ 
ILHF 
- C3 
# 2 In
Q2 Chopper Drive 
without trimming and no adjustments are required. 
Closed-loop bandwidth is set by the R14, C9 time-
constant to 40 kHz. Capacitor CS is matched to C9 
for increased high frequency rejection. 
Notes: 
1. Test results indicate that the amplifier has a 3-dB 
noise figure for source impedance of 15,000 ohms, 
improving with higher impedance. Common mode 
rejection is 94 to 100 dB without trimming. The 
power requirement is low, 0.6 mA from plus and 
minus 14-volt sources (17 mW).
(continued overleaf) 
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 owned rights.
https://ntrs.nasa.gov/search.jsp?R=19720000165 2020-03-17T03:31:37+00:00Z
2. Requests for additional information may be di-
rected to: 
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP 72-10165
Patent status: 
No patent action is contemplated by NASA. 
Source: Paul W. Callaway of
Santa Barbara Research Center 
under contract to
Ames Research Center 
(ARC-10489) 
Brief 72-10165	 Category 01


Differential input preamplifier

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