Circuit utilizing a transistorized differential amplifier is developed for biomedical use. This low voltage operating circuit provides adjustable cancellation at the input for unbalanced noise signals, and automatic temperature compensation is accomplished by a single active element across the input-output ends

Smith, J. R., Jr.

English

NASA Technical Reports Server

March 1964	 Brief 63-10003 
NASA TECH BRIEF 
This NASA Tech Brief is issued by the Technology Utilization Division to acquaint industry 
with the technical content of an innovation derived from the NASA space program. 
New Low-level A-C Amplifier Provides Adjustable Noise Cancellation
and Automatic Temperature Compensation 
The problem: In the field of medical instrumenta-
tion it is necessary to amplify low-level signals in the 
presence of relatively large variable noise signals. The 
size and complexity of the amplifier must be kept to a 
minimum. 
The solution: The transistorized differential am-
plifier represented in the circuit diagram amplifies 
signals received from electrodes attached to the body 
of a person for the purpose of supplying an input to 
an electrocardiograph. The circuit provides a very-
high-ratio common-mode rejection (approximately 
10,000 to 1, referred to the input) and adjustable 
cancellation at the input for unbalanced noise signals. 
Cancellation for unbalanced noise is particularly im-
portant in biomedical applications, because the input 
terminals are often widely separated on the body 
where the noise levels may differ. It operates on a 
very-low-voltage supply to assure safety and includes a 
single active element across the input-output ends for 
automatic temperature compensation, which is partic-
ularly important where widely-varying ambient tem-
peratures are encountered. An adjustable gain feature 
in the circuit does not require the use of matched input 
transistors. Operating on the 4-volt d-c supply, the 
amplifier requires 8 milliwatts of power to produce a 
I-volt peak-to-peak output with a 3.6-millivolt input 
signal. 
How it's done: The d-c power supply (not shown) is 
(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 
rig hts; or B. Assumes any liabilities with respect to the use of, or for 
damag es resulting from the use of, any information, apparatus, method, 
or process disclosed in this document.
https://ntrs.nasa.gov/search.jsp?R=19630000001 2020-03-11T16:45:25+00:00Z
connected to input terminal 1. Input terminals 2 and 3 
may be connected by balanced leads to separate elec-
trodes attached to the body of a person. The two 
terminals (2 and 3) are coupled through separate 
capacitors to the bases of transistors Qi and Q2, 
respectively. A diode network 4 is connected between 
each of the input terminals 2 and 3 and ground in 
order to prevent saturation of the input transistors Q  
and Q2 from overly large noise signals possibly ap-
pearing in the input of the circuit. These diodes form 
low-resistance paths to a common ground lead 5 for 
large input signals. 
Like input signals impressed upon the transistors 
QI and Q2 will produce corresponding voltage varia-
tions at the collectors of these transistors, so that 
no voltage change occurs between the base and emitter 
of transistor Q3. This arrangement provides for con-
version from a double-ended input to a single-ended 
output and at the same time provides for a common 
mode rejection; i.e., like voltage variations of the 
input terminals 2 and 3 with respect to ground do not 
produce signals passing to the output of the amplifier. 
Adjustable noise cancellation is afforded by poten-
tiometer 6 which has a resistor coupled to the collec-
tors of the input transistors Q  and Q2 and a variable 
contact connected to the power supply line of terminal 
1. By changing the position of the variable contact, 
it is possible to increase the gain of one amplifier 
transistor while reducing the gain of the other and 
thus to balance the circuit when the noise levels at the 
two input terminals 2 and 3 are not equal. This ar-
rangement allows the signal gain to be kept constant 
while providing for equalizing the noise gain. 
Transistor Q3 in effect operates as a current source 
producing at its collector a current proportional to the 
signal from the bridge formed by the transistors Q  
and Q2. Following transistor Q3 is a direct-coupled 
amplifier including transistors Q4, Q5, and Q6. 
An emitter-follower connection is employed to 
supply the output signal from the amplifier, and in this 
respect a resistor 7 is connected to the emitter of the
last transistor Q6, with output terminals 8 and 9being 
connected across this resistor. An additional resistor 
10 may be connected in parallel with the resistor of 
the potentiometer 11, and an additional output ter-
minal 12 may be coupled through a resistor 13 to the 
emitter of transistor Q5. 
The circuitry for automatic temperature compen-
sation includes a transistor Q7 having its base coupled 
through a resistor to the collector of the output trans-
istor Q6. A capacitor 14 is connected from the base of 
the feedback transistor Q7 to the power supply for 
bypassing alternating current signals, so that only 
direct current signals are fed back to the input of the 
amplifier. Negative feedback is employed, so that the 
direct current signals returned to the input transistor 
QI are of opposite polarity to the direct current out-
put variations of the amplifier, and consequently, 
serve to cancel these variations. In this way compen-
sation is provided to prevent direct-current drift of the 
circuit with variations in temperature of the circuit 
components. 
Notes: 
I. A unit constructed with particular circuit-com-
ponent values was operational in approximately 
20 seconds and was 3db down in gain at 0.3 cps 
and at 70,000 cps. Its low frequency response is 
determined by the temperature-compensating net-
work. 
2. This amplifier should prove useful in many appli-
cations, including hi-fl equipment. 
Patent status: NASA encourages the immediate 
commercial use of this invention. It is owned by 
NASA, and a patent application has been filed. When 
patented, royalty-free non-exclusive licenses for its 
commercial use will be available. Inquiries concerning 
license rights should be made to NASA Headquarters, 
Washington, D.C. 20546.
Source: Joseph R. Smith, Jr. 
Ames Research Center (ARC-2) 
Brief 63-10003


New low-level a-c amplifier provides adjustable noise cancellation and automatic temperature compensation

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