Temperature compensated solid state differential amplifier with application in bioinstrumentation circuit

Smith, J. R., Jr.

English

NASA Technical Reports Server

July 20, 1965 J. R. SMITH, JR 3,196,362 
TEMPERATURE COMPENSATED SOLID STATE DIFFERENTIAL AMPLIFIER 
Filed Jan. 4. 1962 
INVENTOR. 
JOSFPH R. SMITH JR . 
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3,196,362 United States Patent Ofice ,, , ,, ,, 
TEWERATURE COMPENSATED SOLID STATE 
1)IFFERENTIAL AMPLIFIER 
Josenh R. Smith. Jr.. Milpitas. Calif., assignor to the 
7Jhited States o'f ~merica-as  re~resented bvthe Admin- 
istrator of the National Aeronautics" and Space 
Administration 
Filed Jan. 4,1962, Ser. No. 164,428 
2 Claims. (Cl. 330-14) 
(Granted under Title 35, U.S. Code (19523, sec. 266) 
The invention described herein may be manufactured 
and used by or for the Government of the United States 
of America for governmental purposes without the pay- 
ment of any royalties thereon or therefor. 
The present invention relates to a differential amplifier 
having features and capabilities particularly adapting it to 
utilization in circuitry associated with biomedical equip- 
ment such as an electrocardiograph. 
In the field of medical instrumentation it is necessary 
to amplify relatively low-level signals in the presence of 
substantial and variable noise signals, while at the same 
time minimizing the size and complexity of the amplifica- 
tion means. One example of the applicability of the 
present invention lies in the amplification of signals re- 
ceived from electrodes attached to the body of a person 
for the purpose of supplying an input to an electrocardio- 
graph. The above-noted requirements are of particular 
importance under the conditions wherein the identified 
person may be confined in a limited space and subjected 
to substantial stresses as might occur in supersonic flight. 
In the measurement of cardiovascular functions by an 
electrocardiograph, electrodes may be secured at  appro- 
priate points on the body of a person by suitable ad- 
hesives, and the amplifier hereinafter described may be 
employed to amplify output voltage from such electrodes 
for actuation of an electrocardiograph. In this type of 
application, it is necessary that the input voltages to the 
amplifier be maintained quite low in order not to endanger 
the person upon which the electrodes are secured. Be- 
cause of these low input voltages, the elimination of noise 
pickup on the body of the person becomes a serious prob- 
lem and the present invention is particularly directed to 
the provision of a very high ratio common mode rejection 
and adjustable cancellation for unbalanced noise signals. 
This adjustable cancellation is important inasmuch as 
noise pickup from the body of a person is not always 
balanced even though the input lines to the amplifier are 
themselves balanced. The present invention furthermore 
operates upon a very low voltage supply to maximize 
safety and includes an automatic temperature compensa- 
tion which is particularly important for applications 
wherein widely varying ambient temperatures are en- 
countered. 
I t  is the object to  the present invention to provide an 
improved difierential amplifier circuit having adjustable 
noise cancellation for the elimination of unbalanced noise 
signals. 
It  is another object of the present invention to provide 
an improved differential amplifier having an automatic 
temperature compensation network therein. 
It is a further object of the present invention to provide 
a transistorized differential amplifier circuit having both 
low and high frequency response. 
It is yet another object of the present invention to pro- 
vide an improved differential amplifier having a very high 
ratio of common mode rejection of input noise signals. 
There is illustrated in the accompanying drawing a cir- 
cuit diagram of a preferred embodiment of the present 
invention. 
Considering now the invention hereof in some detail 
and referring to the attached drawing, there will be seen 
to be illustrated input terminals 11, 12, 13, and 14, the 
latter of which being grounded. Terminal 11 is adapted 
tu be connected to a power supply, not shown, which 
produces a direct current voltage of a relatively low order, 
as for example, four volts. Terminals 12 and 13 may be 
ti connected by balanced leads to separate electrodes at- tached to the body of a person and these two terminals 
are coupled through separate capacitors to the bases of 
separate transistors 16 and 17, respectively. A diode net- 
work 18 is connected between each of the input terminals 
12 and 13 and ground, as indicated, in order to prevent 
lo saturation of the input transistors 16 and 17 from overly 
large noise signals possibly appearing in the input of the 
circuit. These diodes form low resistance paths to a com- 
mon ground lead 19 for large input signals. 
The double-ended input of the amplifier circuit here- 
l5 of is afforded by the transistors 16 and 17 which, as illus- 
trated, have the emitters thereof connected through like 
resistors to the ground line 19 and the collectors thereof 
connected to the base and emitter of a second transistor 
20 21. With this circuitry it will be seen that like input signals 
impressed upon the transistors 16 and 17 will produce cor- 
responding voltage variations at  the collectors thereof, 
so that no voltage change occurs between the base and 
emitters of the transistor 21. This arrangement provides 
25 for a conversion from a double-ended input to a single- 
ended output and at the same time provides for a com- 
mon mode rejection, wherein like voltage variations of 
the input terminals 12 and 13  with respect to ground do 
not produce signals passing through the amplifier to 
30 the output thereof. These common mode signals are un- 
wanted signals in this circuitry, and it is desired to amplify 
the voltage variations occurring between the terminals 12 
and 13 rather than between these terminals and ground. 
Adjustable noise cancellation is afforded in the circuit 
35 of the present invention by the potentiometer 22 having a 
resistor coupled to the collectors of the input transistors 
16 and 17 and a variable contact connected to the power 
supply line of terminal 11. By varying the position of 
the movable contact of this potentiometer, it will be seen 
40 to be possib!e to increase the gain of one amplifier tran- 
sistor while reducing the gain of the other and thus to 
balance the circuit under conditions wherein the noise 
level a t  the two input terminals 12 and 13 are not equal. 
This arrangement allows the signal gain to be kept con- 
45 stant while providing for equalizing the noise gain. 
The second transistor 21 serves to convert the double- 
ended input system to a single-ended system, and while 
this circuitry does not afford any amplification it does 
provide for input into a wide load range, in that the 
60 transistor 21 in effect operates as a current source pro- 
ducing at the collector thereof a current proportional 
to the signal from the bridge formed by the transistors 
16 and 17. Following the transistor 21 in the circuit 
is a direct coupled amplifier including successive tran- 
55 sistors 26, 27, and 28, The first of these transistors has 
the base thereof directly coupled to the collector of the 
transistor 21, and appropriate resistors couple the col- 
lector of this transistor 26 to the power supply line and 
the emitter to  the ground line. k resistor couples the out- 
60 put signal from the transistor 26 to the base of the tran- 
sistor 27 and a direct connection is provided between the 
collector of the transistor 27 and the base of the final tran- 
sistor 28. Power is supplied to  these transistors 27 and 
28 by a resistive coupling of the collectors thereof to the 
65 power supply line. 
An emitter-follower connection is employed to supply 
the output signal from the amplifier hereof, and in this 
respect a resistor 29 is connected to the emitter of the 
?O last transistor 28, with output terminals 31 and 32 being 
connected across this resistor. Ground connection of the 
transistors 27 and 28 is afforded by a potentiometer 33 
3,196,362 
3 4 
having the n~ovable contact thereof connected to the input terminals may differ from that of the other. A fur- 
ground line 19 and the resistor thereof connected from ther advantage gained from the circuitry including the po- 
the emitter of the transistor 27 to the resistor 29 in the tentiometer 22 is the availabality of matching the outputs 
emitter circuit of the final transistor 28. An additional of the input transistors without the necessity of providing 
resistor 34 may be connected in parallel with the resistor j actual matched transistors. The adjustable gain feature 
of the potentiometer 33 and an additional output terminal hereof thus allows the utilization of conventionally manu- 
36 may be coupled through a resistor 37 to the emitter factured transistors without requiring that these be in- 
of the transistor 27 dividually matched to balance same under operating con- 
The direct coupled amplifier described immediately ditions. Balancing of the output "zero" level is not ef- 
above may operate in a relatively conventional manner 10 fected by balancing of the noise voltages with the po- 
to amplify signals received from the transistor 21, and tentiomelcr 22, inasmuch as the temperature compensat- 
appropriate connections to the output terminals of the ing network is included herein as a feedback circuit. 
circuit hereof provide a choice of output voltages for en- An actual amplifier c~rcuit constructed as described 
crgizing equipment such as medical instrumentation which above and illustrated in the accompanying drawing has 
may be attached hereto. Unwanted signals in the form 15 been found to have a common mode rejection of approxi- 
of various types of noise signals are canceled out in the mately ten thousand to one, referenced to the input. This 
circuit hereof so that they do not appear in the output of amplifier employing a four volt direct current supply re- 
the amplifier. Voltage differences existing between ground quired eight milliwatts of power to produce a one volt 
and the two input terminals 12 and 13 are balanced out in peak-to-peak output with a 3.6 millivolt input signal. 
the input circuitry so as not to appear as driving signals to 20 The circuit exhibited a high input impedance and a very 
the transistor 21, while adjustable noise level cancellation good low frequency response which was down only three 
is afforded by the potentiometer 22 so that this possible decibels at 0.5 cycle per second and a similar frequency 
source of error signal is prevented from passing to the response at 70,000 cycles per second. Both high and low 
output of the circuit. output impedances are available, and this circuit pro- 
In addition to the foregoing, the amplifier hereof in- 25 duced an output of one volt peak-to-peak feeding into a 
cludes circuitry for automatic temperature compensation. 10,000 ohm load and/or 300 microamperes feeding into a 
This circuitry is illustrated in the accompanying drawings 33 ohm load for a 3 millivolt input. 
as including a transistor 41 having the base thereof cou- Although the present invention has been described above 
pled through a resistor to the collector of the output tran- with reference to a single preferred embodiment thereof, 
sistor 28. The emitter of this transistor 41 is coupled 30 it will be apparent to those skilled in the art that various 
through a resistor to the power supply line and also through modifications and alternatives may be made without de- 
a resistor 42 to the base of the input transistor 16. The parting from the true spirit of the invention. It  is thus 
collector of the transistor 41 is directly coupled to the not intended to limit the invention by the terms of the 
emitter of the input transistor 16 so that there is formed description or details of the drawing, but instead reference 
a feedback circuit from the output transistor 28 to the in- 35 is made to the following claims for a precise definition of 
put transistor 16. A capacitor 43 is connected from the the actual scope of the invention. 
base of the feedback transistor 41 to the power supply What is claimed is: 
line for bypassing alternating current signals so that only 1. An alternating current amplifier comprising a pair 
direct current signals are fed back to the input of the am- of input transistors having the bases thereof separately 
plifier. Negative feedback is herein employed, so that 40  coupled to a pair of input terminals and the emitters there- 
the direct current signals returned to the input transistor of coupled to a common terminal for input signals, a 
16 are of opposite polarity to the direct current output potentiometer connected between collectors of said input 
variations of the amplifier, and consequently, serve to can- transistors and to power supply means for adjustable can- 
cel these variations. Alternating current signals at the cellation of unwanted input signals, a conversion tran- 
output of the amplifier are not fed back, and consequently, 45 sistor, means coupling the collectors of said input tran- 
these desired amplified signals appear in the output and sistors between the base and emitter of said conversion 
no negative feedback is provided for cancellation thereof. transistor whereby the output of said latter transistor is 
In  this manner the direct current voltage level and the al- proportional only to variations in voltage between said in- 
ternating current gain of the amplifier are held constant. put terminals, at least amplifying transistor having the 
This then provides for a temperature compensation to 50 base thereof coupled to the collector of said conversion 
prevent direct current drift of the circuit with variations transistor and producing an amplified output across a re- 
in temperature of the circuit components which change sistor connected to the emitter thereof, and a negative 
the operating parameters of these components. feedback circuit including a transistor having a base cou- 
The above-described circuit of the present invention will pled to the collector of said amplifier transistor and the 
be seen to provide for the amplification of small alternat- 55 collector and emitter connected across the base and emit- 
ing current signals to the exclusion of large or small input ter of one of said input transistors with a bypass capaci- 
noise signals while maintaining a constant alternating cur- tor in the base circuit of said feedback transistor, where- 
rent gain and providing temperature stabilization of the by only direct current amplifier output signals are fed 
circuitry. Variations in voltages appearing at the input back to provide temperature stabilization of the circuit. 
terminals 12 and 13 produce a voltage change between 60 2. A biomedical amplifier comprising a pair of input 
the base and emitter of the second transistor 21 to there- terminals adapted to have impressed thereon voltages with 
by provide input signals to the direct coupled amplifier respect to ground, a diode circuit between each of said in- 
feeding the output terminals of the circuit. Like varia- put terminals and ground for bypassing large input volt- 
tions of the input signals with respect to ground, produce ages, first and second like transistors having the bases 
voltage variations at the collectors of the input transistors 65 thereof coupled to separate input terminals and the emit- 
16 and 17 which are equal, and consequently, do not ters coupled to ground, a potentiometer coupled between 
establish any voltage change between the base and emitter collectors of said first and second transistors and to power 
of the subsequent transistor 21 so that same do not pro- supply means for varying the relative gains of said tran- 
duce signals in the direct coupled amplifier. The po- 70 sistors, a third transistor having a base connected to a 
tentiometer 22 provides for increasing the gain of one collector of the first of said transistors and an emitter con- 
of the input transistors while decreasing the gain of the nected to the collector of said second transistor for pro- 
other to maintain a constant signal gain while equalizing ducing an output current proportional to voltage difference 
the noise gain. This is particularly important in bio- variations at said input terminals, a plurality of direct 
medical applications, wherein the noise level at one of the 75 coupled transistors connected between the collector of 
3,186,362 
5 6 
said third transistor and ot~tput erminal for producing am- 3,046,487 7/62 Matzen ct al. -------- 330-30 X 
plified alternating current output signals, and a direct cur- 3,077,566 2/63 Vosteen ------------ 330-19 X 
rent negative feedback system including a transistor cou- 3,101,437 8/63 Photinos ------------ 330-14 X 
pled to  the collector of the last of said direct coupled 
transistors with a capacitor bypassing alternating current 6 OTHER REFERENCES 
signals thereto and connected in emitter-follower arrange- Conrad "Differential Amplifier Null Detector," Elec- 
ment across the input of said first transistor. tronics, vol. 23, February 1950, pages 96-97. 
Flynn. M. J. "CTRL Latch," IBM Technical Disclosure References Cited by the Examiner Bulletin, vol. 1,  No. 6, April 1959, page 20. 
UNITED STATES PATENTS 10 
2.954.530 9/60 Haskell --------------- 330-19 ROY LAKE. Primary Examiner. 
219991169 9/61 Feiner ------------- 330-28 X JOHN KOMINSKI, Examiner. 


Temperature compensated solid state differential amplifier Patent

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