A circuit is described for adjusting the amplitude of a reference signal to a predetermined level so as to permit subsequent data signals to be interpreted correctly. The circuit includes an operational amplifier having a feedback circuit connected between an output terminal and an input terminal; a bank of relays operably connected to a plurality of resistors; and a comparator comparing an output voltage of the amplifier with a reference voltage and generating a compared signal responsive thereto. Means is provided for selectively energizing the relays according to the compared signal from the comparator until the output signal from the amplifier equals to the reference signal. A second comparator is provided for comparing the output of the amplifier with a second voltage source so as to illuminate a lamp when the output signal from the amplifier exceeds the second voltage

Holley, L. D.

Ward, J. O.

English

NASA Technical Reports Server

United States Patent 
Holley et al. 
[54] DIGITAL AUTOMATIC GAIN AMPLIFIER 
[76] Inventors: James C. Fletcher, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of Larry D. Holley, 
Merritt Island; James 0. Ward, 
Titusville, both of Fla. 
[21] Appl. No.: 780,729 
[22] Filed: Mar. 24,1977 
[51] Int. (3.2 ........................... H03F 1/00; H03F 1/36 
[52] U.S. Cl. ..................................... 330/2; 324/99 D; 
324/123 C; 330/51; 330/86 
[58] Field of Search ............ 324/99 D, 123 C, 123 R, 
324/130; 330/2, 51, 86 
[561 References Cited 
U.S. PATENT DOCUMENTS 
3,159,787 12/1964 Sexton et al. ................ 324/123 R X 
3,539,936 11/1970 McGhee ................................ 330/29 
3,760,287 9/1973 Harris ................................ 330/51 X 
Primary Examiner-Lawrence J. Dah1 
4,109,213 
[45] Aug. 22, 1978 
Attorney, Agent, or Firm-James 0. Harrell; John R. 
Manning 
[571 ABSTRACT 
A circuit for adjusting the amplitude of a reference 
signal to a predetermined level so as to permit subse- 
quent data signals to be interpreted correctly. The cir- 
cuit includes an operational amplifier having a feedback 
circuit connected between an output terminal and an 
input terminal; a bank of relays operably connected to a 
plurality of resistors; and a comparator comparing an 
output voltage of the amplifier with a reference voltage 
and generating a compared signal responsive thereto. 
Means is provided for selectively energizing the relays 
according to the compared signal from the comparator 
until the output signal from the amplifier equals to the 
reference signal. A second comparator is provided for 
comparing the output of the amplifier with a second 
voltage source so as to illuminate a lamp when the 
output signal from the amplifier exceeds the second 
voltage. 
6 Claims, 1 Drawing Figure 
46 
UAUUAL SET 
so - Jc++--+ CLOCK 1- 
EMOTE SET 0 
I . SEQUENCE CONTROL SIGNAL 
https://ntrs.nasa.gov/search.jsp?R=19790014202 2020-03-20T19:01:01+00:00Z
U.S. Patent Aug. 22,1978 4,109,2 13 
4,109,2 13 
a 1 
DIGITAL AUTOMATIC GAIN AMPLIFIER 
ORIGIN OF THE INVENTION 
The invention described herein was made in perfor- 
mance of work under a NASA contract and is subject to 
the provisions of Sections 305 of the National Aeronau- 
tics and Space Act of 1968, Public Law 85-568 (72 Stat. 
435, 402 USCP 2457). 
BACKGROUND OF THE INVENTION 
The present invention relates to a digital automatic 
gain amplifier, and more particularly to a device for 
adjusting the amplitude of a reference signal to a prede- 
termined level so as to permit subsequent data signals to 
be interpreted correctly. 
A variable gain amplifying apparatus wherein the 
output of an amplifier may be veried by means of digital 
signals is disclosed in U.S. Pat. No. 3,470,487 granted to 
P. J. Lukas et al, on Sept. 30, 1969. Another automatic 
range changing circuit for attenuating stepwise the 
voltage output of a-current-to-volts amplifier is dis- 
closed in U.S. Pat. No. 3,539,936 granted to John D. 
McGhee. Both of these devices teach varying the resis- 
tance in the feedback loop by means of electronic cir- 
cuitry such as transistor switches. Normally, prior to 
analyzing signals stored on tapes the analyzing equip- 
ment being used has to be calibrated so that a calibration 
reference signal which preceded the data signals pro- 
duces a predetermined voltage. Heretofore, usually this 
required manual adjustment by means of potentiome- 
ters, which was time consuming. 
SUMMARY O F  THE INVENTION 
The invention includes an apparatus for adjusting the 
amplitude of a reference signal to a predetermined level 
so as to permit subsequent data signals to be interpreted 
correctly. The apparatus includes an operational ampli- 
fier which has a feedback circuit connected between an 
output terminal and an input terminal. A plurality of 
relays are used for selectively placing respective resis- 
tors in said feedback circuit upon being energized. A 
comparator is connected to the output of the opera- 
tional amplifier for comparing the output voltage with a 
reference voltage and generating a compared signal 
responsive thereto. This compared signal is fed to a 
logic circuit, which in turn, automatically energizes and 
de-energizes the relays for placing different resistors in 
the feedback circuit until the output voltage of the oper- 
ational amplifier equals a reference voltage. 
Accordingly, it is an important object of the present 
invention to provide an apparatus for adjusting an oper- 
ational amplifier until a calibration reference sAtnal 
produces a predetermined output signal. 
Still another important object of the present inven- 
tion is to provide an apparatus which can be automati- 
cally adjusted so that a calibration signal from a tape or 
the like produces an output voltage of a predetermined 
value. 
These and other objects and advantages of the inven- 
tion will become apparent upon reference to the follow- 
ing specification, attendant claims and drawing. 
BRIEF DESCRIPTION O F  THE DRAWING 
L 
DESCRIPTION O F  A PREFERRED 
EMBODIMENT 
In analyzing signals carried on magnetic tapes such as 
5 acoustic data, normally a calibration signal of a prede- 
termined voltage is placed on the tape prior to record- 
ing the acoustic data. When reproducing this signal or 
this information so that such can be analyzed, it is neces- 
sary to set the gain of the equipment being used to ana- 
10 lyze the signal with a reference voltage. Normally this 
is accomplished by adjusting potentiometers for vary- 
ing the gain of an amplifier until an output signal pro- 
duced by the calibration reference signal is equal to the 
predetermined voltage. 
In the device constructed in accordance with the 
present invention a signal which is to be analyzed is 
applied to an input terminal 10 of an operational ampli- 
fier 12 through a resistor 14. This input signal may be 
taken off of a tape and prior to analyzing the remaining 
20 information carried on the tape, it is necessary to vary 
the gain of the operational amplifier 12 until the output 
voltage produced by the operational amplifier is equal 
to a given reference voltage, such as produced by a 
reference voltage source 16. 
The operational amplifier 12 may be any suitable 
conventional operational amplifier which has one termi- 
nal grounded as by lead 18. The feedback loop of the 
operational amplifier includes lead 20, the resistive net- 
work generally designated by the reference character 
30 22, lead 24, and resistor 26. The feedback loop is con- 
nected through junction 28 back to the input of the 
operational amplifier 12. By varying the number of 
resistors R1 through RN that are inserted in the feed- 
back loop the gain of the operational amplifier can be 
35 adjusted until the output voltage produced by the cali- 
bration signal is equal to the reference voltage source 
16. The gain produced by the operational amplifier 12 is 
equal to the feedback resistance divided by resistor 14. 
The feedback loop extending between the output of 
40 the operational amplifier 12 and the input terminal 28 is 
in the form of a straight line extending through the 
resistance network 22. This is because all of the relays 
30 through 44 are de-energized and accordingly, none 
of the resistors R1 through RN are in the feedback loop. 
45 In one particular embodiment the resistive value of 
resistor R1 is twice that of resistor R2, and each of the 
succeeding resistors is half the value of the preceding 
resistor. Therefore, by automatically and selectively 
placing the various valued resistors R1 through RN into 
50 the feedback circuit of the operational amplifier, the 
gain of the operational amplifier can be adjusted. 
The apparatus is controlled by a clock 46 which 
when turned on by a manual pusbutton 48 through a set 
circuit 50 is activated to produce a plurality of pulses on 
55 its output. It is to be understood, of course, that the 
clock 46 could be remotely turned on by any suitable 
remote control system. The clock pulses generated by 
the clock 46 are, in turn, fed to a sequence control cir- 
cuit which generates a signal on output leads 52 through 
60 59 in sequence. These signals are fed through a respec- 
tive logic control circuit 60 to a respective relay 30 
through 44. As the signals are sequentially placed on the 
output terminals 52 through 59 they, in turn, energize 
the relays 30 through 44. For example, the first signal 
65 when applied to lead 52 passes through the logic circuit 
15 
25 
The drawing illustrates in block form an apparatus 
including an operational amplifier for adjusting a Cali- 
bration reference signal to a predetermined level. 
60 which may be any suitable logic circuit to energize 
relay 30. When relay 30 is energized, contact 30' is 
shifted so as to place resistor R1 in the feedback loop. 
3 
4,109,2 13 
4 
This, in turn, causes the gain of the operational amplifier 
12 to increase in voltage. 
The output voltage of the operational amplifier is fed 
through an AC-to-DC converter 62 through lead 64 to 
one input 66 of a comparator 70. Another input 71 of the 5 
comparator 70 is connected to the reference voltage 
source 16. If the signal appearing on input lead 66 ex- 
ceeds the voltage of the reference voltage 16 then a 
compared signal is produced on output terminal 72 of 
the comparator and fed back to the logic circuit 66 for 10 
causing relay 30 to be again de-energized removing 
resistor R1 from the feedback circuit. 
The next sequential pulse produced by the sequence 
control circuit 51 causes relay 32 to be energized and 
the cycle of comparing the output signal from the oper- 15 
ational amplifier 12 by the comparator 70 with refer- 
ence voltage is repeated. If the output signal from the 
operational amplifier 12 exceeds the voltage of the ref- 
erence voltage source 16, then resistor 32 is removed 
from the feedback network and on the next cycle resis- 20 
tor 34 is placed therein. This cycle is repeated until the 
output voltage of the operational amplifier 12 equals the 
voltage of the reference voltage source 16. 
If, on the other hand, the output voltage for the oper- 
ational amplifier 12 is less than the reference voltage 25 
source 16, then the particular resistor R1 through RN 
which was placed in the feedback circuit by energizing 
one of the relays 30 through 44 is left in the feedback 
circuit and the next sequential pulse produced by the 
sequence control circuit 51 energizes the next relay. 30 
The gating of the signals from the sequence control 
circuit 51 and the comparator takes place through the 
logic circuit 60 which may be any suitable logic circuit 
to perform such a function. The output of the AC-to- 
DC converter 62 is also fed through an adjustable pot 35 
74 for being fed into a digital panel meter 76. The digital 
panel meter 76 produces a visual indication as to the 
value of the output voltage of the operational amplifier 
12. 
amplifier exceeds a predetermined voltage level, such as 
set by the voltage source 78, then a comparator 80 
compares this overvoltage signal coming in on lead 81 
with the voltage source 78 and generates an overscale 
signal. This overscale signal is used for illuninating 45 
signal lamp 84. 
In order to produce a low impedance from the circuit 
on output terminal 86, the output signal from opera- 
tional amplifier 12 is fed through an operational ampli- 
fier 88 which is similar in construction to the operation 50 
of amplifier 12. The operational amplifier 88 has a feed- 
back loop 90 provided therewith as well as an input 
resistor in its input circuit 92. 
While a preferred embodiment of the invention has 
been described using specific terms, such description is 55  
for illustrative purposes only, and it is to be understood 
that changes and variations may be made without de- 
parting from the spirit or scope of the following claims. 
(d) a reference voltage source; 
(e) a comparator comparing an output voltage of said 
amplifier with said reference voltage and generat- 
ing a compared signal; 
(f) a bank of relays; 
(g) each of said relays operably connected to a re- 
spective resistor of a plurality of resistors; 
(h) means for selectively energizing said relays ac- 
cording to said compared signal from said compar- 
ator for connecting selected ones of said resistors in 
said feedback circuit until said output signal from 
said amplifier equals said reference voltage; 
(i) said means for selectively energizing said relays 
in c 1 u d i n g : 
(i) a sequence control circuit having a plurality of 
output terminals, 
(ii) means for enabling said sequence control circuit 
sequencially generating a signal on said plurality 
of output terminals, 
(iii) a plurality of logic control circuits each of 
which is connected between a respective output 
terminal of said sequence control circuit and a 
respective relay for energizing a respective relay 
responsive to receiving a signal from said se- 
quence control circuit, and 
(iv) means for applying said compared signal to 
said logic control circuits for deenergizing said 
relays when said output voltage of said amplifier 
exceeds said reference voltage. 
2. The apparatus as set forth in claim 1 further com- 
(a) an AC-to-DC converter connected between the 
output of said amplifier and said comparator for 
converting an AC signal produced by said ampli- 
fier to a D C  signal to be compared with said refer- 
ence voltage source in said comparator. 
3. The apparatus as set forth in claim 2 further com- 
(a) a digital panel meter connected to the output of 
said AC-to-DC converter for producing a visual 
indication as to the value of the output voltage of 
said amplifier. 
4. The apparatus set forth in claim 3 further compris- 
(a) a second comparator; 
(b) a lamp connected to the output of said compara- 
tor; 
(c) a second reference voltage source connected to 
said second comparator; 
(d) means for connecting said DC signal produced by 
said AC-to-DC converter to said second compara- 
tor; 
whereby when said DC signal exceeds said second 
reference signal said lamp is illuminated. 
5. An apparatus for adjusting the amplitude of a refer- 
ence signal to a predetermined level so as to permit 
subsequent data signals to be interpreted correctly, said 
apparatus comprising: 
prising: 
prising: 
After the system is set, if the output of the operational 40 
ing: 
What is claimed is: 
1. An apparatus for adjusting the amplitude of a refer- 60 
(a) an operational amplifier; 
(b) a feedback circuit connected between an output 
terminal and an input terminal of said operational 
amplifier; 
ence signal to a predetermined level so as to permit 
subsequent data signals to be interpreted correctly, said 
apparatus comprising: (c) a plurality of resistors; 
(d) a reference voltage source; 
(e) a comparator comparing an output voltage of said 
amplifier with said reference voltage and generat- 
(a) an operational amplifier; 
(b) a feedback circuit connected between an output 65 
terminal and an input terminal of said operational 
amplifier; ing a compared signal; 
(c) a plurality of resistors; (f) a bank of relays; 
4,109,213 
5 6 
(g) each of said relays operably connected to a re- 
spective resistor of said plurality of resistors; 
(h) means for selectively energizing said relays ac- 
cording to said compared signal from said compar- 
equals said reference voltage. prising: 
(1) means for connecting said DC signal produced by 
said AC-to-DC converter to said second compara- 
tor; 
whereby when said DC signal exceeds said second 
reference signal said lamp is illuminated. 5 ator said Output from said amp1ifier 6. The apparatus as set forth in claim 5 further com- 
(i) a second comparator; 
u) a lamp connected to the output of said comparator; 
(k) a second reference voltage Source connected to 10 
(a) a display panel connected to the output of said 
amplifier for producing a visual indication of the 
output voltage being produced by said amplifier. 
said second comparator; and * * * * *  
15 
20 
25 
30 
35 
40 
45 
50 
55 
60 
65 


Digital automatic gain amplifier

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