A stable excitation supply for measurement transducers is described. It consists of a single-transistor oscillator with a coil connected to the collector and a capacitor connected from the collector to the emitter. The output of the oscillator is rectified and the rectified signal acts as one input to a differential amplifier; the other input being a reference potential. The output of the amplifier is connected at a point between the emitter of the transistor and ground. When the rectified signal is greater than the reference signal, the differential amplifier produces a signal of polarity to reduce bias current and, consequently, amplification

Hamlet, J. F.

English

NASA Technical Reports Server

REPLY TO
ATTN OF GP
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON, D C 20546
JUN 2 1 1974
TO: KSI/Scientific & Technical Information Division
Attn: Miss Winnie M. Morgan
FROM: GP/Office of Assistant General
Counsel for Patent Matters
SUBJECT; Announcement of NASA-Owned U.S. Patents in STAR
In accordance with the procedures agreed upon by Code GP
and Code KSI, the attached NASA-owned U.S. Patent is being
forwarded for abstracting and announcement in NASA STAR.
The following information is provided:
U.S. Patent No.
Government or
Corporate Employee
Supplementary Corporate
Source (if applicable)
NASA Patent Case No.
NOTE - If this patent covers an invention made by a corporate
employee of a NASA Contractor, the following is applicable:
YES NO
Pursuant to Section 305 (a) of the National Aeronautics and
Space Act, the name of the Administrator of NASA appears on
the first page of the patent; however, the name of the actual
inventor (author) appears at the heading of column No. 1 of
the Specification, following the words "...with respect to
an invention of ..."
Bonnie L. Woerner
Enclosure
https://ntrs.nasa.gov/search.jsp?R=19740018619 2020-03-17T14:45:33+00:00Z
United States Patent
Hamlet
[19] [in 3,815,048
[45] June 4, 1974
[54]
175]
[73]
LC-OSCILLATOR WITH AUTOMATIC
STABILIZED AMPLITUDE VIA BIAS
CURRENT CONTROL
3,649,929 3/1972 Thompson 331/109 X
Inventor: John F. Hamlet, Huntsville, Ala
Assignee: The United States of America as
represented by the Administrator of
the National Aeronautics and Space
Administration, Washington, D C.
[22] Filed June 15, 1973
Primary Examiner—Herman Karl Saalbach
Assistant Examiner—Siegfried H. Grimm
Attorney, Agent, or Firm—L D. Wofford, Jr ,
Porter; J R Manning
G J.
[57] ABSTRACT
Appl No 370,505
[52] U.S. Cl
[ 5 1 ] Int. Cl
[58] Field of Search.
[56]
331/109,331/117 R, 331/183
H03b 3/02
331/109, 117 R, 183
References Cited
UNIT
3,134.947
3,213,390
A stable excitation supply for transducers having a sin-
gle-transistor oscillator with a coil connected to the
collector and a capacitor connected from the collector
to the emitter. The base is connected through a resis-
tor to ground The output of the oscillator transistor is
connected to a buffer circuit of two transistors of op-
posite conductivity type That output is connected to a
standard rectifier and then as one input to a differen-
tial amplifier The output of the differential amplifier
„ PATFMTC ls connected to a resistor which is connected betwe'
1 tu 5> 1 A 1 fci> PA 1 fcN I b
 thg emjtter of tne oscjnator transistor and ground.
64 Charasz 331/109
65 Faith et al 331/109 6 Claims, 5 Drawing Figures
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SHEET 1 OF 2
FIG. I.
•-OUTPUT
FIG. 2.
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. LOAD
(TRANSDUCER)
PATENTEDJUH 41974 3,815,048
SHEET 2 OF 2
FIG. 3.
OSCILLATOR
i
FIG. 4.
OUTPUT
FIG. 5.
OSCILLATOR
1
OUTPUT
3,815,048
LC-OSCILLATOR WITH AUTOMATIC
STABILIZED AMPLITUDE VIA BIAS CURRENT
CONTROL
ORIGIN OF THE INVENTION
The invention described herein was made by an em-
ployee of the United States Government 'and may be
manufactured and used by or for the Government for
governmental purposes without the payment of any
royalties thereon or therefor.
BACKGROUND OF THE INVENTION
This invention relates to circuit design for the genera-
tion of oscillations with stability of amplitude as well as
frequency of the signals generated, which is useful as a
power excitation supply to transducers and for diverse
other purposes.
A stable excitation supply for transducers provides
controlled inputs for use in excitation of capacitance
and inductance transducers used for measuring such
parameters as pressure, vacuum, liquid level, displace-
ment, fluid flow, and fluid density. Such stable supplies
are required when the transducers are not incorporated
into a null balance readout system.
Previous methods to stabilize the AC supplies have
included regulation of the DC power supply to the os-
cillator or an external automatic gain control (AGC)
circuit controlling the oscillator, typically employing
devices such as incandescent lamps or field effect tran-
sistors. Disadvantages of these techniques are that reg-
ulation of DC power requires excessive numbers of ad-
ditional circuit parts and the tuning out of distortion,
that AGC by incandescent lamps provides limited dy-
namic range and is temperature sensitive; and that
AGC by field effect transistor has inherent distortion.
The advantages of this invention over previous de-
signs are that it provides amplitude regulation with low
distortion. Stability is accomplished by changing the
bias on the oscillator transistor instead of with control
devices. In an optional feature, amplitude can be se-
lected over a large range
U.S. Pat. No. 3,134,947- to J. G. Charasz describes a
method of stabilizing the amplitude of a multi-
transistor oscillator by controlling the bias on one tran-
sistor. This invention is different in that it comprises a
method to control the amplitude of a single-transistor
oscillator, thereby minimizing the complexity of the
basic oscillator. The patent in its present form also re-
quires a crystal to control frequency. Also, the patent
design is not suited for use as a measurement trans-
ducer source.
U.S. Pdt. No. 3,213,390 to W. O. Faith et al describes
a method of the kind indicated above to control oscilla-
tor amplitude by controlling the DC voltage supply to
the oscillator. The instant invention differs in that the
amplitude is controlled by controlling only the bias of
the oscillator
U.S. Pat. No. 3,278,860 to R. Winn, U.S. Pat. No.
3,369,193 to J. G. Nordahl, U.S Pat. No. 3,378,791 to
W. T. Towner, and U.S. Pat. No. 3,388,345 to W. T.
Towner describe methods to control oscillator ampli-
tude via external AGC control using a diode circuit ex-
ternal to the oscillator. The instant invention differs in
that it does not require an external element, but utilizes
the oscillator transistor only and controls amplitude by
controlling the bias on that transistor.
U.S. Pat. No. 3,443,245 to V. Whittaker and U.S.
Pat. No. 3,480,880 to D. A. Starr, Jr. describe methods
of controlling oscillator amplitude by use of a transistor
in addition to that required for the basic oscillator. The
5 instant invention does not require this external element
and controls the oscillator amplitude by controlling the
bias on the oscillator transistor.
SUMMARY OF THE INVENTION
10 It is a primary-object of this invention to provide a
practical and useful oscillation supply having good sta-
bility.
It is a related object of this invention to provide an
oscillation supply having low distortion.
15 It is, similarly, an object of this invention to provide
such an oscillation supply for use with measurement
transducers.
It is a related object of this invention to provide a sta-
ble oscillation supply the output amplitude of which
20 may be changed by the simple change of an input sig-
nal.
In accordance with this invention a first transistor is
connected in a feedback with a tuned RC circuit to
form an oscillator. The output of the oscillator is recti-
25 fied, which may be accomplished in a conventional cir-
cuit. The rectified signal is connected as one input to
a differential amplifier, the other input of which is a ref-
erence potential. The output of the differential ampli-
fier is connected at a point between the emitter of the
30
 transistor and ground. When the rectified signal is
greater than the reference signal, the differential ampli-
fier produces a signal of polarity to reduce bias current
(and, consequently, amplification) in the first transis-
tor. In one embodiment various reference voltage lev-
35
 els may be supplied by connection to different inputs
to vary the amplitude level at which the oscillator is sta-
bilized.
Other objects, features, advantages, and characteris-
tics of the invention will be apparent from the following
40
 description of preferred embodiments, as illustrated
from the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram indicating the major ele-
ments of the preferred circuit.
FIG. 2 is a circuit diagram of the preferred circuit.
FIG. 3 is a block diagram showing the preferred sys-
tem connected by cable to a remote output.
FIG. 4 is a block diagram showing illustratively a fea-
ture in which various reference voltages may be em-
ployed in the basic system.
FIG. 5 is a block diagram of the invention without the
buffer, which is not an essential element
55 DESCRIPTION OF THE PREFERRED
EMBODIMENTS
As shown in FIG. 1, the preferred embodiment com-
prises an oscillator 1, the output of which drives a
60 buffer 3. The output of buffer 3 drives the load, which
may be a conventional measurement transducer. The
buffer 3 is essentially a conventional feature which
holds the center line of the oscillations to the zero level.
65 The output of buffer 3 is also fed back through recti-
fier 5, then through a differential amplifier system 7 to
the oscillator 1. The output of amplifier system 7, when
the rectified signal is larger than a preselected refer-
45
3,815,048
3
 4 . •
ence' signal, reduces the bias in the active element of The rectifier circuit is essentially conventional* in
oscillator 1, as is discussed in detail in connection with which signals of one polarity are passed by diode 84
FIG. 2. and amplitude smoothed by the interaction of capaci-
FIG. 2 is a circuit diagram of the system shown gener- tor 88 and resistor 90.
ally in FIG. 1. The oscillator 1 is seen to be comprised 5 Differential amplifier 100 is a conventional and well
of a conventional transistor 20 having an emitter, a known circuit, and is not shown in detail. A +12 volt
base, and a collector. The base of transistor 20 is con- source of potential is connected at one input through
nected through a 10K ohm resistor 22 to ground. The a 460 ohm resistor 102, which is connected through 1 K
collector of transistor 20 is connected through a coil 24 ohm resistor 104 to the amplifier and to ground across
to a +12 volt source of reference potential. A capacitor '0 diode 106. Zener diode 106 is poled to block signals
26 is connected directly across the collector and emit- driven by the +12 volt source. Resistor 108, which is
ter of transistor^20. connected across the input terminal of amplifier 100
Coil 24 and capacitor 26 form, a tuned circuit at the connected to resistor 104 and the ouiput of amplifier
preselected frequency of oscillations, and the feedback 100, is of relatively very large magnitude ( 100K ohms).
through capacitor 24 to the input of transistor 20 is suf- 15 Accordingly, substantially the same +12 volts as is ap-
ficient to sustain oscillations. P''ed ^ Tom tne reference source appears as the refer-
The emitter of transistor 20 is connected to ground ence inPut to amplifier 100. The output of amplifier
through a series connection of 1 OK ohm resistor 28 and 10° IS connected through 400 ohm resistor 1 1 0 to the
2.7K ohm resistor 30. A relatively large (0.33Mf) ca- ,A Junction of resistors 28 and 30.
pacitor 32 is connected between the emitter of transis- 20 In operation, when the signal from resistor 90 m rec-
tor 20 and ground to provide appropriate impedance tifier 5 ls greater than the reference signal appl.ed to
across wh.ch the feedback voltage is developed . differential amplifier 100 through resistor 104, the sig-
The output of oscillator 1 ,s connected on line 40 to nal af the outPut °fn amplifier 100 opposes the bias cur-
the buffer 3, which comprises two, parallel lines 42 and „ f"1 .'" transistor 20 and thereby reduces the bias. Re-
44, each with a capacitor 46 and 48 to block DC com- 25 ducl™ of <he *>™ reduce« the ampl.fication by transis-
ponents. Transistors 50 and 52 are of opposite conduc- tor 20' and the amplitude of the output signal de-
uvity types, thereby forming a complementary circuit. «««?• Conversely, when the ratified signal ,s less
The emitter of transistor 50 ,s connected directly to the thfan th*refe™ce s'g"al to. amPllfler 100' the outP"|
ermtter of transistor 52, while the base of trans.stor 50
 30
 of
 ™PllfieJ ^° lnc"a*fs b™ T? '". translstor 20
... :. .,
 c. .. . .. , f
 JU
 and the output amplitude of oscillations increases.is connected to capacitor 46. S.milarly the base of
 The ^ ^ ^ demonstrated stab|, of
transistor 52 ,s connected to capacitor 48 ^ ^ ^
 f to , t ,„ test/be.
The collector of transistor 50 is connected to a +12 4--jrpr * H — TIVT4--jrpr
. . . i • i i /* ff*% iCI r / \J \^
volt reference source and the collector of transistor 52
is connected to a -12 volt reference source. The col- 35 MODIFIED EMBODIMENTS
lector of transistor 50 ,s connected through a large re-
 A remote em wnergb |oadj |s ehminated b
sistor 54 (47K ohm) to the base of transistor 50, and
 usg of , cab,es K shown\n F[G e3-
that point is connected to the base of trans.stor 52
 An illustrative circult to provlde various reference
through diodes 56 and 58, which are in series and poled
 vo,tages js shown jn F,G 4 Jhus by sjm , mov {he
to conduct current driven by the +12 volt source. The 40
 switch 120 between the terminals 122, 124, and 126,
base of transistor 52 is connected through a large resis-
 whjch are electrically separated from a reference volt-
tor 60 (5 1 K ohm) to the -12 volt reference source
 age +v by resistors ,28 and 130, preselected, different
The circuit comprising resistors 54 and 60 serves to voltages may be applied as the reference signal to am- '
bias the transistors 50 and 52 at the desired operating
 p|ifier 100 The level of arr)pijtude of the oscillator 1 is
point 45 then stabilized around the selected reference voltage
The structure and operation of this buffer circuit are automatically in the manner described in connection
essentially conventional, with each transistor 50 and 52
 wj th pjG 2.
conducting on alternate half cycles of the output signal
 F1G 5 jnustrates the basic circuit without the buffer
from oscillator 1. 3, since that is not an essential element. Such a circuit
The output of buffer 3 appears on line 70 at the June- 5l) ,s suggested for applications requiring only light loads.
tion of transistors 50 and 52. In the preferred applica-
tion, that is connected through capacitor 72, which Other variations of the invention described will be '"
blocks DC components, to a transformer 74. The out- apparent, and variations may well be developed which
put of the transformer 74 drives a transducer (not employ more than ordinary skill in this art, but never-
 (
shown) used for measuring, in particular, a capacitive theless employ the basic contribution and elements of '
transducer used in a liquid level probe. this invention. Accordingly, patent protection should
The output of buffer 3 is also connected through ca- not be essentially limited by the preferred embodiment
pacitor 80, which blocks DC components, through 10K disclosed, but should be as provided by law, with par-
ohm resistor 82 to the junction of 1 7K ohm resistor 83 ticular reference to the accompanying claims.
and diode 84. The other end of resistor 83 is connected What is claimed is:
to ground. The side of diode 84 away from resistor 83 1. A stabilized oscillator circuit comprising a first
is connected to diode 86, which is connected to ground transistor having an emitter, a base, and a collector; a
and poled to reject current which diode 84 is poled to capacitor and an inductor connected in a tuned circuit
pass. A 0.1 p.f capacitor 88 is connected to ground in
 fi<. between said collector and said emitter; a first resistor
parallel with diode 86. A IK ohm resistor 90 is con- " connected from said base to ground; a rectifying circuit
nected between diode 84 and the input of differential having its input connected to receive signals from said
amplifier 100. collector; a differential amplifier having one of its in-
3,815,048
puts connected to the output of said rectifying circuit;
a source of reference potential connected to the other
input of said differential amplifier; a second resistor
having one terminal connected to the junction of said
emitter and said tuned circuit and the other terminal
connected to one terminal of a third resistor, the other
terminal of said third resistor being connected to
ground; a fourth resistor having one terminal con-
nected to the junction of said second and third resis-
tors, the output of said differential amplifier being con-
nected to the other terminal of said fourth resistor
2. The oscillator as in claim 1 in which said inductor
is connected from said collector to a source of refer-
ence potential and said capacitor is connected between
said emitter and the junction of said inductor and said
collector.
3. The oscillator as in claim 2 also comprising a sec-
ond transistor and a third transistor* each having an
emitter, a base, and a collector and being of opposite
conductivity type, connected emitter-to-emitter, with
bases connected in parallel to said collector of said first
transistor, the collector of said second transistor being
connected to one source of reference potential and the
5 collector of said third transistor being connected to a
second source of reference potential to form a buffer
circuit, the junction of the emitters of said second and
third transistors being connected to the input of said
rectifier circuit.
10 4. The circuit as in claim 1 in which said other termi-
nal of said differential amplifier is adapted to be selec-
tively connected to different reference potentials
5. The circuit as in claim 2 in which said other termi-
nal of said differential amplifier is adapted to be selec-
15 lively connected to different reference potentials
6. The circuit as in claim 3 in which said other termi-
nal of said differential amplifier is adapted to be selec-
tively connected to different reference potentials
20
25
30
35
40
45
50
55
60
65



LC-oscillator with automatic stabilized amplitude via bias current control

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