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Патент USA US3021499

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Feb. 13, 1962
J. |_. NIELSEN
3,021,489
DOUBLE TIME-CONSTANT AGC FOR SPEECH AMPLIFIER
Filed Dec. 14, 1959
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States Patent 0 "r
Patented Feb. 13, 1962
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1
signal having a fast attack time and a slow release time
3,021,489
without adversely aifecting the temperature stability of
Jorgen Louis Nielsen, Rochester, N.Y., assignor to Gen
the circuit.
The invention has for a further object the provision of
an automatic gain control circuit of the character de
scribed above which employs junction type transistors in
order to realize all of the well known advantages of the
DGUBLE TiMEuCDNSTANT AGC FOR
SPEECH AMPLIFIER
eral Dynamics Corporation, Rochester, N.Y., a corpo
ration of Delaware
Filed Dec. 14, 1959, Ser. No. 859,391
7 Claims. (Cl. 330-29)
latter devices.
The foregoing and other objects are achieved, in ac
The present invention relates generally to a new and 10 cordance with the present invention, by providing a speech
improved automatic gain control circuit and is particu
larly concerned with a circuit of this type for controlling
the gain of a speech ampli?er used in radio transmitting
equipment and utilizing junction transistors.
The transmission of human speech gives rise to 'a num
ber of problems directly concerned with the inherent char
acteristics of the speech signals. Thus, for example, the
' very high peak-to-average ratio of speech signals makes
it di?icult to obtain optimum use of the available trans
ampli?er having its gain automatically controlled by a
circuit having time constants larger than those of com
pressor ampli?ers referred to above. Thus, the gain of
the speech ampli?er in the present invention'is controlled
by the average speech signal rather than by, the speech
- envelope and, as a consequence, the output signal has a
substantially constant mean level regardless of the input
signal but, at the same time, the output possesses the
same peak-to-average ratio as the input signal. Thus,
mitter power since the peaks cannot be permitted to 20 the speech ampli?er accommodates both soft spoken and
loud voiced operation without serious distortion by re
exceed the capabilities of the transmitter. In high ei?~
ciency communications equipment, it is, therefore, de
taining the quality or naturalness of the input regardless
sirable to employ some means of speech processing to
of its strength.
minimize the effects of the peak power signals without,
detector-ampli?er followed by two gated stages for pro
viding widely different time constants for the attack and
release of the gain control signal. The attack time con
at the same time, materially impairing the quality or
naturalness of the voice transmission.
Among the proposed solutions to the problem is the
use of gain control circuits known as compressors in
To this end, the AGC loop includes a
stant is very fast to provide a quick reduction in gain
for the high level input speech signals while the release
time constant is slow, thus preventing the gain from pop
the audio ampli?cation stages of the transmitter. Such
a compressor is actually an ampli?er having its gain
automatically varied in response to a change in input
level so that the gain is reduced for peak signals. How
hence, avoiding frequent bursts. This provides for reg
ever, these compressors are subject to distortion due to
the input signal, but still provides rapid gain reduction
ping back up again as soon as a peak has passed and,
ulation of the gain in accordance with the mean level of
on peak signals. A very wide dynamic range can thus be
velope and have very short time constants, the gain con’ 35 obtained. The gated stages include junction type tran
the fact that, since they operate from the signal en
trol signal contains distortion components having fre
quencies falling within the audio range which compo
nents enter the signal paths of the transmitter where
they appear as popping sounds accompanied by signal
sistors and provision is made for clamping these tran
sistors to stable operating potentials in order to maintain
the temperature stability.
The speech ampli?er is a common emitter type am
distortion. Furthermore, due to the insufficient fre 40 pli?er having its intrinsic gain controlled by varying the
emitter current but, since the base bias voltage is made
quency separation between the speech signal and the con
very stilt, the emitter potential does not shift appreciably
trol signal, regeneration is often an acute problem with
as the gain is varied.
the result that the dynamic range of transmitters using
The invention, both as to its organization and method
compressors is generally very limited. Moreover, due
of operation, together with further'objects and advantages
to the very short time constants of the control circuit,
the gain is reduced and increased at a syllabic rate, thus
giving rise to frequent bursts which overload the trans
thereof, will best be understood by reference to the spe
ci?cation taken in connection with the accompanying
mitter and/or necessitates operation at a lower average
drawing in which:
level of drive. The ideal automatic gain control circuit
would provide signals having very fast attack time for
the high level signals but, at the same time, having a slow
FIG. 1 is a block diagram showing the gain control cir
cuit of the present invention as used in conjunction with
release time in order to avoid or minimize the bursts.
A second well known circuit for minimizing the e?ects
of the peak signals is a clipper which cuts off the posi
tive and negative peaks beyond predetermined upper
and lower limits. The clipping action is, of course, ac
companied by distortion in an amount dependent upon
the degree of clipping. While this distortion can be
reduced by various techniques such as by use of a low
pass ?lter to eliminate higher order harmonics of most
frequencies in the pass band, it cannot be completely
a speech ampli?er;
FIG. 2 is a schematic diagram showing several of the
stages illustrated in block form in FIG. 1;
FIG. 3 is a simpli?ed diagram which is useful in ex
plaining the operation of the gain control circuit of the
present invention; and
FIG. 4 is a typical curve of collector current versus gain
for the speech ampli?er shown in FIGS. 1 and 2 and will
be useful in explaining the present invention.
Referring now to the drawing and ?rst to FIGS. 1 and 2,
the gain control circuit of the present invention is there
indicated by reference numeral 10 and is used to control
eliminated and, hence, it may impair the quality of the
the gain of a typical speech ampli?er comprising a plu
speech transmission.
rality of amplifying stages 11, 12 and 13. While three
It is an object of the present invention to provide a
new and improved circuit for automatically controlling 65 such stages are shown in the drawing, it should be under
stood that any desired number may be employed depend
the gain of a speech ampli?er without, at the same time,
ing, of course, upon the desired ampli?cation, the gain of
increasing the distortion.
each stage, and other similar factors. Each of the ampli
A further object of the invention resides in the pro
fying stages is of the resistance coupled type and is oper
vision of an automatic gain control circuit characterized
by a fast attack time and a slow release time.
> Another object of the invention is to provide an auto
matic gain control circuit for developing a DC. control
70 ated Class A with the circuit 10 being illustrated as ef
fective to control thegain of only the ?rst stage 11.
The ?rst stage 11 is illustrated as including a junction
8,021,489
3
v 4
type transistor 14 having its base 15 supplied with speech
network consisting of resistor 54} and condenser 51 con
input signals derived from a microphone 16 or the like.
The transistor 14 is illustrated as being of the p-n-p type
but the gain control circuit of the present invention is also
gated stage 39.
-
nected in the base circuit of a transistor 52 in the release
The low output impedance of the emit
ter follower including transistor 44 causes the condenser
applicable to n-p-n type transistors. These input signals
51 to charge very rapidly through the gating diode 49 to
pass through a coupling capacitor 17 and are supplied to
substantially the same voltage as the condenser 43 and
at about the same rate. The signal passed by the diode
the junction between a resistor 18 and a resistor 19' con
nected in a voltage divider network across the positive
and negative terminals 29 and 21 of a DC. source. The
49 also drives the base 53 of the transistor 52 in a nega
tive direction thereby increasing the flow of emitter cur
use‘ of the term positive and negative merely indicates 10 rent through a resistor 54 connected between the emitter
55 of the transistor 52 and the emitter 22 of the transistor
relative polarity since the terminal 21 may actually be at
14 in the ?rst speech ampli?er stage 11. The collector
zero or ground potential while the terminal 2%} is positive
53 of the transistor 52 is connected directly to the nega
or, on the other hand, the terminal Ztl'rnay be at ground
tive terminal 21.
potential while the terminal 21 is’negative. The emitter
The increased emitter current in the transistor 52 re
22 of the transistor 14 is connected to the positive ter 15
sults in decrease of the gain of the first amplifying stage
minal 20 through a resistor 23 bypassed by a capacitor
11. As was previouslyvindicated, the bias on the transistor
24., The collector 25 of the transistor 14 is connected
14 is so stiff that its emitter potential, i.e., the potential of
to the negative terminal 21 through a' load resistor 26
across which the output of the amplifying stage 11 is de
junction 60, remains substantially constant despite varia
veloped. This output is, of course, passed through the
tions in emitter current of the transistor 52." This is ac
amplifying stages 12 and 13.
complished by well known means of-D.C. stabilization.
More speci?cally, the impedance of the voltage divider
.
A portion of the output of the amplifying stage 13 is
applied to the gain control circuit or loop 16' which in
cludes a power detector stage 28 and a pair of gating
circuits 29 and 30 providing widely di?erent time con
stants for the attack and release of the gain control sig
nal. More specifically, the output of the amplifying
stage 13 is coupled through a capacitor 31 to therbase 32
network formed by resistors 18 and 19 is selected so that
the voltage at' the base 15 is substantially equal to the
voltage at the emitter 22 and is stiff in order to provide
a stable operating point. In this connection, it should
be observed that a stable emitter potential establishes
being of the p-n-p type although, here again, it might
stable voltage reference levels throughout the ampli?er.
The operating point for the-transistor 14 is given in terms
of its D.C. collector current and the D'.C. voltage between
be also of the n-p-n type. The input signals to the latter
stage are applied between the emitter 34 and the base
its collector and emitter. When the latter voltage ex
ceeds a predetermined level, usually 2 or 3 volts, the in
32 of the transistor and aredeveloped across a resistor
trinsic, gain of the transistor is virtually independent of
35 which is connected in series with a resistor 35 to
the voltage and becomes primarily a function of the col
of a junction transistor 33 which is again illustrated as
‘form a voltage divider network across the terminals 2%) 35 lector current which is, in turn, made up of emitter cur
and 21. The transistor 33 has its emitter 34 connected
rent and base current. Since the base current is only a
to the positive terminal 2a through a resistor 37 by
small fraction of the collector current, the latter is, for
passed by a condenser 38. The collector 39 is connected
all practical purposes, equal to the emitter current. There
to the negative terminal 21 through a load resistor 40
fore, by controlling the level of the emitter current in the
across which the output of the detector 28 is developed. 40 transistor 14 it is possible to control the gain. The emit
The voltage divider network formed by resistors 35 and
ter current is controlled by bleeding o? controlled amounts
36 biases ‘the base of the transistor 33 to saturation so
through the resistor 54 and the transistor 52. A simpli?ed
that even small output signals from the stage 13 are de
circuit of the components used in the gain control is
tected.
shown in FIG. 3 from which it will be apparent that, since
'_ Assuming that signals are present at the output of the 45 the emitter voltage remains substantially constant, the total
stage 13 and that it is, therefore, desirable to reduce the
current ?ow I0 through the resistor 23 is also maintained
gain of the stage 1.1 in order to control these signals, the
substantially constant. The current In is obviously made
power detector 28 produces‘ an output signal across the
up of the emitter current 12 of the transistor 14 and the
load resistor 40. The output signal developed across the
emitter current I1 of the transistor 52.‘ Since the total
resistor 40 is passed through a diode 41, which passes only
current 10 remains substantially constant, the increased
negative-going signals, to an RC network consisting of
?ow of emitter current 11 through the resistor, 54 in re
resistor 42 and condenser 43 in the base circuit of a tran
sponse to the signal gated through diode 49 is accom
sistor 44 in the attack gated circuit 29. - The RC network,
panied by a decrease in the ?ow of emitter current I2
which is connected between the base 45 of the latter tran
through the transistor 14, thus decreasing the gain of the
sistor and the positive terminal 20, serves the dual pur
amplifying stage 11.
pose of providing a fast attack time constant for the AGC
vIn view of the foregoing description it will be recognized
circuit 10 and, of temperature stabilizing the transistor
that the peak signals at the output of the ampli?er 13
44. More speci?cally, the latter transistor is connected as
result in the development of a gain control signal which
an emitter follower with its collector 46 connected di
reduces the gain of the stage 11. The fast time constants
rectly to the negative terminal 21 and it's emitter 47 con 60 of the network 42-43 and the accompanying fast charg
nected through a load resistor 48‘, to the positive ter
ing of the condenser 51 through the diode 49 give a very
minal 20.’ The negative signal passed by the diode 41,
fast attack time for the circuit. The time constant of
of ‘course, charges the condc'nser'43 very rapidly and
the RC network 50——51 is an order of magnitude larger
drives the-base ‘45 in a- negative direction to increase the
than that of the network 42-43 and, hence, the network
emitter current ?ow through resistor ‘48, thus’ developing 85 50-51 is effective to maintain increased emitter current
a negativesignal at the output of the' emitter follower
?ow through the transistor 52 for an interval following
stage. The time constant of» the RC (network, 42——43
the attack instant. The long time constant of the net
is very fast with the result that the ‘signal developed across
work 50-51, therefore, provides a slow release for the
the resistor 48 follows closely the signal ‘passed by the
AGC circuit 10. The slow release time, of course, pre
diode 41 and this, coupled’ with the fact that the con
vents frequent signal bursts through the'amplifying stages
denser 43 is charged from a low impedance, small time 70. and, hence, improves the. quality of they transmission.
constant circuit with large voltage excursion/(across re
An initial burst or overshoot of, the gain control, for ex
sistor 49), produces the fast attack time ofv the gain con
ample, at the beginning of a sentence is unavoidable even
trol circuit 10.‘ The negative signal appearing across the
with the AGC circuit described. This initial burst is of
resistor-.48 is gated through a diode 49 to a second RC
short duration and since the gain’ is kept down for a
3,021,489
6
while as a result of the slow release time, bursts are not
repeated at a syllabic rate. However, such an initial burst
might overload the transmitter and, hence, the output of
the speech ampli?er from the stage 13 may be passed
through a clipper (not shown) designed to clip at or
slightly below the overload level. The distortion gener
ated by such a clipper is not oifensive because it is of
such short duration that it is virtually indiscernible to the
human ear.
A capacitor 61 connected from the emitter 55 to the
in view of the foregoing discussion, it will be recog
rized that the gain control circuit described is e?ective
to accomplish the enumerated objects of the invention.
This circuit controls the gain of a speech ampli?er with
out increasing the distortion and is, at the same time,
capable of providing a control signal having fast attack
and slow release time constants without adversely affect
ing the temperature stability. While a particular embodi
ment of the invention has been shown, it will be under
10 stood, of course, that the invention is not limited thereto
positive terminal 2!) prevents motor-boating of the circuit
by ?ltering any residual AG. in the control signal at the
since many modi?cations Will occur to those skilled in
output of the stage 39.
claims to cover any such modi?cations as fall within the
the art and it is, therefore, contemplated by the appended
The resistor 54 stabilizes the
control current I, by preventing small changes in the
true spirit and scope of the invention.
gain control signal from causing large variations in the 15
What is claimed as new and desired to be secured
emitter current flow through transistor 14. Speci?cally,
by Letters Patent of the United States is:
the curve shown in FIG. 4 of the gain reduction in the
amplifying stage 11 versus D.C. collector current of the
transistor 14 is very steep at large gain reductions and,
ampli?er of the type having an input signal applied to
the gain.
bias the ampli?er, and the collector being connected to
1. In an automatic gain control circuit for a transistor
its base and an output signal derived from its collector, a
as a consequence, the bleed current I; must be very stable 20 source of DC. power having positive and negative ter~
and carefully controlled to‘ obtain an accurate control of
minals, means connected to the base of the transistor to
Thus, it is desirable that the existence of a
given voltage at the base 53 in response to a signal from
one terminal of said source through a load resistor, a
power detector circuit responsive to speech signals de
the stage 13 of predetermined level will produce a well
de?ned value of bleed current 1,. Small variations in 25 rived from the ampli?er, a’ gated circuit excited by the
sign? output from said power detector circuit and in
the voltage at the base 53 should not be allowed to pro
cluding a diode coupled to pass only signals of one polar
duce large excursions of the bleed current I1. This is
accomplished by the resistor 54 and the negative D.C.
ity from said power detector, said gated circuit including
an emitter-follower transistor circuit having a slow time
feedback in the stage 30. Such an arrangement requires
that the control signal at the base 53 have a ‘large volt 30 constant resistor-capacitor network in the base circuit of
said emitter-follower transistor, a stabilizing circuit in
age and, as a result, the power detector 28 must provide
cluding said diode and the resistance of said slow time
a large voltage output in response to signals from the
constant network coupled across said terminals for main
amplifying stage 13. This accounts for the use of a
taining said diode in conducting condition throughout the
power detector transistor 33 initially biased to saturation.
The need for a large voltage change at the base 53 to 35 temperature range of the equipment, a resistor, the tran
sistor of said emitter-follower having an emitter con
effect signi?cant gain reduction in the amplifying stage
nected through said resistor to the emitter electrode of
11 also means that small, instantaneous variations in the
said ampli?er, means including a resistance connected
input signal will not cause ?uctuations in the gain, thus
between the other terminal of said D.C. source and the
providing What may be termed a delayed AGC action.
Resistors 42 and 53 are of such value that under no
40 emitter electrode of said ampli?er for maintaining sub
stantially constant the total current through the emitter
circuits of said ampli?er and the emitter-follower for
holding the emitter electrode voltage of the ampli?er sub
and 49 remain conducting throughout the temperature
stantially constant irrespective of variations in emitter
range of the circuit to provide the desired fast attack
and slow release time constants without compromising the 45 current ?ow through the transistor of said emitter-fol
lower, whereby the signal passed by said diode di?'eren
temperature stability of the transistor circuits. More spe
tially increases and decreases, respectively, the currents
ci?cally, as is well known, the cuto? collector current
?owing in the emitter circuits of said emitter-follower and
I00 of a transistor includes the thermally generated reverse
said ampli?er.
current ?owing through the collector to base junction and
2. In an automatic gain control circuit for a transistor
the leakage current. The thermally generated current 50
ampli?er of the type having an input signal applied to its
increases exponentially with temperature. It ?ows through
base and an output signal derived from its collector, a
the collector to base junction and, hence, does not directly
source of DC. power having a voltage divider network
aifect the collector current but, when the voltage divider
connected across its positive and negative terminals, the
connected to the base has a high impedance, the current
Ico shifts the base voltage and indirectly a?ects the col 55 base of the transistor being connected to the voltage
divider network to bias the ampli?er, and the collector
lector current by varying the base current. At room
being connected to one terminal through means including
temperatures the current 100 for most transistors has
a load resistor, a power detector circuit responsive to
negligible eiiect but as the temperature increases, this
speech signals derived from ‘the ampli er and including
current becomes bothersome. Therefore, in order to
avoid the e?ects of the current 100, it is important to keep 60 a transistor biased to yield maximum detection gain, a
rst gated circuit at the output of the power detector and
the base circuit impedance as low and the emitter resist
including a ?rst diode connected to pass only signals of
ance as large as the remaining circuit considerations will
one polarity, said ?rst gated circuit further including a
permit. This accounts for the desirability of keeping the
?rst emitter follower transistor circuit having in its base
diodes '41 and 49 conducting under no-signal conditions
signal conditions, i.e., in the absence of drive signals from
the ampli?er 13 to the power detector 28, the diodes 41
throughout the temperature range. Thus, if the diodes
circuit a fast time constant resistance-capacitor network
were cut off under no-signal conditions, the DC. resist
supplied with signals passed by said ?rst diode, a ?rst
ance in the base circuit of the transistor 44 would con -
stabilizing circuit including said ?rst diode and the re
sistance of said ?rst network connected between the posi
tive and negative terminals of the source to maintain said
prise only the large resistor 42. The current 1C0 of the
transistor 44 ?owing into the base 4'5 would drive the
base in a negative direction, thereby decreasing the gain
of the amplifying stage 11 even though no input speech
signal is present. The same situation would prevail in
the transistor 52. By keeping the diodes 41 and 49 con
ducting, the above disadvantages are avoided and tem'
perature stability of the transistor circuits is thus achieved. 75
?rst diode conducting throughout the temperature range
of operation, a second gated circuit excited by the signal
output from the ?rst emitter follower circuit and includ
ing a second diode connected to pass only signals of one
polarity, said second gated circuit including a second
enutter follower transistor circuit having a slow time
3,021,4sa
7
constant resistor’ capacitor network in‘ its base circuit, a
second stabilizing circuit including said second diode and
the resistance of said slow time constant network con
nected for maintaining the second diode conducting
throughout the temperature range of the equipment, the
transistor of said second emitter follower having an
emitter connected through at least one resistor to the
emitter of said ampli?er, whereby the signal passed by
the second diode increases the emitter current flow
through the transistor of the second emitter follower,
,means including a resistance connected between the other
terminal of the D0. source and the emitter of the am
it
circuit excited by the signal output from said ?rst emitter
follower circuit md including a second diode connected
to pass only negative-going signals, said second gated cir
cuit including a'second emitter follower transistor circuit
having a slow time constant resistor-capacitor network in
its base circuit, the transistor of said second emitter fol
lower having an emitter connected through at least one
resistor to the emitter of the ampli?er, whereby the signal
passed by the second diode increases the emitter current
flow through the transistor of the second emitter follower,
and means including a resistance connected between one
terminal of the D.C. source and the emitter of the ampli
pli?er for maintaining the emitter voltage of the ampli
?er substantially constant irrespective of variations in
?er for maintaining the emitter voltage of theampli?er
emitter follower so that the latter current ?ow is effective
follower so that the latter current ?ow is effective to de
crease the flow of emitter current in the ampli?er.
5. In an automatic gain control circuit for a transistor
substantially constant ir"espective of variations in emitter
emitter current ?ow through the transistor of the second 15 current flow through the transistor of the second emitter
to decrease the ?ow of emitter current in the ampli?er,
and means including a bypass condenser connected to
bypass A.C. signals from the emitter of the second emit
ter follower. ,
3. In an automatic gain control circuit for a transistor
ampli?er of the type having an input signal applied to
its base and an output signal derived from its collector,
ampli?er of the‘ type having‘ an input signal applied to
20 its base and an output signal derived from its collector,
means including a source of DC. power for biasing the
base of the transistor in the ampli?er, a power detector cir
cuit responsive to speech vsignals derived from the ampli
?er and including a transistor biased to yield maximum
and negative terminals for’ biasing the base of the tran 25 detection gain, a ?rst gated circuit at the output of the
‘power detector, said ?rst gated circuit further including
sistor in the ampli?er, a power detector circuit responsive
a ?rst; emitter follower transistor circuit having a fast
to speech signals derived from the ampli?er and including
time constant resistance-capacitor network in its base cir
a transistor biased to yield relatively high detection gain,
cuit supplied with signals from'the detector, a second
a ?rst gated circuit at the output of the power detector
and including a ?rst diode connected to pass only signals 30, gated circuit’ excited by the signal output from said ?rst
emitter follower circuit and including a second emitter
of one polarity, said ?rst gated circuit further including
follower transistor circuit having a slow time constant
a ?rst emitter follower transistor circuit having in its base
resistor-capacitor network in its base circuit, the tran
circuit a’ fast time constant resistance-capacitor network
means including a source of DC. power having positive 7
sistor of said second emitter follower having an emitter
supplied with signals passed by said ?rst diode, a ?rst
stabilizing circuit including said ?rst ‘diode and the re 35 connected through at least one resistor to the emitter
of the ampli?er, whereby the signal passed by the second
sistance of said ?rst network-connected between the posi
tive and negative terminals of the source to maintain the
?rst diode conducting throughout the temperature range
of operation, a second gated circuit excited by the signal
gated circuit increases the emitter current ?ow' through
the transistor of the second emitter follower, and means
including a resistance connected between one terminal of
output from said ?rstemitter follower circuit and in 40 the DC. source and the emitterof the ampli?er for main
taining the emitter, voltage of the ampli?er substantially
cluding a second diode connected to pass only signals of
constant irrespective of variations in emitter current flow
one polarity, said second gatedtcircuit including a second
through the transistor of the second emitter follower so
emitter follower transistor circuit having a slow time con
that the latter current ?ow is effective to decrease the flow
stant resistor-capacitor network in its base circuit, a sec
of emitter current in the ampli?er.
ond stabilizing circuit connected to be energized by said
6. In an automatic gain control circuit for a transistor
source'and including the second diode and the resistance
ampli?er of the type having a control electrode, an input
of the slow time constant network for maintaining the
signal applied to an input electrode and an output signal
second diode conducting throughout the temperature
derived from an output electrode, a source of DC.
range of the equipment, the transistor of said second emit
ter follower having an emitter connected through at least 50 power having a voltage divider network connected across
its positive and negative terminals, the input electrode
one resistor to the emitter of saidampli?er, whereby the
of the transistor being connected to the voltage divider
signal passed by the second diode increases the emitter
network to bias the ampli?er and the output electrode
current ?ow through the transistor of the second emitter
being connected to one terminal through means includ
follower, and means including a resistance connected be
tween one terminal of the DC. source and the emitter of 55 ing a load resistor, a power detector circuit responsive
the ampli?er for maintaining the emitter voltage ‘of the
ampli?er substantially constant irrespective of variations
to speech signals derived from the ampli?er and includ
ing a transistor biased to yield relatively high detection
gain, a ?rst gated circuit at the output of the power detec
in emitter current ?ow through the transistor of the sec
tor and including a ?rst diode connected to pass only sig
ond emitter follower so that the latter current flow is
effective to decrease the ?ow of emitter current in the 60 nals of one polarity, said ?rst gated circuit further in
cluding a ?rst emitter follower transistor circuit having
ampli?er.
in its base circuit a fast time constant resistance-capacitor
4. In an automatic gain control circuit for a transistor
ampli?er of the type having an input signal applied to
network supplied with signals passed by said ?rst diode,
to speech signals derived from the ampli?er and including
operation, a second gated circuit excited by the signal out
a ?rst circuit including said ?rst diode and the resistance
its base and an output signal derived from its collector,
means including a source of DC. power having positive 65 of said ?rst network connected between the positive and
negative terminals of the source to maintain the ?rst
and negative terminals for biasing the base of the tran
diode conducting throughout the temperature range of
sistor in the ampli?er, a power detector circuit responsive
put from said ?rst emitter follower circuit and including
a transistor biased to yield maximum detection gain, a
?rst gated circuit at the output of the power detector and 70 a second diode connected’ to pass only signals of one
polarity, said second gated circuit including a second
including a ?rst diode connected to pass only negative
emitter follower transistor circuit having a slow time con
going signals,~said ?rst gated circuit further including a
stant resistor-capacitor network in its base circuit, a sec
?rst emitter follower transistor circuit having in its base
ond circuit connected between the positive and negative
circuit a fast- time constant resistance-capacitor network
supplied with signals passed by the diode, a second gated 75 terminals of the source and including the second diode
3,021,489
9
and the resistance of the slow time constant network for
10
maintaining the second diode conducting throughout the
of the source to maintain the ?rst diode conducting
through the temperature range of the equipment, a sec
temperature range of operation, the transistor of said
second emitter follower having an emitter connected
ond gated circuit excited by the signal output from said
7. In an automatic gain control circuit for a transistor
tween one terminal of the DC. source and said control
ampli?er of the type having a control electrode, an in
put signal applied to an input electrode and an output
signal derived from an output electrode, means including
a source of D.C. power for biasing the input electrode, a
power detector circuit responsive to speech signals derived
from the ampli?er and including a transistor biased to 25
yield maximum detection gain, a ?rst gated circuit at
the output of the power detector and including a ?rst
diode connected to pass only negative-going signals, said
?rst gated circuit further including a ?rst emitter follower
transistor circuit having in its base circuit a fast time
electrode of the ampli?er for maintaining the control elec
trode voltage of the ampli?er substantially constant ir
respective of variations in emitter current ?ow through
?rst emitter follower circiut and including a second diode
through at least one resistor to the control electrode of
connected to pass only signals of one polarity, said second
the transistor in the ampli?er, whereby the signal passed
gated circuit including a second emitter follower transis
by the second diode increases the emitter current ?ow
tor circuit having a slow time constant resistor-capacitor
through the transistor of the second emitter follower,
network in its base circuit, a second circuit connected be
and means including a resistance connected between one
tween the positive and negative terminals of the source
terminal or‘ the DC. source and the control electrode of 10 and including the second diode and the resistance of the
the ampli?er for maintaining the control electrode poten
slow time constant network for maintaining the second
tial of the ampli?er substantially constant irrespective of
diode conducting throughout the temperature range of
variations in emitter current ?ow through the transistor
operation, the transistor of said second emitter follower
of the second emitter follower so that the latter current
having an emitter connected through at least one resistor
flow is effective to decrease the ?ow of current through 15 to the control electrode of the ampli?er, whereby the
the control electrode in the ampli?er, and means includ
signal passed by the second diode increases the emitter
ing a bypass condenser connected to bypass A.C. signals
current flow through the transistor of the second emitter
from the emitter of the second emitter follower.
follower, and means including a resistance connected be
constant resistance-capacitor network supplied with sig
nals passed by said ?rst diode, a ?rst circuit including
said ?rst diode and the resistance of said ?rst network
connected between the positive and negative terminals
the transistor of the second emitter follower so that the
latter current ?ow is effective to decrease the ?ow of
control electrode current in the ampli?er.
References Cited in the ?le of this patent
UNITED STATES PATENTS
2,489,126
Fay et a1. ___________ __ Nov. 22, 1949
2,673,899
2,773,945
2,862,046
2,866,015
Montgomery _________ __ Mar.
Theriault ___________ __ Dec.
Relis ________________ __ Nov.
Sailor _______________ __ Dec.
30,
11,
25,
23,
1954
1956
1958
1958
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