Automatic gain controller

An automatic gain controller includes a signal detector receiving an output signal from an amplifier for converting the signal into a DC current changing according to the output signal; first and second time-constant controllers receiving the DC current for removing an AC component to produce first and second pure DC components with different, predetermined time-response characteristics; a voltage subtracter receiving and subtracting the first and second pure DC components to produce a resultant voltage; a voltage comparator for comparing the resultant voltage with a predetermined reference voltage; a switch responsive to the voltage comparator for controlling transmission of the first pure DC component; a voltage sink for discharging the first pure DC component when the switch is closed; and a control voltage amplifier receiving the first pure DC component when the switch is open for producing an output signal to control the gain of the receiving amplifier. Accordingly, high-speed and reliably stable gain control can be achieved.

BACKGROUND OF THE INVENTION 
The present invention relates to an automatic gain controller and, more 
particularly, to an automatic gain controller operating at high speed. 
An automatic gain controller is an apparatus for controlling the output 
signal of a receiver such that a constant amplitude is output at all times 
for subsequent circuitry regardless of the amplitude of the input signal. 
The amplitude of the input signal to the receiver may fluctuate due to the 
inhibitory factors of a transmission channel of radio or wire 
communication. 
During communication, sudden unexpected inhibitory influences in the 
transmission channel often occur, such that the received signal cannot but 
vary according to certain abnormalities. Such signal variations are caused 
by the loss of the signal path due to changes in weather conditions such 
as clouds or rain, due to sudden changes in the atmosphere or ionosphere, 
or due to a change in the length of a transmission line. Unless these 
changes in the amplitude of the received signal are actively dealt with, 
many errors and other problems are generated when recovering information 
contained in the received signal. 
When the amplitude of the received signal due to the inhibitory factors of 
a transmission line changes, an automatic gain controller detects 
amplitude changes in a received signal, to thereby control the gain of the 
receiver such that if signal strength diminishes, the automatic gain 
controller increases the receiver gain and, conversely, if signal strength 
rises, the automatic gain controller decreases the gain. Accordingly, the 
automatic gain controller controls the output signal of the receiver to 
have a constant amplitude at all times. This automatic gain controller is 
widely used for a variety of receivers in the fields of satellite 
communications, ground network communications, mobile communications, etc. 
The conventional automatic gain controller receives a signal, and then 
detects a change of its amplitude to control the gain accordingly. 
Therefore, the automatic gain controller cannot expect the change in the 
received signal, and then, the automatic gain controller is required to 
operate at high speed to control the gain of the rceiver for outputting an 
accurate signal. 
FIG. 1 is a circuit diagram illustrating a construction of a conventional 
automatic gain controller for controlling the gain of a receiving 
amplifier. 
Referring to FIG. 1, a conventional automatic gain controller comprises a 
receiving amplifier 44, signal detecting means 41, time constant 
controlling means 42, and control voltage amplifying means 43. Receiving 
amplifier 44 amplifies a signal having various amplification factors and 
received via a receiving signal input S1 and output as a receiving signal 
output S2. In general, most receiving amplifiers 44 include an automatic 
gain control node for controlling their overall gain, to prevent the 
output from being changed due to varying losses in transmission lines as 
described above. A gain control voltage S3 is supplied via the automatic 
gain control node so that output signal S2 can be stabilized without being 
influenced by a change of the input signal S1. 
Signal detecting means 41 receives a signal from receiving signal output S2 
of receiving amplifier 44 or from some other source, and then converts the 
signal into a corresponding direct current (DC) level. Here, the signal is 
rectified by means of a diode and converted to a DC level which varies in 
amplitude according to the magnitude of the received signal. The voltage 
components detected by signal detecting means 41 are supplied to time 
constant controlling means 42. 
Time constant controller 42 receives the detected signal from signal 
detecting means 41 and removes the AC component included in the detected 
signal, to thereby produce a pure DC component and simultaneously provide 
a predetermined time response function characteristic. Time constant 
controlling means 42 includes a resistor R and a capacitor C and the 
time-response characteristic is determined by the RC time-constant. Since 
the gain is controlled at all times and whenever the amplitude of the 
detected signal changes, if the output signal of receiving amplifier 44 
undergoes an inordinate amount of change, the change is buffered with a 
predetermined duration of time response (attack time). Accordingly, the 
signal at receiving signal output S2 is prevented from being changed 
erratically, and thereby receiving amplifier 44 can precisely produce a 
signal having the desired amplitude. 
Control voltage amplifying means 43 receives the output of time-constant 
controlling means 42 and changes the output to desired gain control 
voltage S3. Gain control voltage S3 is then supplied to the aforementioned 
gain controlling node of receiving amplifier 44. The voltage level 
detected at signal detecting means 41 and time-constant controlling means 
42 is determined according to the output of receiving amplifier 44, but 
may differ from the voltage level for controlling the desired gain of the 
gain voltage controlling node of receiving amplifier 44. Thus, control 
voltage amplifying means 43 offsets the difference. The control voltage 
amplifying means 43 also controls the direction of voltage change detected 
from the automatic gain controller to be consistent with the gain 
controlling direction of receiving amplifier 44 unless both of those 
directions are identical. In more detail, when the voltage detected from 
the automatic gain controller is in proportion to the amplitude of 
receiving signal S1 (i.e., receiving signal S1 diminishes and accordingly 
the detected DC current diminishes, or vice versa), if the receiving 
amplifier 44 operates in such a manner that the overall gain thereof 
becomes smaller when voltage S3 supplied to the gain controlling node 
becomes smaller, or, conversely, the overall gain becomes greater when 
voltage S3 supplied to the gain controlling node becomes greater, the 
output of time-constant controlling means 42 cannot be used as just being 
amplified, and thus, the output is inverted and then amplified by control 
voltage amplifier 43, and supplied to the gain controlling node of 
receiving amplifier 44. 
The conventional automatic gain controller is classified as a high-speed 
automatic gain controller or a low-speed automatic gain controller, 
according to the response time of time-constant controlling means 42. 
These two types of gain controllers are selectively used according to 
design criteria. 
FIG. 2 is a graphic diagram showing the change of voltage signals S3 
according to time in high-speed response and low-speed response in the 
conventional automatic gain controller. Referring to FIG. 2, the low-speed 
automatic gain controller which sets a slow response time in time-constant 
controller 42 (FIG. 1) cannot cope actively with the quick change in 
receiving signal input S1 since the transition to the predetermined 
control voltage is carried out at a low speed. However, the low-speed 
automatic gain controller has a slower transient response characteristic 
to reach the predetermined voltage, so that it exhibits a more stable 
operation characteristic after the control voltage reaches its 
predetermined level, and thereby acts as a buffer for minute instantaneous 
changes in the received signal. Accordingly, the controller will not 
operate poorly, and output signal S2 of receiving amplifier 44 can be 
stabilized. 
On the contrary, the high-speed automatic gain controller sets a fast 
response time in time-constant controlling means 42. Here, the controller 
can cope actively with a quick change of the receiving signal input S1 
because the transition to a predetermined control voltage is performed at 
a fast speed, e, and the control voltage cannot be stabilized at the 
predetermined voltage level. Moreover, since the high-speed automatic gain 
controller has a greater tendency to control the gain for minute 
instantaneous changes of the received signal, and the output signal S2 of 
receiving amplifier 44 cannot be stable. 
Accordingly, when the conventional automatic gain controller, to cope with 
the changes of the input signal, performs a transition from one control 
voltage level to a higher control voltage level, the response time 
characteristic of time-constant controlling means 42 is determined 
according to the charge reaching capacitor C via resistor R and its output 
load. In addition, when the gain controller performs a transition from a 
higher control voltage level to a lower one to cope with level changes in 
the input signal, the response time of time-constant controlling means 42 
is determined according to the capacitance of capacitor C and the output 
load of the automatic voltage controller, such as the internal resistance 
of control voltage amplifier 43 or the control voltage node of receiving 
amplifier 44. 
The conventional automatic gain controller shows a longer response time for 
discharging than for charging, since the load of the automatic gain 
controller. i.e., the DC resistance of the gain control voltage node of 
receiving amplifier 44, is greater than input resistor R. In other words, 
when the gain control voltage changes from a lower level to a higher 
level, to cope with the change of the input signal, the rise time is 
mainly determined according to the response time characteristic of 
time-constant controller 42. On the contrary, if the change in the 
received input causes the gain control voltage to change from a higher 
level to a lower level, the fall time is determined according to the 
output load of the automatic gain controller. Therefore, the fall time is 
longer than the rise time. 
Accordingly, the transition from a lower control voltage level to a higher 
control voltage level shortens gain control time. However, a transition 
from a higher control voltage level to a lower control voltage level 
lengthens gain control time, which slows the response time of receiving 
amplifier 44. 
To improve certain drawbacks due to the difference between the rise time 
and fall time in the prior art, it has been suggested that an additional 
load resistor be installed at the output of time-constant controlling 
means 42 (FIG. 1). This method is intended to shorten the rise time by 
intentionally discharging the charge accumulated in time-constant 
controlling means 42, but the response time is reduced by the additional 
resistor, and the rise time is also shortened, and thereby the control 
voltage becomes unstable at a predetermined voltage level. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an automatic gain 
controller having a high-speed response characteristic by providing an 
apparatus for shortening the all time from a control voltage level to a 
lower control voltage level due to a input signal variation. 
Another object of the present invention is to provide a stable automatic 
gain controller which alleviates the drawbacks of the conventional 
high-speed automatic gain controller which tends to be unstable by 
performing a transition to a pretermined voltage, and providing a fast 
response characteristic by shortening the response time of the discharging 
period. 
To accomplish the above-described objects, the present invention provides 
an automatic gain controller for producing at all times a signal at a 
constant amplitude, although the amplitude of the input signal to a 
receiving amplifier changes according to the lapse of time, the controller 
comprising signal detecting means for receiving the signal from the 
receiving amplifier and converting the signal to a DC current changing 
according to the received signal; first and second time-constant 
controlling means for receiving the detected signal from the signal 
detecting means and removing an AC component from the detected signal to 
thereby produce only a DC component and simultaneously providing a 
predetermined time-response functional characteristic; voltage subtracting 
means for receiving the outputs of the first and second time-constant 
controlling means and subtracting the output of the second time-constant 
controlling means from the output of the first time-constant controlling 
means to provide the resultant voltage; voltage comparing means for 
comparing the output voltage of the voltage subtracting means with a 
predetermined reference voltage; switching means for controlling the 
transmission of the output signal of the first time-constant controlling 
means is responsive to the output of the voltage subtracting means; 
voltage sinking means :for discharging the output voltage of the first 
time-constant controlling means, the voltage relayed via the switching 
means; and control voltage amplifying means for receiving the output 
signal of the first time-constant controlling means to be converted to the 
amplitude of the control voltage to control the receiving amplifier.

DETAILED DESCRIPTION OF THE INVENTION 
An automatic gain controller according to the present invention is to be 
described, hereinafter, in reference with the attached drawings. 
FIG. 3 is a block diagram showing an automatic gain controller according to 
the present invention. Referring to FIG. 3, the automatic gain controller 
comprises a signal detector 1 for receiving the signal S2 output from a 
receiving amplifier 44 and detecting a signal therefrom; first and second 
time-constant controllers 2 and 3 for receiving the output signal of 
signal detector 1 and controlling the time-constants thereof: a voltage 
subtracter 4 for receiving the output signals of the first and second 
time-constant controllers 2 and 3 and subtracting one from the other: a 
voltage comparator 5 for comparing the output signal of voltage subtracter 
4 with a reference voltage V.sub.c ; a switch 6 for transmitting the 
output signal of first time-constant controller 2, in response to the 
output signal of voltage comparator 5; a voltage sink 7 for forcibly 
discharging the voltage output from switch 6: and a control voltage 
amplifier 8 for amplifying the output signal of first time-constant 
controller 2 to thereby control the gain of receiving amplifier 44. 
Hereinbelow, the operation of the automatic gain controller is described. 
Signal detector 1 generally utilizes diodes or semiconductor devices for 
receiving output signal S2 of receiving amplifier 44 or any other signal 
to thereby convert the signal into a DC voltage whose amplitude fluctuates 
according to the amplitude of the received signal. The voltage detected by 
signal detector 1 is supplied to each of first and second time-constant 
controlling means 2 and 3. First and second time-constant controllers 2 
and 3 receive the detected signals from signal detector 1 and remove the 
AC component from the signals, to thereby provide a pure DC component. 
Also, for the first and second time-constant controlling means 2 and 3 to 
have a predetermined response time characteristic they may have an analog 
structure, utilizing resistors and capacitors as in the conventional 
means, or a digital circuit which samples the input voltage at a 
predetermined high frequency, digitalizes the sampled data, and utilizes a 
memory device. First and second time-constant controllers 2 and 3 are of 
same construction, but their response times are set differently, that is, 
first time-constant controller 2 is set to have the desired response time 
for the entire receiving amplifier 44, and second time-constant controller 
3 is set to have a faster response time than that of first time-constant 
controller 2. 
The output (S.sub.A) of first time-constant controller 2 is supplied to one 
input node of voltage subtracter 4, control voltage amplifier 8, and 
switch 6. The output (S.sub.B) of second time-constant controller 3 is 
supplied to the other input node (opposite polarity) of voltage subtracter 
4 and to voltage sink 7. 
Voltage subtracter 4 receives the outputs of first and second time-constant 
controller 2 and 3 and produces an output voltage V.sub.o : 
EQU V.sub.o =V1-V2 
wherein V.sub.o is the output voltage of voltage subtracter 4, and V1 and 
V2 are the output voltages of first and second time-constant controllers 2 
and 3, respectively. 
Voltage comparator 5 receives two inputs, i.e., output voltage V.sub.o from 
voltage subtracter 4 and a comparing voltage V.sub.c, and produces a 
control signal for turning on switch 6 if V.sub.o is greater than V.sub.c. 
At this moment, comparing voltage V.sub.c can be set variably according to 
a user's requirements. Switch 6 opens or closes the path between first 
time-constant controller 2 and voltage sink 7, according to the control 
voltage output from voltage comparator 5. 
Voltage sink 7 forcibly discharges the output voltage of first 
time-constant controller 2 which is supplied as an input via switch 6. 
Here, the discharging should be accomplished faster than the discharging 
carried out by way of the load at the gain control voltage node of 
receiving amplifier 44. For analog circuits, resistors can be used, and 
for digital circuit a digital subtractor can be adopted. 
Control voltage amplifier 8 is the same as in the prior art. The control 
voltage amplifier 8 receives the output of first time-constant controller 
2 and converts the output into a control voltage whose magnitude is proper 
for receiving amplifier 44, to thereby supply a gain control voltage S3 to 
receiving amplifier 44. Also, control voltage amplifier 8 adjusts the 
direction of change of voltage detected from the automatic gain controller 
so that it is identical with the direction of the gain control of 
receiving amplifier 44 by using an inverting amplifying means, if 
directions of those changes are not identical. A controller 9 comprises 
the voltage subtracter 4, the voltage comparator 5, and the switch 6. 
FIG. 4 is a graphic diagram illustrating the change of signal voltage S3 
according to time of the automatic gain controller as shown in FIG. 3. 
Referring to FIG. 4, it can be assumed that if switch 6 (FIG. 3) is opened 
and gain controlling voltage S3 increases, receiving amplifier 44 operates 
to have a greater gain, and conversely, if gain controlling voltage S3 
decreases, the receiving amplifier 44 operates to have a smaller gain. In 
the case of a receiving amplifier 44 having the exact opposite operation, 
an inverting amplifier can be used as the control voltage amplifying means 
8. 
When receiving signal input S1 decreases, gain control voltage S3, which is 
an output voltage of the automatic gain controller, should maintain the 
signal amplitude at the predetermined constant level by increasing the 
gain of receiving amplifier 44. Such sharply falling slopes can occur due 
to natural causes, for example, sudden variations in the ionosphere which 
are generated instantaneously by the external influence of sunspot 
explosions, or artificial causes such as burst transmissions by which 
large pieces of information are instantly transmitted. Generally speaking, 
the natural causes affect short wave communication system, and the 
artificial causes affect satellite communication systems. 
When receiving input signal S1 which enters receiving amplifier 44 
decreases as above-described, receiving signal output S2 produced by 
receiving amplifier 44 diminishes, and therefore, the output voltage of 
signal detector 1 decreases. 
At this moment, as described above, since response time of second 
time-constant controller 3 is shorter than that of first time-constant 
controller 2, the accumulated charge diminishes faster than in first 
time-constant controller 2. Accordingly, a difference voltage occurs 
between outputs of first and second time-constant controller 2 and 3. The 
voltage difference V.sub.o is calculated by voltage subtracter 4 and 
thereby applied to voltage comparator 5. 
The voltage difference V.sub.o caused by the discharging speed is compared 
with comparing voltage V.sub.c by means of voltage comparator 5 (FIG. 3). 
If output voltage V.sub.o of first and second time-constant controllers 2 
and 3 is greater than the predetermined comparing voltage V.sub.c, as in a 
point A of FIG. 4, switch 6 is closed, and thereby the output voltage of 
first time-constant controller 2 is supplied to voltage sink 7 as well to 
control voltage amplifier 8, so that the output voltage of first 
time-constant controller 2 is quickly discharged. Therefore, the output 
voltage of first time-constant controller 2 changes to the level at a 
point B'. 
When the output voltage of first time-constant controller 2 moves to point 
B', output voltage V.sub.o of first and second time-constant controllers 2 
and 3 becomes smaller than preset comparing voltage V.sub.c, thereby 
opening switch 6 and discharging only the output voltage of first 
time-constant controller 2 through the load resistor. Therefore, the 
output voltage of first time-constant controller 2 is reduced from the B' 
voltage level to follow the curve between points B and C, which coincides 
with the discharge curve of first time-constant controller 2. Accordingly, 
the output voltage of first time-constant controller 2 changes to the 
level at a point C'. 
At point C', if output voltage V.sub.o of first and second time-constant 
controllers 2 and 3 becomes greater than voltage V.sub.c, the output 
voltage of first time-constant controller 2 is discharged at a faster 
speed. Also, at a point D', if output voltage V.sub.o is smaller than the 
voltage V.sub.c, switch 6 opens as described above, so that the output 
voltage of first time-constant controller 2 continues decreasing along its 
original discharge curve. 
Control voltage amplifier 8 amplifies the output of first time-constant 
controller 2 at a predetermined ratio so as to provide the amplified 
output for the gain control node of receiving amplifier 44. While the 
output decreases to the desired voltage level, the transition to the 
desired voltage level is performed very quickly so as to provide a faster 
automatic gain controlling characteristic than that of first time-constant 
controller 2. 
After reaching the desired voltage level E of FIG. 4, the difference 
voltage V.sub.o between the voltages of first and second time-constant 
controllers 2 and 3 rarely exceeds comparing voltage V.sub.c, since 
automatic gain controlling voltage S3 becomes completely dependent on the 
output voltage of first time-constant controller 2, which therefore 
removes the transient response and the instability usually occurring in 
the high-speed gain controlling apparatus. 
FIG. 5 is a graphic diagram showing the operation when received signal S1 
input to receiving amplifier 44 rises sharply. Gain controlling voltage S3 
which is an output voltage of the automatic gain controller should be set 
large to reduce the gain of receiving amplifier 44 and thereby to maintain 
the consistency in amplitude of the output signal. 
As described above, if received signal S1 input to receiving amplifier 44 
becomes large, received signal output S2 from receiving amplifier 44 
increases, and also, the output voltage of signal detector 1 increases. 
Therefore, first and second time-constant controllers 2 and 3 each receive 
the output of signal detector 1, so that they begin to accumulate charges 
in their accumulating means (capacitors) at different response time. 
Second time-constant controller 3 has a short response time, so that from 
the beginning to point A, it is charged faster than first time-constant 
controller 2. When the voltage difference V.sub.o between the time 
constant controlling means is obtained by voltage subtracter 4, if the 
output voltage of second time-constant controller 3 is greater, output 
voltage V.sub.o has a negative value. At this time, if voltage V.sub.o is 
compared with voltage V.sub.c, voltage V.sub.o proves to be smaller than 
V.sub.c, and therefore, switch 6 opens, and gain controlling voltage S3 
becomes dependent on the output voltage of first time-constant controller 
2. That is, the gain controlling voltage S3 moves toward a desired 
time-constant of the overall amplifier. 
When the transient response characteristic of first time-constant 
controller 2 is large and output voltage V.sub.o of the first and second 
time-constant controlling means is greater than the voltage V.sub.c (at 
point B), switch 6 closes so as to provide the output voltage of first 
time-constant controller 2 to voltage sink 7 as well as to control voltage 
amplifier 8. Accordingly, the output voltage of first time-constant 
controller 2 is forcibly discharged, and thus the output voltage of first 
time-constant controller 2 shifts to the level at point C. 
When the output voltage of first time-constant controller 2 shifts to point 
C, output voltage V.sub.o of first and second time-constant controllers 2 
and 3 becomes smaller than voltage V.sub.c, and switch 6 opens. Therefore, 
the output voltage of first time-constant controller 2 is discharged only 
by the load resistor, and then decreases from the voltage level of point C 
to follow the original discharge curve of the first time-constant 
controlling means. 
When voltage V.sub.o approaches the desired voltage level E, the voltage 
difference between first and second time-constant controller 2 and 3 
rarely exceeds comparing voltage V.sub.c, and automatic gain control 
voltage S3 becomes completely dependent on the output voltage of first 
time-constant controller 2, which can remove the transient response and 
the instability which usually occur in the high-speed controlling 
apparatus. 
As the voltage changes to the desired voltage level according to the gain 
controlling operation, the transition is performed at a designed overall 
response time. However, in the case of a transient response, the voltage 
is quickly restored to the original voltage, so that a more reliable and 
accurate automatic gain control can be accomplished without 
malfunctioning. 
FIG. 6 is a circuit diagram showing an embodiment of the automatic gain 
controller as shown in FIG. 3. Referring to FIG. 6, a diode 31 corresponds 
to signal detector 1 (FIG. 3), a resistor 32 and a capacitor 33 correspond 
to first time-constant controller 2, a resistor 34 and a capacitor 35 
correspond to second time-constant controller 3. The controller 9 includes 
a diode 36 corresponding to voltage subtracter 4, voltage comparator 5 and 
switch 6. A resistor 37 corresponds to voltage sink 7 and amplifier 38 
corresponds to control voltage amplifier 8. 
In greater detail, diode 31 receives and rectifies the output signal of the 
receiving amplifier, that is supplied to resistors 32 and 34. First 
time-constant controller 2 comprising of resistor 32 and capacitor 33 
receives the rectified signal and eliminates the AC component included in 
the rectified signal via capacitor 33. Thus, the rectified signal is 
converted into a pure DC component, supplied to diode 36 and control 
voltage amplifying means 38. An inherent response time is determined by 
the values of resistor 32 and capacitor 33. Second time-constant 
controller 3 comprising resistor 34 and capacitor 35 receives the 
rectified signal, and the AC component included in the rectified signal 
via capacitor 35. Thus, the rectified signal is converted into a pure DC 
component. The resultant DC component is supplied to a diode 39. First 
time constant controlling means has a time response characteristic 
determined by the values of resistor 34 and capacitor 35. Here, the 
cathode of diode 36 is connected to one end of resistor 37 and the other 
end of resistor 37 is grounded. Further, the output voltage terminal of 
control voltage amplifying means 38 is connected to the gain controlling 
node of receiving amplifier 44. 
In the above-described embodiment, diode 39 prevents the voltage passing 
through diode 36 from being fed back to the second time-constant 
controlling means, while providing the discharge path of the second 
time-constant controlling means. The anode of diode 39 is commonly 
connected to resistor 34 and capacitor 35, and its cathode is commonly 
connected to that of diode 36 and to resistor 37. 
Operation with the circuit of FIG. 6 is described in more detail with 
reference to Figs.4 and 5. 
Operation of the circuit is described with respect to the case when signal 
input S1 to receiving amplifier 44 falls sharply. Since received signal 
output S2 produced by receiving amplifier 44 becomes smaller in this case, 
accordingly, the output rectified voltage of diode 31 decreases. 
Therefore, resistors 32 and 34 begin to discharge the charges accumulated 
in capacitors 33 and 35 at respectively differing response-time speeds. 
That is, capacitor 33 discharges charges to the gain control voltage node 
via control voltage amplifying means 38, and capacitor 35 discharges 
charges to resistor 37 via diode 39. 
As described above with respect to the circuit construction, the second 
time-constant controlling means having resistor 34 and capacitor 35 has a 
relatively short response time, and thus discharges the accumulated 
charges faster than the first time-constant controlling means comprising 
of resistor 32 and capacitor 33, which creates a voltage difference across 
diode 36. If the voltage difference is greater than the voltage generated 
by resistor 32 and capacitor 33 (as at point A of FIG. 4) and if the 
forward voltage of diode 36 exceeds 0.7 V (or 0.2 V in the case of a 
germanium diode), diode 36 conducts so as to provide the output voltage of 
resistors 32 and 34 to resistor 37 performing as a voltage sink, as well 
as to the control voltage amplifying means 38, to thereby discharge the 
output voltage at a fast speed. Accordingly, gain control voltage S3 
changes to point B' as shown in FIG. 4. 
When the output voltage of resistors 32 and 34 change to the level at point 
B' of FIG. 4 and the forward voltage across diode 36 thus becomes less 
than 0.7 V, diode 36 does not conduct, so that the output voltages of 
resistors 32 and 34 are discharged only via load resistors. Therefore, the 
output voltage level at point B' decreases to follow the curve between 
points B and C, so that output voltage S3 moves to point C' of FIG. 4. 
At point C of FIG. 4, if the voltage generated by first time-constant 
controlling means 2 is greater than the voltage generated by second 
time-constant controlling means 3, and if the voltage difference obtained 
between first and seconde time-constant controllers 2 and 3 exceeds 0.7 V 
(i.e., the forward voltage of diode 36), then diode 36 conducts. 
Conversely, if the voltage difference between first and second 
time-constant controllers 2 and 3 becomes less than 0.7 V (as at point D' 
of FIG. 4), diode 36 does not conduct. This operation is repeated over 
time. 
On the other hand, operation of the circuit is described hereinbelow with 
respect to the case when received input signal S1 to receiving amplifier 
44 rises sharply, with reference to FIG. 5. 
Gain voltage S3 of automatic gain controller increases to reduce the gain 
of the receiving amplifier 44, so as to maintain the constant signal 
amplitude of the output. As described above, an increase of received 
signal S1 input to receiving amplifier 44 results in an increase of 
received signal output S2 output therefrom, thereby raising the output 
voltage of diode 31. Therefore, the input signals to first time-constant 
controlling means 2 and second time-constant controlling means 3 increase, 
so that capacitors 33 and 35 begin to accumulate charges therein according 
to different time-response speeds. 
As described, the time constant controller having resistor 34 and capacitor 
35 has a short response time, and therefore charges faster than the time 
constant controller having resistor 32 and capacitor 33, from the 
beginning to point A as shown in FIG. 5. Thus, a reverse bias develops 
across diode 36, such that it does not conduct. Accordingly, gain control 
voltage S3 becomes dependent on the output voltage of resistors 32 and 34. 
That is, the gain control voltage S3 increases to a desired time-constant 
of the overall amplifier. 
However, resistor 32 and capacitor 33 of first time-constant controller 2 
have a large transient response characteristic. Therefore, if the 
difference of the output voltages between first and second time-constant 
controllers 2 and 3 (as at point B of FIG. 5) is greater than 0.7 V, diode 
36 conducts. Accordingly, the output voltage of first time-constant 
controlling means 2 is applied to voltage sinking means 37 as well as 
control voltage amplifying means 38, so as to forcibly discharge the 
accumulated charges. Therefore, the control signal voltage moves to point 
C of FIG. 5. 
If the output voltage of first time-constant controlling means 2 transits 
the point C of FIG. 5, the difference between the two voltages becomes 
smaller than 0.7 V and thus diode 36 stops conducting and the output 
voltage of resistor 32 and capacitor 33 is discharged only via the load 
resistor. Thus, the signal voltage output from the first time-constant 
controlling means is reduced from the voltage level of point C to follow 
the original curve of resistor 32 and capacitor 33. 
In the above embodiment, a single diode 36 is used, but if the voltage 
difference between first and second time-constant controllers 2 and 3 
should be preset to a larger value, diode 36 could be replaced with a 
plurality of diodes connected in series. 
Also, when a sufficient voltage difference cannot be obtained to turn on 
diode 36 due to a decrease of the output voltage of first and second 
time-constant controllers 2 and 3, each time-constant controlling means 
can adopt voltage amplifying means at its own output node, as shown in FIG 
7. 
The automatic gain controller according to the present invention gives the 
following advantages: 
1) Since two time-constant controlling means each having a different 
time-constant are included, a forcible discharging path can be established 
by using the difference voltage between these time-constant controlling 
means, and therefore, a faster falling transition to the desired voltage 
level can be accomplished; 
2) As the automatic gain control voltage approaches the desired voltage 
level, the automatic gain controller provides a low-speed response 
characteristic which can remove the transient response and instability 
which occur in the high-speed automatic gain controller; and 
3) The rising transition to the desired voltage level is performed at an 
designed overall time-response speed, and thus if a transient response 
occurs, high-speed restoration to the original voltage level is performed 
so as to reduce malfunctions of the gain controller. 
While the present invention has been shown and described with reference to 
particular embodiments, it will be understood by those skilled in the art 
that various changes in form and details may be effected therein without 
departing from the spirit and scope of the invention as defined by the 
appended claims.