JP2000101374A - Automatic level control circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an automatic level control circuit capable of level- controlling the signal level of a wide range. SOLUTION: An RF signal is attenuated by a variable resistor 10, amplified by an amplification factor different according to the size of an attenuation signal by an amplifier circuit part, including an amplifier 11 and an amplifier 15 and outputted from an ALC(automatic level control) circuit 1. An output from the ALC circuit 1 is fixed by utilizing feedback operation, when the output from the amplifier circuit part is smaller than a prescribed output voltage value outputted from the ALC circuit 1, the attenuation factor for the resistor 10 is reduced to increase the output from the amplifier circuit part, and when the output from the amplifier circuit part is larger than a prescribed output voltage value, the attenuation factor of the resistor 10 is increased to reduce the output from the amplifier circuit part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic level control (ALC) for controlling the level of an output signal to be constant.
Level Control) circuit.

[0002]

2. Description of the Related Art An ALC circuit is a circuit which always obtains a constant value output from an ALC circuit by changing an amplification degree of an input signal by a feedback action. The following shows an example of a conventional ALC circuit. This will be described with reference to FIG.

FIG. 3 is a circuit diagram showing an example of a conventional ALC circuit 100. The ALC circuit 100 of FIG.
Is a variable resistor 101, an amplifier 102, a detection circuit 10
3, capacitor 104, error amplifier 105, and resistor 1
06.

The variable resistor 101 is constituted by a PIN diode or the like, and its attenuation rate changes according to the value of a control current (control current) input from the error amplifier 105 via the resistor 106. By the attenuation rate that changes according to the value of the control current input from
The signal is attenuated and output to the amplifier 102. As shown in FIG. 4B, the attenuation factor of the variable resistor 101 due to the PIN diode changes depending on the magnitude of the control current, and as the value of the control current increases, the attenuation factor of the variable resistor 101 due to the PIN diode increases. Becomes smaller. In FIG. 4B, each solid line in the figure shows the relationship between the frequency of the variable resistor 101 formed by the PIN diode and the attenuation factor when the control current value is constant.

The amplifier 102 amplifies a signal input from the variable resistor 101 at a predetermined amplification rate, and
It is output as an output signal from the C circuit 100. Amplifier 1
As shown in FIG. 4A, the gain of 02 decreases as the frequency increases. The amplification factor of the amplifier 102 is constant if the frequency is the same.

[0006] The detection circuit 103 detects the output signal of the amplifier 102, and the capacitor 104 holds the voltage of the output signal of the amplifier 102, which is the detection result of the detection circuit 103.

The error amplifier 105 amplifies the difference between the value of the comparison voltage and the value of the voltage between the plates of the capacitor 104, and the amplified voltage becomes a control current via the resistor 106, and becomes a control current.
1 is input. When the value of the output signal of the amplifier 102 is equal to or higher than a predetermined output voltage value (a target value of the voltage level obtained by level control), the value of the control current is reduced. Is set such that the value of the control current increases when is less than the predetermined output voltage value.

The circuit operation of the ALC circuit 100 whose circuit configuration is described above will be briefly described. The RF signal input to the ALC circuit 100 is attenuated by the variable resistor 101,
The signal is further amplified by the amplifier 102 and output as an output signal from the ALC circuit 100.

In order to make the output value of the signal output from the ALC circuit 100 constant, the voltage value of the output signal output from the amplifier 102 is adjusted to a predetermined output voltage value of the ALC circuit 100 (a voltage level obtained by level control). Is larger than the target value), the output voltage from the error amplifier 105 decreases, and the current value of the control current input to the variable resistor 101 decreases. And the variable resistor 10
1 increases, and the output signal from the amplifier 102 decreases.

On the other hand, when the voltage value of the output signal output from the amplifier 102 is smaller than the predetermined output voltage value of the ALC circuit 100, the output voltage from the error amplifier 105 increases, The current value of the control current is increased. Then, the attenuation rate of the variable resistor 101 decreases, and the output signal from the amplifier 102 increases.

With the above-described feedback action, the output voltage value from the ALC circuit 100 is controlled to a predetermined output voltage value of the ALC circuit 100, and a constant output voltage value is obtained from the ALC circuit 100. Become.

[0012]

The above-mentioned conventional ALC circuit 100 utilizes the fact that the attenuation factor of the variable resistor 101 changes when the value of the control current to the variable resistor 101 changes, and the output signal Level control.

However, as shown in FIG. 4B, the attenuation of the variable resistor 101 due to the PIN diode decreases as the frequency increases, so that the variation of the attenuation of the variable resistor 101 decreases. There is a problem that the range of the signal level that can be obtained is small.

An object of the present invention is to provide an automatic level control circuit capable of controlling the level of a wide range of signal levels.

[0015]

According to the first aspect of the present invention,
An attenuation circuit for attenuating an input signal at an attenuation rate corresponding to the magnitude of a control signal, and detecting a signal input from the attenuation circuit, and amplifying a signal input from the attenuation circuit at a different amplification factor according to the detection result. An automatic level control circuit comprising: an amplification circuit for amplifying; and a control signal generation circuit for detecting an output of the amplification circuit and outputting a control signal corresponding to the detection result to the attenuation circuit. The output voltage from the automatic level control circuit is made constant by changing and controlling the attenuation factor and the amplification factor of the amplifier circuit.

According to the first aspect of the present invention, the output voltage of the automatic level control circuit is not controlled only by changing the attenuation rate of the attenuation circuit, but the attenuation rate of the attenuation circuit and the amplification rate of the amplification circuit are adjusted. Since the output voltage of the automatic level control circuit is controlled by changing the level, the level control can be performed for a wide range of signal levels even when the amount of change in the attenuation rate of the attenuation circuit is small, for example.

According to a sixth aspect of the present invention, there is provided an amplifying circuit for detecting an input signal and amplifying the input signal at a different amplification factor in accordance with the detection result, and converting a signal input from the amplifying circuit into a control signal having a magnitude of a control signal. An automatic level control circuit comprising: an attenuation circuit that attenuates at an attenuation rate according to the control circuit; and a control signal generation circuit that detects an output of the attenuation circuit and outputs a control signal corresponding to the detection result to the attenuation circuit. The output voltage from the automatic level control circuit is made constant by changing and controlling the attenuation factor of the amplification circuit and the amplification factor of the attenuation circuit.

According to the sixth aspect of the invention, the output voltage of the automatic level control circuit is not controlled only by changing the attenuation rate of the attenuation circuit, but the attenuation rate of the attenuation circuit and the amplification rate of the amplification circuit are adjusted. Since the output voltage of the automatic level control circuit is controlled by changing the level, the level control can be performed for a wide range of signal levels even when the amount of change in the attenuation rate of the attenuation circuit is small, for example.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of an automatic level control (ALC) circuit according to the present invention will be described in detail with reference to the drawings.

(First Embodiment) Hereinafter, an embodiment of an ALC circuit according to a first embodiment of the present invention will be described in detail with reference to FIG. First, the configuration will be described.

FIG. 1 shows an ALC according to the first embodiment.
1 is a circuit configuration diagram illustrating a circuit 1. FIG. The ALC circuit 1 shown in FIG. 1 includes a variable resistor 10, an amplifier 11, a detection circuit 12, a comparator 13, a switch 14, an amplifier 15, a detection circuit 16,
It comprises a capacitor 17, an error amplifier 18, and a resistor 19.

The variable resistor 10 is constituted by a PIN diode or the like, and its attenuation rate changes according to the value of a control current (control current) input from the error amplifier 18 via the resistor 19. The RF signal is attenuated by an attenuating rate that changes according to the value of the control current input from the resistor 18 via the resistor 19 and the amplifier 11 or the switch 1
4 to the amplifier 15. As mentioned above, P
FIG. 4 shows the attenuation rate of the variable resistor 10 using the IN diode.
As shown in (b), the current varies depending on the magnitude of the control current. As the control current increases, the attenuation factor of the variable resistor 10 due to the PIN diode decreases.

The amplifier 11 amplifies the signal input from the variable resistor 10 at a predetermined amplification factor, and outputs the amplified signal to the amplifier 15 via the switch 14. As shown in FIG. 4A, the amplification factor of the amplifier 11 decreases as the frequency increases.

The detection circuit 12 detects the output signal of the variable resistor 10 and outputs the detected signal to the comparator 13.

The comparator 13 compares the voltage of the detection signal input from the detection circuit 12 with a reference voltage, and outputs the comparison result to the switch 14 as a control signal. Here, the comparator 13 outputs a signal of the voltage level of “L” when the voltage of the detection signal is smaller than the reference voltage, and outputs a signal of the voltage level of “H” when the voltage of the detection signal is higher than or equal to the reference voltage. Output to the switch 14 as a control signal. The value of the reference voltage is set in consideration of the amount of change in the attenuation rate of the variable resistor 10.

The switch 14 controls the connection between the terminals A1 and A3 and the connection between the terminals A2 and A3 in accordance with a control signal from the comparator 13 by a control circuit (not shown). More specifically, when the control signal from the comparator 13 is a signal of a voltage level of “H”, the terminals A1 and A
3 is connected, and the output of the variable resistor 10 is input to the amplifier 15. On the other hand, when the control signal from the comparator 13 is a signal of the voltage level of "L", the terminal A2 and the terminal A
3 is connected, and the output of the amplifier 11 is input to the amplifier 15.

The amplifier 15 amplifies a signal input from the variable resistor 10 or the amplifier 11 at a predetermined amplification factor.
The amplified signal is output as an output signal from the ALC circuit 1. The amplification factor of the amplifier 15 is, as shown in FIG.
It decreases as the frequency increases.

The amplifier 11, the detection circuit 12, and the comparator 1
3, the switch 14 and the amplifier 15 constitute an amplifier circuit unit for amplifying the voltage value of the output signal of the variable resistor 10, and the amplifier circuit unit amplifies the output voltage of the variable resistor 10 at different amplification factors. Will be done. More specifically, when the output voltage of the variable resistor 10 is equal to or lower than the reference voltage, the output voltage of the variable resistor 10 is amplified by the amplifiers 11 and 15, while the output voltage of the variable resistor 10 is Is equal to or higher than the reference voltage, the output voltage of the variable resistor 10 is amplified only by the amplifier 15.

The detection circuit 16 detects the output signal of the amplifier 15, and the capacitor 17 holds the voltage of the output signal of the amplifier 15 which is the detection result of the detection circuit 16.

The error amplifier 18 amplifies the difference between the comparison voltage and the voltage between the plates of the capacitor 17, and the amplified voltage is input to the variable resistor 10 as a control current via the resistor 19. . When the value of the output signal of the amplifier 15 is equal to or higher than a predetermined output voltage value (a target value of the voltage level obtained by level control), the value of the control current is reduced.
Further, the value of the comparison voltage is set so that the value of the control current increases when the value of the output signal of the amplifier 15 is less than the predetermined output voltage value.

The circuit operation of the above-described ALC circuit 1 will be briefly described. The RF signal input to the ALC circuit 1 is attenuated by the variable resistor 10, and is further amplified by the amplifier 15 when the voltage value of the output signal of the variable resistor 10 is equal to or higher than a predetermined reference voltage value. It is output as an output signal from the ALC circuit 1. On the other hand, when the voltage value of the output signal of the variable resistor 10 is less than the predetermined reference voltage value, the output signal output from the variable resistor 10 is amplified by the amplifiers 11 and 15 and the ALC circuit 1
Is output as an output signal.

In order to make the output value of the signal output from the ALC circuit 1 constant, the voltage value of the output signal output from the amplifier 15 is adjusted to a predetermined output voltage value of the ALC circuit 1 (a voltage level obtained by level control). Is larger than the target value), the output voltage from the error amplifier 18 decreases, and the current value of the control current input to the variable resistor 10 decreases. Then, the attenuation rate of the variable resistor 10 increases, and the output signal output from the variable resistor 10 decreases. As a result, the output signal output from the amplifier 15 decreases.

On the other hand, when the voltage value of the output signal output from the amplifier 15 is smaller than the predetermined output voltage value of the ALC circuit 1, the output voltage from the error amplifier 18 increases, The current value of the control current is increased. Then, the attenuation rate of the variable resistor 10 decreases, and the output signal output from the variable resistor 10 increases. As a result, the output signal output from the amplifier 15 increases.

With the above-described feedback action, the output voltage value from the ALC circuit 1 is controlled to a predetermined output voltage value of the ALC circuit 1, and a constant output voltage value is obtained from the ALC circuit 1. Become.

According to the ALC circuit 1 of the first embodiment, the output signal of the variable resistor 10 can be amplified at different amplification factors by the amplifier circuit section including the amplifier 11 and the amplifier 15, and RF that could not be level controlled only by changing the attenuation rate of the resistor 10
Level control is also possible at the signal level of the signal.

That is, in the ALC circuit 1 of the first embodiment, the variation of the attenuation factor of the variable resistor 10 and the variation of the amplification factor of the amplifier circuit section including the amplifiers 11 and 15 are combined. Level control can be performed on the level of the RF signal in the range corresponding to the amount of change.

Further, as in the conventional ALC circuit 100, P
Even when the amount of change in the attenuation of the variable resistor 10 composed of an IN diode or the like decreases as the frequency increases, the attenuation of the variable resistor 10 and the amplifier 1
Since the level control is performed by changing and controlling the amplification factor of the amplification circuit unit including the amplifier 1 and the amplifier 15,
Level control can be performed for a wide range of RF signal levels.

(Second Embodiment) Hereinafter, an ALC circuit according to a second embodiment of the present invention will be described in detail with reference to FIG. First, the configuration will be described.

FIG. 2 shows an ALC according to the second embodiment.
FIG. 2 is a circuit configuration diagram showing a circuit 2. The ALC circuit 2 shown in FIG. 2 includes an amplifier 20, a detection circuit 21, a comparator 22, a switch 23, an amplifier 24, a variable resistor 25, a detection circuit 26,
It comprises a capacitor 27, an error amplifier 28, and a resistor 29.

The amplifier 20 amplifies the RF signal at a predetermined amplification rate, and outputs the amplified signal to the amplifier 24 via the switch 23. As shown in FIG. 4A, the amplification factor of the amplifier 20 decreases as the frequency increases.

The detection circuit 21 detects the input RF signal, and outputs the detected signal to the comparator 22.

The comparator 22 compares the voltage of the detection signal input from the detection circuit 21 with a reference voltage, and outputs the comparison result to the switch 23 as a control signal. Here, the comparator 2
2 is “L” when the voltage of the detection signal is smaller than the reference voltage.
Is output to the switch 23 when the voltage of the detection signal is higher than or equal to the reference voltage. The value of the reference voltage is set in consideration of the amount of change in the attenuation rate of the variable resistor 25.

The switch 23 controls the connection between the terminal B1 and the terminal B3 and the connection between the terminal B2 and the terminal B3 according to a control signal from the comparator 22 by a control circuit (not shown). More specifically, when the control signal from the comparator 22 is a signal of the voltage level of "H", the terminal B1 and the terminal B
3 and the RF signal is directly input to the amplifier 24. On the other hand, when the control signal from the comparator 22 is a signal of the voltage level of "L", the terminal B2 and the terminal B
3 is connected, and the output of the amplifier 20 is input to the amplifier 24.

The amplifier 24 receives the RF signal or the amplifier 2
The signal input from 0 is amplified at a predetermined amplification rate and output to the variable resistor 25. The amplification factor of the amplifier 24 is shown in FIG.
As shown in (a), the frequency decreases as the frequency increases.

The above-mentioned amplifier 20, detection circuit 21, comparator 2
2, an amplifier circuit for amplifying the voltage value of the RF signal is configured by the switch 23 and the amplifier 24, and the RF signal is amplified by the amplifier circuit at a different amplification factor. More specifically, when the voltage of the RF signal is lower than the reference voltage, the signal is amplified by the amplifier 20 and the amplifier 24. On the other hand, when the voltage of the RF signal is higher than the reference voltage, the signal is amplified only by the amplifier 24. Will be.

The variable resistor 25 is constituted by a PIN diode or the like, and its attenuation rate changes according to the value of a control current (control current) input from the error amplifier 28 via the resistor 29. The signal input from the amplifier 24 is attenuated by an attenuation rate that changes according to the value of the control current input from the resistor 28 through the resistor 29, and the attenuated signal is output as an output signal from the ALC circuit 2. As described above, the attenuation factor of the variable resistor 25 using the PIN diode changes depending on the magnitude of the control current as shown in FIG. 4B. As the control current increases, the variable resistor 25 using the PIN diode increases. Becomes smaller.

The detection circuit 26 detects the output signal of the variable resistor 25, and the capacitor 27 holds the voltage of the output signal of the variable resistor 25 which is the detection result of the detection circuit 26.

The error amplifier 28 amplifies the difference between the comparison voltage and the voltage between the plates of the capacitor 27, and the amplified voltage is input to the variable resistor 25 as a control current via the resistor 29. When the value of the output signal of the variable resistor 25 is equal to or higher than a predetermined output voltage value (a target value of a voltage level obtained by level control), the value of the control current is reduced. The value of the comparison voltage is set such that the value of the control current increases when the value of the output signal is less than the predetermined output voltage value.

The circuit operation of the above-described ALC circuit 2 will be briefly described. The RF signal input to the ALC circuit 2 is amplified by the amplifier 24 when the voltage value of the RF signal is equal to or higher than a predetermined reference voltage value, further attenuated by the variable resistor 25, and As an output signal. On the other hand, when the voltage value of the RF signal is lower than the predetermined reference voltage value, the RF signal is
Is amplified by the amplifier 24, further attenuated by the variable resistor 25, and output as an output signal from the ALC circuit 2.

In order to keep the output value of the signal output from the ALC circuit 2 constant, the voltage value of the output signal output from the variable resistor 25 is adjusted to a predetermined output voltage value of the ALC circuit 2 (obtained by level control). If it is larger than the target value of the voltage level), the output voltage from the error amplifier 28 decreases, and the value of the control current input to the variable resistor 25 decreases. Then, the attenuation rate of the variable resistor 25 increases, and the output signal output from the variable resistor 25 decreases.

On the other hand, when the voltage value of the output signal output from the variable resistor 25 is smaller than the predetermined output voltage value of the ALC circuit 2, the output voltage from the error amplifier 28 increases and the variable resistor 25 , The current value of the control current input to is increased. Then, the attenuation rate of the variable resistor 25 decreases, and the output signal output from the variable resistor 25 increases.

By the above-described feedback action, the output voltage value from the ALC circuit 2 is controlled to a predetermined output voltage value of the ALC circuit 2, and a constant output voltage value is obtained from the ALC circuit 2. .

According to the ALC circuit 2 of the second embodiment, the RF signal can be amplified at different amplification factors by the amplifier circuit section including the amplifier 20 and the amplifier 24, and the attenuation of the variable resistor 25 is reduced. The level control can be performed even at the signal level of the RF signal, which cannot be controlled only by changing the rate.

That is, in the ALC circuit 2 of the second embodiment, the variation of the attenuation of the variable resistor 25 and the variation of the amplification of the amplifier circuit section including the amplifiers 20 and 24 are combined. Level control can be performed on the level of the RF signal in the range corresponding to the amount of change.

Further, as in the conventional ALC circuit 100, P
Even if the amount of change in the attenuation of the variable resistor 101 composed of an IN diode or the like becomes smaller as the frequency increases, the attenuation of the variable resistor 25 and the amplification circuit section including the amplifiers 20 and 24 described above. The level control is performed by changing and controlling the amplification factor of the RF signal, so that the level control can be performed for a wide range of RF signal levels.

[0056]

According to the first aspect of the present invention, the output voltage of the automatic level control circuit is not controlled only by changing the attenuation rate of the attenuation circuit. Since the output voltage of the automatic level control circuit is controlled by changing the rate, the level can be controlled for a wide range of signal levels even when the amount of change in the attenuation rate of the attenuation circuit is small, for example.

According to the second aspect of the present invention, since the attenuation circuit is configured by the PIN diode, it is possible to configure a circuit whose attenuation rate changes.

According to the third aspect of the present invention, the amplifier circuit amplifies the signal input from the attenuating circuit at a predetermined amplification factor, and detects the signal input from the attenuating circuit to detect the signal from the attenuating circuit. A comparison circuit for comparing a signal input from the attenuating circuit and a signal input from the first amplifier based on a comparison result by the comparison circuit; And a second amplifier that amplifies the obtained signal at a predetermined amplification factor, so that it is possible to configure an amplification circuit whose amplification factor changes.

According to the fourth aspect of the present invention, when the voltage of the signal input from the attenuation circuit is equal to or higher than the predetermined voltage value, the selection circuit selects the signal input from the attenuation circuit by the comparison circuit. And when the voltage of the signal input from the attenuating circuit is less than a predetermined voltage value by the comparison circuit, the signal input from the first amplifier is selected and output to the second amplifier. This makes it possible to control the amplification factor of the amplifier circuit according to the output signal from the attenuation circuit.

According to the fifth aspect of the present invention, the control signal generation circuit amplifies the difference between the voltage value as the detection result of the detection circuit and the predetermined comparison voltage value, and transmits the amplified voltage via the resistor. By outputting a control current corresponding to the amplified voltage to the attenuation circuit as a control signal, the attenuation rate of the attenuation circuit can be controlled according to the output signal of the amplification circuit.

According to the sixth aspect of the invention, the output voltage of the automatic level control circuit is not controlled only by changing the attenuation rate of the attenuation circuit, but the attenuation rate of the attenuation circuit and the amplification rate of the amplification circuit are adjusted. Since the output voltage of the automatic level control circuit is controlled by changing the level, the level control can be performed for a wide range of signal levels even when the amount of change in the attenuation rate of the attenuation circuit is small, for example.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram illustrating an ALC circuit according to a first embodiment.

FIG. 2 is a circuit configuration diagram illustrating an ALC circuit according to a second embodiment.

FIG. 3 is a circuit configuration diagram showing an example of a conventional ALC circuit.

FIG. 4 is a diagram for explaining an amplification factor of an amplifier and an attenuation factor of a PIN diode;

[Explanation of symbols]

 Reference Signs List 1 ALC circuit 10 Variable resistor 11 Amplifier 12 Detection circuit 13 Comparator 14 Switch 15 Amplifier 16 Detection circuit 17 Capacitor 18 Error amplifier 19 Resistance 2 ALC circuit 20 Amplifier 21 Detection circuit 22 Comparator 23 Switch 24 Amplifier 25 Variable resistor 26 Detection Circuit 27 Capacitor 28 Error amplifier 29 Resistance 100 ALC circuit 101 Variable resistor 102 Amplifier 103 Detection circuit 104 Capacitor 105 Error amplifier 106 Resistance

Claims (6)

[Claims] An attenuation circuit for attenuating an input signal at an attenuation rate according to a magnitude of a control signal, a signal inputted from the attenuation circuit being detected, and a signal inputted from the attenuation circuit according to a result of the detection. An automatic level control circuit comprising: an amplification circuit that amplifies the output signal at a different amplification factor; and a control signal generation circuit that detects an output of the amplification circuit and outputs a control signal corresponding to the detection result to the attenuation circuit. An automatic level control circuit, wherein an output voltage from the automatic level control circuit is made constant by changing and controlling an attenuation rate of the attenuation circuit and an amplification rate of the amplification circuit. 2. The automatic level control circuit according to claim 1, wherein said attenuation circuit comprises a PIN diode. 3. The amplifying circuit comprises: a first amplifier for amplifying a signal input from the attenuating circuit at a predetermined amplification factor; and detecting a signal input from the attenuating circuit and comparing the detected signal with a predetermined reference voltage. A comparison circuit; a selection circuit that selects a signal input from the attenuation circuit and a signal input from the first amplifier based on a comparison result by the comparison circuit; 3. The automatic level control circuit according to claim 1, further comprising: a second amplifier that amplifies at an amplification factor. 4. The second amplifier selects a signal input from the attenuation circuit when a voltage of a signal input from the attenuation circuit by the comparison circuit is equal to or higher than a predetermined reference voltage value. Selecting a signal input from the first amplifier and outputting the signal to the second amplifier when a voltage of a signal input from the attenuating circuit is lower than a predetermined reference voltage value by the comparison circuit. 4. The automatic level control circuit according to claim 3, wherein: 5. The control signal generation circuit amplifies a difference between a voltage value as a detection result of the detection circuit and a predetermined comparison voltage value, and amplifies the amplified voltage via a resistor to correspond to the amplified voltage. 5. The automatic level control circuit according to claim 1, wherein a current is output to the attenuation circuit as a control signal. 6. An amplification circuit for detecting an input signal and amplifying the input signal at a different amplification rate according to the detection result, and a signal input from the amplification circuit at an attenuation rate according to a magnitude of a control signal. An automatic level control circuit comprising: an attenuating circuit for attenuating; and a control signal generating circuit for detecting an output of the attenuating circuit and outputting a control signal corresponding to the detection result to the attenuating circuit. An automatic level control circuit, wherein an output voltage from the automatic level control circuit is made constant by changing and controlling an attenuation factor and an amplification factor of the attenuation circuit.