JPH0261805B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an amplification device advantageously used for amplifying audio frequency signals such as audio signals.
More specifically, the present invention relates to an amplification device that amplifies an input signal without causing distortion even when the amplitude of the input signal becomes abnormally large.

Such amplification devices have traditionally been called
It is known as having an ALC (automatic level control) function. Amplifiers with ALC function are
Normally, when an input signal with excessive amplitude is applied, the gain is rapidly reduced, and then when the excessive input signal disappears, the gain is slowly restored.
Return to the original high gain amplification state. In this way, input signals with excessive amplitudes are amplified without waveform distortion, and small input signals are amplified with a large gain. The reason why the time required for gain recovery, that is, the recovery time, is made larger than the time required for gain reduction, that is, the attack time, is because when an audio signal with excessive amplitude is input, auditory problems such as the so-called breathing phenomenon occur. This is to eliminate unnaturalness. According to such prior art, when an interfering signal such as a click that is inputted instantaneously and is much larger than the audio signal is given, the ALC responds to the interfering signal and immediately returns to normal amplitude. There is a problem in that an input signal having a signal having an input signal is dropped due to excessive level reduction and becomes inaudible for a certain period of time related to the recovery time.

The interference signal may occur, for example, when lightning occurs,
This occurs when an object collides with the microphone or when operating the press talk switch.

An object of the present invention is to make it possible to smoothly amplify a signal with a normal amplitude immediately after an interference signal such as a click that has an amplitude larger than the excessive amplitude that can be expected in an audio signal. An object of the present invention is to provide an improved amplification device.

The present invention includes an attenuation circuit 3 that attenuates the level of an input signal and whose attenuation amount is variable; and an amplification circuit 4 that amplifies the output of the attenuation circuit 3.
and a level that responds to the output signal from the amplifier circuit 4 and rapidly increases the amount of attenuation of the input signal by the attenuation circuit 3 when the output signal increases, and then gently decreases the amount of attenuation when the output signal decreases. a control circuit 6; a level discrimination circuit 33 that discriminates the level of the input signal at a predetermined discrimination level larger than a possible excessive amplitude of the audio signal;
a detecting means 57 for detecting that the input signal once exceeds the discrimination level and then returns to below the discrimination level; This amplifying device is characterized in that it includes a control circuit 58 that rapidly reduces the amount of attenuation of the attenuation circuit 3 to a minimum value when the amount of attenuation of the attenuation circuit 3 is restored to below the discrimination level after exceeding the above-mentioned discrimination level.

FIG. 1 is an electrical circuit diagram of one embodiment of the present invention. For example, an audio frequency signal from the input terminal 1 is applied to a high gain amplifier circuit 4 via a coupling capacitor 2 and a resistor 57, and an attenuation circuit 3. The signal amplified by this amplifier circuit 4 is derived from an output terminal 5. In connection with the attenuation circuit 3, a level control circuit 6 is provided. The output signal from the amplifier circuit 4 is applied from a coupling capacitor 7 in a level control circuit 6 to resistors 8 to 10 connected in series. A diode 12 is connected between a connection point 11 between the resistors 8 and 9 and the ground. Resistance 9, 10
The signal from the connection point 13 is given to the differential amplifier circuit 14. Differential amplifier circuit 14 includes a pair of transistors 15 and 16 and resistors 17-20. The output from the differential amplifier circuit 14 passes through the diode 21 and quickly charges the capacitor 22. The capacitor 22 is connected to the resistor 2 when the voltage at the collector of the transistor 16, that is, the connection point 30 decreases.
3, 24, 25, etc. are discharged slowly.

The resistors 23 to 26 connected to the cathode side of the diode 21 control the transistor 27 so as to bring the transistor 27 closer to the active state when the collector voltage of the transistor 16 is at its lowest level, that is, when there is no signal. Apply bias voltage to the base.

The output from the capacitor 22 via the resistor 23 is
It is applied to the transistor 27 of the attenuation circuit 3. The output from transistor 27 is provided to transistor 28. In this attenuation circuit 3, the input signal via the coupling capacitor 2 is voltage-divided by the resistor 29 and the transistor 28, and the voltage-divided and attenuated signal is provided to the amplifier circuit 4.

When the amplitude of the input signal input from input terminal 1 is not excessively large, that is, it is normal,
The potential at the connection point 13 is at a low level to cut off the transistor 15, and therefore the transistor 16 included in the differential amplifier circuit is conductive.
Therefore, the connection point 30 between the resistor 18 and the transistor 16 is at a low level, and the capacitor 2
The terminal voltage of 2 is low and the transistor 27 in the attenuation circuit 3 is cut off. Therefore, the impedance between the collector and emitter of transistor 28 is extremely large. Therefore, the input signal is not attenuated by the attenuation circuit 3, but is amplified by the amplification circuit 4.

Next, assume that the level of the input signal applied to input terminal 1 increases. The output signal from the amplifier circuit 4 is connected to a coupling capacitor 7 and a resistor 8.
The signal is then rectified by the diode 12. At this connection point 11, the AC signal applied to the diode 12 and the DC component of the rectified output are superimposed, and this superimposed voltage is divided by a voltage dividing circuit consisting of resistors 9 and 10. from transistor 1
Given on the basis of 5. Therefore, connection point 13
When the voltage at is greater than or equal to a value sufficient to cause transistor 15 to conduct, transistor 15 becomes conductive, thereby causing transistor 16 to turn off.
Therefore, the connection point 30 becomes a high potential, and the capacitor 22 is rapidly charged via the diode 21.
The terminal voltage of capacitor 22 rises rapidly. Therefore, the transistor 27 becomes conductive, and accordingly the transistor 28 becomes conductive, and the impedance between its collector and emitter becomes a value determined by the terminal voltage of the capacitor 22. Therefore, the input signal from the coupling capacitor 2 is connected to the resistor 29 and the transistor 28.
The voltage is divided and attenuated by and input to the amplifier circuit 4. In this way, the attenuation circuit 3 from the input terminal 1
The signal attenuated by
will be derived.

When the amplitude of the input signal from input terminal 1 returns from an excessive state to a normal value, the potential at connection point 13 decreases, transistor 15 is cut off, and transistor 1
6 is conductive. Therefore, the diode 21 is cut off, the charge in the capacitor 22 is discharged by the resistors 23, 24, etc., and the output voltage of the capacitor 22 gradually decreases. This capacitor 2
When the voltage across transistor 2 slowly decreases and turns off transistor 27, transistor 28 turns off. As a result, the input signal via the coupling capacitor 2 is applied to the amplifier circuit 4 without being attenuated by the attenuation circuit 3.

In summary, when an input signal with excessive amplitude is applied, the voltage at the terminals of capacitor 22 rises rapidly, transistors 27, 28 are activated, and the input signal from input terminal 1 is therefore attenuated. . When the input signal with the excessive amplitude returns to the normal amplitude value, the charge in the capacitor 22 is gradually discharged, and therefore, the attenuation state by the input signal attenuation circuit 3 is such that the amplitude of the input signal has returned to normal. It will continue for a while after that.

The time during which the transistor 28 is kept in a conductive state due to the slow discharge of the capacitor 22 is the so-called recovery time from when a large amplitude signal is no longer input until it returns to the high gain state, and this recovery time is , is selected so that it is sufficiently larger than the so-called attack time from when a signal with a large amplitude is input until the gain is reduced, so that the so-called breathing phenomenon of the audio signal is sufficiently reduced. Therefore, if a disturbance input such as a click with an amplitude even larger than the excessive amplitude considered in the audio signal is input, the audio signal will be excessively attenuated for a certain period related to the recovery time immediately after the interference input disappears, and the sound at that time will be If the input signal level is low, the audio signal will be audibly missing.

The input signal from input terminal 1 is also input to level discrimination circuit 33 via coupling capacitor 32 . The output from the coupling capacitor 32 is applied to a non-inverting input terminal 34a of a differential amplifier 34 in a level discrimination circuit 33. A resistor 35 is interposed between the output terminal 34c and the inverting input terminal 34b of the differential amplifier 34. This input terminal 34b
is grounded via a resistor 36 and a capacitor 37. Therefore, the connection point 38 at the differential amplifier 34c and the connection point 3 at the inverting input terminal 34b
The DC voltages at node 9 are equal, but the amplitude of the AC component is smaller at connection point 39 than at connection point 38 due to the action of capacitor 37. The time constant of the resistor 36 and capacitor 37 is sufficiently larger than the period of the frequency component of the input signal, and therefore the terminal voltage of the capacitor 37 is selected to a value that does not allow it to follow the frequency of the input signal. In this way, the resistor 36 and the resistor 35 determine the gain between the input terminals 34a and 34c.

The connection point 38 is connected to the differential amplifier 4 via a resistor 42.
It is connected to the inverting input terminal 41b of No. 1, and
It is connected to the output terminal 41c via a resistor 43.
The resistance values of the resistors 42 and 43 are chosen to be equal, so the gain between the connection point 38 and the output point 41c is -
It is 1. Non-inverting input terminal 41 of differential amplifier 41
a is grounded by a capacitor 44 and a resistor 45. This capacitor 44 has a resistor 40
Bias the signal applied through the Therefore, the connection point 46 of the non-inverting input terminal 41a has only a DC component, and has a potential determined by the resistor 45 and the resistor 47, which is lower than the potential of the non-inverting input terminal 50a of the differential amplifier 50.

The output signals from the differential amplifiers 34 and 41 are applied to the inverting input terminal 50b of the differential amplifier 50 via diodes 48 and 49. Non-inverting input terminal 50
A predetermined reference voltage Vcc/2 is applied to a. The DC bias voltage of the non-inverting input terminal 34a of the differential amplifier 34 is connected to the connection point 46 via the resistor 40.
Since the potential of the non-inverting input terminal 41a of the differential amplifier 41 is the same as that of the non-inverting input terminal 41a of the differential amplifier 41, the potential of the output terminals 34c and 41c when there is no signal is also the potential of the connection point 46.
The potential is almost the same as that of . Further, the potential of the inverting input terminal 50b of the differential amplifier 50 when there is no signal is higher than the potential of the output terminals 34c and 41c by the diode 4.
The potential is lower by the forward voltage drop of 8 and 49, and this potential is the potential of the non-inverting input terminal 50a.
Since it is lower than Vcc/2, the potential of the output terminal 50c becomes high level. The input applied to the non-inverting input terminal 34a via the capacitor 32 causes the output terminal 3 to
The potential of 4c fluctuates and Vcc/2 and diode 4
When the potential of the output terminal 50c of the differential amplifier 50 becomes higher than the sum of forward voltage drops of 8, the logic of the potential of the output terminal 50c of the differential amplifier 50 is inverted and becomes a low level. When the potential of the output terminal 34c decreases, the potential of the output terminal 41c increases and becomes Vcc/2.
The logic at the output terminal 50c is also inverted when the voltage becomes higher than the sum of the forward direction voltage drop of the diode 49. That is, when the potential of the non-inverting input terminal 34a increases due to the input applied via the capacitor 32, and exceeds the predetermined upper threshold value l1, and becomes below the lower threshold value l2, the output terminal 50
c, that is, the output of the level discrimination circuit 33 is inverted in logic and becomes a low level.

The output from the level discrimination circuit 33 is on line 51
Only a positive pulse is applied from the differential capacitor 52 to the diode 53 to the base of the transistor 54 via the resistor 55. The differential capacitor 52 and the diode 53 respond to the output from the level discrimination circuit 33 to determine whether the input signal from the input terminal 1 is above the discrimination level threshold l1 and below the threshold l2. A detecting means 57 is configured to detect that after reaching the discrimination level l1, l2, that is, within the range of thresholds l1 to l2.

The output of this detection means 57 is applied to the base of the transistor 54 via the resistor 55 as described above, and the resistor 56 is connected to the base of the transistor 54.
is connected. The collector and emitter of the transistor 54 are connected in parallel to the capacitor 22. The resistors 55 and 56 and the transistor 54 are
A control circuit 58 is configured to respond to the output of the detection means 57 and rapidly reduce the amount of attenuation of the attenuation circuit 3.

Referring to FIG. 2, assume that an interference signal having an excessive amplitude, such as a critter, is input from the input terminal 1. The output waveform from coupling capacitor 32 is shown in FIG. The output from the coupling capacitor 32 is given to the level discrimination circuit 33, and the level discrimination circuit 33 operates as described above, so that the waveform shown in FIG. 2 is derived from the output terminal 50c of the comparison circuit 50. be done. The output waveform from differential capacitor 52 is shown in FIG. The diode 53 supplies only the positive pulse of the output from the differential capacitor 52 to the base of the transistor 54, as shown in FIG. Transistor 54 is momentarily conductive only while the output from differential capacitor 52 exceeds threshold value l3. The switching state of transistor 54 is as shown in FIG. In this way, when a disturbance signal such as a click having an excessive amplitude is input from the input terminal 1, the amplitude of the disturbance signal is at the discrimination level l1, l.
Immediately after deviating from the range of discrimination levels l1 and l2 and returning to the range of discrimination levels l1 and l2, the transistor 5
4 becomes conductive instantaneously. This discharges and reduces the charge on capacitor 22 to the extent that transistors 27 and 28 are inactive. Therefore, immediately after the extinction of the interference signal with excessive amplitude (the peak-to-peak level decreases to l1-l2) and after the transistor 54 has become conductive, the transistors 27, 28 are cut off, thereby causing The input signal from the input terminal 1 is not attenuated by the attenuation circuit 3, but is applied to the amplifier circuit 4 and amplified. This prevents the input signal immediately after interference signals such as clicks from being lost without being amplified.

As described above, according to the present invention, when an input signal having an excessive amplitude that is considered to be an audio signal and has an amplitude lower than a predetermined discrimination level is input to the level discrimination circuit, the level control circuit Automatic level control is performed in a conventional manner. In addition, when an input signal having an amplitude larger than the excessive amplitude that is considered to be an audio signal is given and exceeds the discrimination level determined by the level discrimination circuit, the attenuation circuit is activated immediately after falling below the discrimination level. Since the amount of attenuation is rapidly reduced, it is possible to prevent the audio signal from being lost immediately after an interfering signal such as a click is input and exceeds the discrimination level, and the normal audio signal remains amplified at a high gain. This can be obtained by using an amplifier circuit.

[Brief explanation of the drawing]

Fig. 1 is an electrical circuit diagram of an embodiment of the present invention;
The figure is a waveform diagram for explaining the operation. DESCRIPTION OF SYMBOLS 1...Input terminal, 2...Coupling capacitor, 3...Attenuation circuit, 4...Amplifying circuit, 5...Output terminal, 6...Level control circuit, 31...Differentiating circuit, 33...Level discrimination circuit, 54...Transistor.

Claims (1)

[Claims] 1. An attenuation circuit 3 that attenuates the level of an input signal and whose attenuation amount is variable; and an amplifier circuit 4 that amplifies the output of the attenuation circuit 3.
and, in response to the output signal from the amplifier circuit 4, when the output signal increases, the amount of attenuation of the input signal by the attenuation circuit 3 is rapidly increased, and when the output signal thereafter decreases, the attenuation amount is gently decreased. a level control circuit 6; a level discrimination circuit 33 that discriminates the level of the input signal at a predetermined discrimination level that is larger than a possible excessive amplitude of the audio signal; and responsive to an output from the level discrimination circuit 33;
a detecting means 57 for detecting that the input signal once exceeds the discrimination level and then returns to below the discrimination level; and a control circuit 58 that rapidly reduces the attenuation amount of the attenuation circuit 3 to a minimum value when the level returns to below the discrimination level after exceeding the discrimination level.