JPH0878983A - Alc circuit - Google Patents

Abstract

PURPOSE: To obtain an ALC circuit allowing a large signal to be inputted and having few noise generation by reducing the current flowing in first and second transistors when no signal exists and a small signal is inputted and increasing current when a signal exists. CONSTITUTION: A level detection circuit 32 performs a level detection of the output signal of an output circuit 28 and generates the first and second control signals of a linear output. This second control signal increases the current values flowing in first and second variable current sources 23 and 25, the input dynamic range for a differential amplifier 34 is increased, a gain lowering is sufficiently performed for also a large input signal and an automatic gain control ALC can be performed for the signal. At this time, the absolute amount of the noise generated in a differential amplifier 34 is simultaneously increased, but a larger input signal exists as compared with when no signal exists and the level of the input signal is small. Therefore, the increase is not substantially affected. Therefore, a low noise amplification can be performed when no signal exists and in the low level state of the input signal. And when the large signal is inputted, the input dynamic range is increased, the gain lowering is sufficiently performed for also the large input signal and the AGC is performed for the signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ALC (automatic gain control) circuit for keeping the level of an audio signal constant,
In particular, the present invention relates to an ALC circuit capable of inputting a large signal and generating less noise.

[0002]

2. Description of the Related Art In ALC for converting input signals of various levels into signals of a constant level, the level of an output signal is detected and the gain of an amplifier is changed so that the level becomes constant. FIG. 2 shows such an ALC circuit, in which the signal from the input terminal (1) is amplified by a differential amplifier (4) consisting of first and second transistors (2) and (3). First and second transistors (2) and (3)
Has a first differential amplifier (5) whose operating current source is the first transistor (2) and a second differential amplifier (6) whose operating current source is the second transistor (3). It is connected. The first differential amplifier (5) and the second differential amplifier (6) are connected in a double differential amplifier form, and the collector current of the first and second transistors (2) and (3) The level of the signal from the input terminal (1) is controlled by changing the diversion ratio.

The outputs of the first differential amplifier (5) and the second differential amplifier (6) are output in a double-ended type in a current form by the first to third current mirror circuits (7) to (9). . That is, when the collector current of the transistor (21) forming the second current mirror circuit (8) increases, the collector current of the transistor (10) forming the third current mirror circuit (9) decreases, and the output terminal (11) , An output voltage higher than the bias voltage VB of the bias power supply (12) can be obtained.

On the other hand, the level detection circuit (13) detects the voltage level of the output terminal (11) and generates an "H" level output when the voltage level of the output terminal (11) is low. In this case, the signal from the input terminal (1) is being amplified with full gain. Among the transistors (14) to (17) forming the first differential amplifier (5) and the second differential amplifier (6), the transistors (15) and (16) have a signal of "H" level when there is no signal. Is applied,
"L" when there is no signal in the transistors (14) and (17)
A level signal is applied. Then, the transistor (1
A large current flows through 5) and (16), and the signal from the input terminal (1) is amplified with full gain.

When the voltage level of the output terminal (11) is high, the level detection circuit (13) generates an "L" level output. There is a linear change between "H" and "L". In this case, the signal from the input terminal (1) is in a state where the gain is reduced by the first differential amplifier (5) and the second differential amplifier (6), and the current flowing through the transistors (15) and (16) is Control is performed so that the signal that decreases and is generated at the output terminal (11) becomes a constant level.

Therefore, according to the circuit of FIG. 2, ALC can be applied to a voice signal or the like.

[0007]

By the way, the input dynamic range at the input terminal (1) of FIG. 2 is determined by the emitter internal resistance re of the first and second transistors (2) and (3) and the resistance of the resistor (18). It depends on the product of the value R and the current value I of the first and second current sources (19) and (20).
In the ALC circuit, it is desirable to be able to handle even the highest possible level of input signal. That is, a wide input dynamic range is required at the input terminal (1). In order to widen the input dynamic range of the input terminal (1), the resistance value R of the resistor (18) and the current value I of the first and second current sources (19) and (20) may be increased. If it is set to a large value, the amount of noise generated increases, which is a problem.

Therefore, an ALC circuit having a wide input dynamic range at the input terminal (1) and low noise has been desired.

[0009]

The present invention has been made in view of the above points, and includes a first transistor in which an input signal is applied to the base and a first variable current source is connected to the emitter,
A second transistor having an emitter connected to the emitter of the first transistor and a second variable current source, a first differential amplifier having the first transistor as an operating current source, and the second transistor as an operating current source. A second differential amplifier, an output circuit for extracting the output signals of the first and second differential amplifiers by double-ended output, and level detection of the output signal of the output circuit to generate first and second control signals. A level detection circuit for controlling the base voltages of the transistors forming the first and second differential amplifiers according to the first control signal, and the first and second control signals according to the second control signal. The current flowing through the transistor is controlled so that the current flowing through the first and second transistors is reduced when there is no signal, and the current flowing through the first and second transistors is increased when there is a signal. And wherein the door.

[0010]

According to the present invention, the current flowing through the first and second transistors is controlled according to the second control signal, and the current flowing through the first and second transistors is reduced when there is no signal or a small signal is input. However, since the current flowing through the first and second transistors is increased when there is a signal, the amount of noise generated is reduced when there is no signal, and the input dynamic range at the input terminal can be widened when there is a signal. . When a signal is present, the amount of noise generated increases with an increase in current, but if a signal exists, even if some noise is generated, it becomes relatively inconspicuous and does not pose a problem.

[0011]

FIG. 1 shows an ALC circuit of the present invention.
In (22), the input signal is applied to the base and the first signal is applied to the emitter.
A first transistor to which a variable current source (23) is connected,
(24) has a second emitter whose emitter is connected to the emitter of the first transistor (22) and the second variable current source (25).
A transistor, (26) is the first transistor (2
2) a first differential amplifier using an operating current source, (27) a second differential amplifier using the second transistor (24) as an operating current source, and (28) first to third current mirror circuits ( Two
9) to (31), which is an output circuit for extracting the output signals of the first and second differential amplifiers (26) and (27) by a double end output, and (32) is an output signal of the output circuit (28). It is a level detection circuit that performs level detection and generates first and second control signals that are linear outputs (gradually changing from “H” level to “L” level).

The signal from the input terminal (33) is amplified by the differential amplifier (34) consisting of the first and second transistors (22) and (24). The collectors of the first and second transistors (22) and (24) are connected to a first differential amplifier (2) using the first transistor (22) as an operating current source.
6) and a second differential amplifier (27) using the second transistor (23) as an operating current source. First
The differential amplifier (26) and the second differential amplifier (27) are connected in a double differential amplifier form, and the current shunt ratio of the collector currents of the first and second transistors (22) and (24). The signal level from the input terminal (33) is controlled by changing the.

The outputs of the first differential amplifier (26) and the second differential amplifier (27) are the first to third current mirror circuits (2).
9) to (31), the current type is output as a double end type. That is, when the collector current of the transistor (35) forming the second current mirror circuit (30) increases, the transistor (3) forming the third current mirror circuit (31) is increased.
The collector current of 6) decreases, and an output voltage higher than the bias voltage VB of the bias power supply (38) is obtained at the output terminal (37).

On the other hand, the level detection circuit (32) detects the voltage level of the output terminal (37), and outputs "H" level (first control signal) when the voltage level of the output terminal (37) is low. To occur. In this case, input terminal (3
In this state, the signal from 3) is amplified with full gain. First differential amplifier (26) and second differential amplifier (27)
Of the transistors (39) to (42) constituting the transistor (40) and (41), an “H” level signal is applied to the transistors (40) and (41) when there is no signal (including the case where the input signal level is low), ) And (42), an "L" level signal is applied. Then, the transistor (4
A large current flows through 0) and (41), and the signal from the input terminal (33) is amplified with full gain.

On the other hand, at this time, a second control signal is generated from the level detection circuit (32) to reduce the current value flowing through the first variable current source (23) and the second variable current source (25).
The second control signal can also be used as the first control signal. Then, the absolute amount of noise generated in the differential amplifier (34) becomes small, and there is no problem even when there is no signal or when the input signal level is low. In this state, since the input signal level is originally low, the first variable current source (23) and the second variable current source (2
There is no problem even if the current value flowing in 5) is small and the dynamic range is narrow.

As a result, the circuit of FIG. 1 can amplify the input signal with low noise when there is no signal or when the input signal level is low. Next, when the signal level from the input terminal (33) increases and the voltage level of the output terminal (37) is high, the level detection circuit (32) generates the "L" level first control signal. There is a linear change between "H" and "L". In this case, the signal from the input terminal (33) is in a state where the gain is reduced by the first differential amplifier (26) and the second differential amplifier (27), and the current flowing through the transistors (40) and (41) is The control is performed so that the signal that decreases and is generated at the output terminal (37) has a constant level.

On the other hand, at this time, a second control signal is generated from the level detection circuit (32) to increase the current value flowing through the first variable current source (23) and the second variable current source (25).
Then, the input dynamic range to the differential amplifier (34) is increased, and even a large input signal can be sufficiently lowered in gain to perform ALC. At this time, the absolute amount of noise generated in the differential amplifier (34) increases at the same time, but since there is a larger input signal than when there is no signal or when the input signal level is low, the increase is substantial. Not affected by.

Therefore, according to the circuit of FIG. 1, the input signal can be amplified with low noise when there is no signal or when the input signal level is low, and the input dynamic range increases when a large signal is input, so that a large input signal can be obtained. The gain has been lowered enough
LC can be applied. FIG. 3 shows a specific circuit example of the first variable current source (23) (the same applies to the second variable current source (25)) of FIG. 1. By adjusting the value of the variable resistor (50), The current flowing through the transistors (51) (52) (53) changes, the current flowing through the transistors (54) (55) also changes, and the current flowing through the first transistor (22) changes. The transistors (56) (57) carry a constant current.

By the way, when the current of the transistor (55) changes, the operating current of the first differential amplifier (26) also changes, so that the gain thereof changes. Then it ’s correct,
ALC cannot be applied. Therefore, in the present invention, compensation (supply) of the current change of the transistor (55) is performed by the transistor (53). With this configuration, the current flowing through the first transistor (22) does not change and desired ALC can be performed.

[0020]

As described above, according to the present invention, the current flowing through the first and second transistors is reduced when there is no signal or when a small signal is input, and when there is a signal, the first and second transistors are provided. Since the current flowing through the input terminal is increased, the amount of noise generated is reduced when there is no signal, and the input dynamic range at the input terminal can be increased when there is a signal. When a signal is present, the amount of noise generated increases with an increase in current, but if a signal exists, even if some noise is generated, it becomes relatively inconspicuous and does not pose a problem.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an ALC circuit of the present invention.

FIG. 2 is a circuit diagram showing a conventional ALC circuit.

FIG. 3 is a first variable current source (23) of the ALC circuit of the present invention.
It is a circuit diagram showing.

[Explanation of symbols]

 (22) First transistor (23) First variable current source (24) Second transistor (26) First differential amplifier (27) Second differential amplifier (28) Output circuit (32) Level detection circuit

Claims (2)

[Claims] 1. A first transistor having an input signal applied to a base and a first variable current source connected to an emitter, and a second transistor having an emitter connected to an emitter of the first transistor and a second variable current source. A first differential amplifier using the first transistor as an operating current source, a second differential amplifier using the second transistor as an operating current source, and a double output signal of the first and second differential amplifiers. An output circuit for extracting the end output; and a level detection circuit for level-detecting an output signal of the output circuit to generate first and second control signals.
The base voltage of the transistors forming the first and second differential amplifiers is controlled according to the control signal, and the currents flowing through the first and second transistors are controlled according to the second control signal. An ALC circuit, characterized in that a current flowing through the first and second transistors is reduced and a current flowing through the first and second transistors is increased when a signal is present. 2. A current for canceling out a current that has changed in the first and second transistors in response to the second control signal is supplied to the collectors of the first and second transistors. The ALC circuit according to claim 1, wherein: