JPS644687B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention provides an AGC for receiving amplification connected to a telephone line.
It is related to circuits.

It is said that the difference in reception input level of telephone lines is about 30 to 40 dB depending on the line length. Conventionally, as a means for correcting this level difference, an AGC circuit has been used in the receiving amplification section.

A desired characteristic of this AGC circuit is a long response time (recovery time).

In other words, when a strong reception level signal is input, it is quickly detected, moves to lower the reception level, and maintains the AGC level even if reception input is lost for a short time.

FIG. 1 shows the structure of a conventional AGC circuit.
AMP1 and AMP2 are amplifiers, SP is speaker, Q
1, Q2 are transistors that serve as the detection part and control part of AGC, R1 to R5 are resistors, D1, D2
is a diode, and C1 and C2 are capacitors.
T1 is a terminal to which reception input is applied.

Figure 2 shows the signal waveform of the main part, A is the signal applied to the input terminal T1, B is the capacitor C
The voltage between the terminals of 2 and C indicates the voltage applied to the gate of the transistor Q1. Also, a is a line noise signal, b is a low level audio signal, and c is a high level audio signal.

In the circuit shown in Fig. 1, when a large reception input as shown in Fig. 2 c is applied, it is applied to the amplifier AMP1 through the resistor R1, and the audio signal amplified here is rectified by the diode D2. The capacitor C2 is charged as shown in FIG. Therefore, the rectified output according to the audio input is the transistor Q.
The output of Q2 turns on the transistor Q1. When the transistor Q1 is turned on, the resistor R3 is short-circuited, and the reception input is reduced by the attenuation section of the resistors R1 and R2.

Further, when a line noise signal as shown in FIG. 2B is input from the reception input terminal T1, it is amplified by the amplifier AMP1, rapidly charged to the capacitor C2, and the voltage accumulated in the capacitor C2 passes through the transistor Q2 and turns on the transistor Q1. Therefore, AGC is applied by the attenuation ratio of resistors R1 and R2, and its output is heard from speaker SP through amplifier AMP2. The charge stored in the capacitor C2 is discharged between the gate and source of the transistor Q2, but it takes several tens of seconds. At this time, if you make a call to a person who is far away from the line, the AGC is on, so the sound of the other party will be too low and you will not be able to hear it, which will interfere with the call.

The present invention aims to solve the above-mentioned problems and to provide a more natural listening level.

The present invention will be described below along with one embodiment thereof.

FIG. 3 shows an embodiment thereof, and the same parts as those shown in FIG. 1 are given the same reference numerals.
The difference is that the output of amplifier AMP1 is connected to capacitor C.
1, the capacitor C3 is charged via the diode D3 and the resistor R7, and a circuit consisting of the resistors R8 and R9 and the transistor Q4 is provided to discharge the charging voltage, and the charging The point is that a transistor Q3 is provided to perform AGC control using voltage.

T2 is a terminal to which a signal for discharging the charge in the capacitor C3 is applied during standby.

In this circuit, the AGC circuit with transistor Q2 as the main element is set to have a fast response time and quick recovery time, and
The AGC circuit with 3 as the main element has a slow response time,
The recovery time is set to be slow.

FIG. 4 shows the signal waveforms of the main parts of this embodiment. A is the signal applied to the input terminal T1, and in the figure,
a, b, and c indicate line noise and low-level audio signals, respectively. In the figure, B is the voltage between the terminals of capacitor C2, C is the voltage between the terminals of capacitor C3,
D indicates the gate voltage of the transistor Q1.

Next, the operation of this embodiment will be explained. When line noise enters from the receiver input terminal T1, this noise is amplified by the amplifier AMP1, and is added to the capacitor C2 for rapid charging. However, the charge accumulated in capacitor C2 is rapidly discharged by resistor R6. Also, since the charging time constant is lengthened by resistor R7 and capacitor C3, this capacitor C3
is hardly charged. Therefore, after line noise occurs, AGC will not be applied even if you talk to a person with low reception input, and the phenomenon of inaudibility that occurs in the past will not occur.

Also, when a large reception input is input continuously, capacitor C2 is charged and capacitor C3 is charged.
is also charged, turns on transistor Q1 through transistors Q2 and Q3, and connects resistor R1 and resistor R.
AGC is applied with a damping ratio of 2. Also, during standby, voltage is applied via terminal T2, and transistor Q4
is turned on to discharge the charge in the capacitor C3.

As is clear from the above embodiments, according to the present invention, line noise does not cause AGC, but large reception input causes AGC to be applied, and AGC characteristics suitable for a reception amplification unit connected to a telephone line can be obtained.

[Brief explanation of the drawing]

Figure 1 is a wiring diagram of a conventional AGC circuit for receiving amplification.
Fig. 2 is a signal waveform diagram of the same main part, Fig. 3 is a wiring diagram of the AGC circuit for receiving amplification in one embodiment of the present invention,
FIG. 4 is a signal waveform diagram of the same main part. AMP1...Amplifier, C2, C3...Capacitor, R6, R7...Resistor, Q1-Q4...Transistor.

Claims (1)

[Claims] 1. A receiver amplifier that amplifies the receiver signal, two time constant circuits with different time constants to which the receiver signal is input, a loop that applies AGC to the receiver amplifier using the outputs of the two time constant circuits, and the two time constants. This AGC circuit for receiving amplification is patented as having a rapid discharge loop connected to a time constant circuit with a large time constant.