JPH07202605A - Compressor circuit for compounder - Google Patents

Abstract

PURPOSE:To provide the compressor circuit for compounder which improves the reproducibility of a weak input signal at a high frequency at the output signal of compounder by compressing such an input signal at the same compressibility of a low frequency. CONSTITUTION:When the input signal gets weak and lowered rather than the set value of its voltage VIN, a current is supplied from a comparator circuit 5 to an operational amplifier circuit 4, and the bias current of the operational amplifier circuit 4 is increased. Thus, the opened loop gain of the operational amplifier circuit 4 is increased, the compressibility is not changed by the frequency even in the case of the weak input signal, and the reproducibility of this input signal at the output signal of the compounder is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compressor circuit for a compander used to suppress transmission noise on a communication path.

[0002]

2. Description of the Related Art A compander is a circuit composed of a compressor and an expander connected in series, and is used to suppress transmission noise superimposed on a signal on a communication path.
Recently, it is often used in the circuits of car phones and cordless phones.
FIG. 6 is an explanatory diagram of the compander, in which the dynamic range of the input signal is compressed by the compressor and conversely the dynamic range is expanded by the expander. Usually, the compression ratio is logarithm 1/2 and the expansion ratio is doubled, and the input signal of the compressor is reproduced as the output signal of the expander. For example, if there is -30 dB level transmission noise on the communication path, and if the input signal is directly applied to the communication path, an input signal below that level cannot be distinguished from noise. However, if the input signal exceeds -60 dB due to the compressor, it becomes a signal exceeding -30 dB on the communication path and can be distinguished from noise.

Further, the expander expands the dynamic range of the communication path to widen the level difference between noise and signal, and as a result, transmission noise is suppressed. For example, an input signal of -40 dB is -20 dB on the communication path, which is 10 dB above the noise level of -30 dB, and an output signal of the expander has a transmission noise level of -60 dB.
And a level difference of 20 dB from the output signal of.

FIG. 3 is a block diagram of a conventional compressor circuit for a compander, which is mainly composed of an operational amplifier circuit 4, a variable gain circuit 6, and a rectifying / smoothing circuit 7. The voltage V _IN of the input signal of the input terminal 1 is applied to the inverting input terminal of the operational amplifier circuit 4 via the resistor R3, and the voltage source E1 is connected to the non-inverting input terminal. Then, the operational amplifier circuit 4
Inverts and amplifies the input signal. The voltage V _OUT of the output signal of the operational amplifier circuit 4 is applied as a negative feedback signal to the rectifying / smoothing circuit 7 via the resistor R1 and the variable gain circuit 6 and the resistor R2. The gain of the variable gain circuit 6 is controlled by the output of the rectifying / smoothing circuit 7, and the output of the variable gain circuit 6 is added to the operational amplifier circuit 4 together with the input signal. In addition, R4, R
Reference numeral 5 is a DC feedback resistor, and C1 is an AC signal bypass capacitor. Then, the feedback rate of the negative feedback signal of the operational amplifier circuit 4 is changed by the strength of the input signal, and the closed loop gain is controlled. The feedback factor decreases when the input signal is weak and its level is low, and the closed loop gain becomes high. Further, when the input signal is strong and the level thereof is high, the input signal becomes large and the closed loop gain becomes low.

FIG. 4 is a characteristic diagram showing the relationship between the voltage of the input signal and the voltage of the output signal of such a compressor circuit. The frequency of the input signal is 1 KHz. As shown by the solid line, the voltage V _IN of the input signal is compressed to ½ to become the voltage V _{OUT of the} output signal, but as is clear from comparison with the dotted line showing the relationship when not compressed, the input signal is weak. As the level of the voltage V _IN becomes lower, the closed loop gain becomes higher, and as the input signal becomes stronger, the closed loop gain becomes lower. For example, when the voltage V _IN is −40 dB, the closed loop gain is 20 dB, and when it is −80 dB, the closed loop gain is 40 dB.

Further, FIG. 5 is a characteristic diagram showing the relationship between the frequency of the input signal and the voltage of the output signal using the strength of the input signal as a parameter. Input signal voltage V _IN is 0 dB, -20
At the dB level, the voltage V _{OUT of the} output signal compressed to 1/2 is obtained in a wide frequency range of 0.1 KHz to 20 KHz. However, the voltage V _IN is -40d
When the level is lowered to B and -60 dB, the compression ratio does not reach 1/2 at the frequency exceeding 3 KHz. For example, when the voltage V _IN is -60 dB, the output voltage at -20 dB at -30 dB is compressed to about -38 dB, and is reduced by -8 dB. The reason is that in order to compress the input signal of this level to 1/2, the closed loop gain exceeds the open loop gain of the operational amplifier circuit 4 at a frequency exceeding 3 KHz, and theoretically the closed loop gain becomes open loop. Because the gain cannot be exceeded. Therefore, the weak input signal of the compressor has a drawback that the reproducibility of the output signal of the expander, that is, the output signal of the expander is poor when the frequency becomes high.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to improve the reproducibility of an input signal in a compander output signal by allowing a weak input signal to be compressed at the same compression ratio as a low frequency even at a high frequency. It is to provide a compressor circuit for a compander that can perform.

[0008]

The present invention has an operational amplifier circuit for inverting and amplifying an input signal, a variable gain circuit to which an output signal of the operational amplifier circuit is added as a negative feedback signal, and a rectifying / smoothing circuit. In a compressor circuit for a compander in which the output of a variable gain circuit whose gain is controlled by the output of the operational amplifier is added to the operational amplifier circuit together with an input signal, when the input signal is weaker than a set value, the bias current of the operational amplifier circuit is changed. It has a gain control circuit for increasing the open loop gain.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a compressor circuit for a compander of the present invention will be described below with reference to FIG. 1 which is a circuit diagram. The same parts as those in FIG. 3 are designated by the same reference numerals. The compressor circuit in FIG. 1 has, in addition to the operational amplifier circuit 4, the variable gain circuit 6 and the rectifying / smoothing circuit 7, a comparing circuit 5 whose input side is connected to the rectifying / smoothing circuit 7 and whose output side is connected to the operational amplifier circuit 4. ing. The operational amplifier circuit 4 is mainly composed of a differential amplifier circuit including transistors Q1, Q2, Q3, Q4 and resistors R6, R7.

The emitters of the transistors Q1 and Q2 are connected to each other via resistors R6 and R7, and the connection point of the resistors R6 and R7 is connected to the output side of the current source S1. The input side of the current source S1 is connected to the terminal 3 to which the power supply voltage V _CC is applied. The current source S1 is
It supplies the bias current of the differential amplifier circuit of the main part of the operational amplifier circuit. A current mirror circuit including a transistor Q3 and a transistor 4 is connected as an active load to the collectors of the transistor Q1 and the transistor Q2. A transistor Q5 having a collector connected to the terminal 3 via the current source S2 and the output terminal 2 and a base connected to the collector of the transistor Q4 constitutes an amplifier circuit, and is connected between the collector and the base. The capacitor C2 is a capacitor for preventing oscillation.

The output terminal 2 is connected to a variable gain circuit 6 and a rectifying / smoothing circuit 7 via a resistor R1 and a resistor R2, respectively. The rectifying / smoothing circuit 7 is connected to the variable gain circuit 6 and controls its gain. The output side of the variable gain circuit 6 is connected to the base of the transistor Q1 which is one input terminal of the operational amplifier circuit 4. The voltage source E1 is connected to the base of the transistor Q2 which is the other input terminal. The input terminal 1 is connected to the output terminal 2 via the resistors R3, R4, R5, and is connected to the base of the transistor Q1 via the resistor R3. The connection point between the resistors R4 and R5 is grounded via the capacitor C1. Then, one end on the input side of the comparison circuit 5 is connected to the output side of the rectifying and smoothing circuit 7, and the other end is connected to the voltage source E2. The output side of the comparison circuit 5 is connected to the output side of the current source S1. In this comparison circuit 5, the voltage of the voltage source E2 serving as the reference voltage and the rectifying / smoothing circuit 7
Are compared, and a current is generated on the output side when the output voltage becomes lower than the voltage of the voltage source E2.

In the compressor circuit constructed as described above, the feedback rate of the negative feedback signal of the operational amplifier circuit 4 changes depending on the strength of the input signal, and the closed loop gain is controlled as in the case of FIG. . The relationship between the voltage V _IN of the input signal and the voltage V _OUT of the output signal is expressed by equation (1). V _OUT ² = V _IN · I ₀ · R ₁ · R 2 / R 3 (1) Note that I ₀ is the current value of the bias current source of the amplifier circuit that constitutes the variable gain circuit 6, and R 1, R 2, and R 3 are of the same sign, respectively. It represents the resistance value of the resistor. What is different from FIG. 3 is that the input signal becomes weak and the level of the voltage V _IN thereof becomes low, so that the voltage V _OUT of the output signal also becomes low and the output voltage of the rectifying / smoothing circuit 7 becomes equal to or lower than the voltage of the voltage source E2. In this case, the current is supplied to the operational amplifier circuit 4. This current flows as a bias current into the operational amplifier circuit 4 together with the current of the current source S1, so that its transconductance increases and the open loop gain increases.

The voltage of the voltage source E2 serving as the reference voltage can be determined by setting the lowest level of the voltage V _IN at which the compression ratio does not change depending on the frequency as a set value and corresponding to the set value. Therefore, when the voltage V _IN becomes low enough to change the compression ratio at a high frequency, the open loop gain can be increased by increasing the bias current of the operational amplifier circuit 4, so that the required closed loop gain can be obtained. It is possible to prevent the change of the compression ratio due to the frequency. Although the decrease in the voltage V _OUT indicating the decrease in the voltage V _IN is detected by the output of the rectifying / smoothing circuit 7, a detection circuit different from the rectifying / smoothing circuit 7 may be newly provided.

FIG. 2 is a circuit diagram showing another embodiment of the compander compressor circuit of the present invention. In FIG. 2, the voltage V _{IN of the} input signal is directly applied to the comparison circuit 5 so that the DC source E
3 differs from FIG. 1 in that it is compared with the voltage of 3, but the other configurations are the same as in FIG. By directly applying the voltage V _IN to the comparison circuit 5, the set value can be accurately determined.

[0015]

As described above, the compander compressor circuit of the present invention increases the open loop gain of the operational amplifier circuit even when the input signal strength becomes weaker than the set value, so that even at a high frequency. The required closed-loop gain is obtained because the compression ratio does not change. Therefore, the compression ratio at a high frequency does not decrease even with a weak input signal, so that the reproducibility of the input signal in the output signal of the compander does not change depending on the strength or frequency of the input signal. Since the open loop gain of the operational amplifier circuit is increased only when the input signal is weak, there is an advantage that a phase delay or an oscillation phenomenon does not occur when the input signal is strong.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a compander compressor circuit of the present invention.

FIG. 2 is a circuit diagram showing another embodiment of the compander compressor circuit of the present invention.

FIG. 3 is a circuit diagram of a conventional compander compressor circuit.

4 is a characteristic diagram showing a relationship between an input signal voltage and an output signal voltage of the compressor circuit of FIG.

5 is a characteristic diagram showing the relationship between the frequency of the input signal and the voltage of the output signal of the compressor circuit of FIG. 3 with the strength of the input signal as a parameter.

FIG. 6 is an explanatory diagram of a compander circuit.

[Explanation of symbols]

 4 Operational amplifier circuit 5 Comparison circuit 6 Variable gain circuit 7 Rectification smoothing circuit

Claims (3)

[Claims] 1. An operational amplifier circuit for inverting and amplifying an input signal,
It has a variable gain circuit to which the output signal of the operational amplifier circuit is added as a negative feedback signal and a rectifying / smoothing circuit, and the output of the variable gain circuit whose gain is controlled by the output of the rectifying / smoothing circuit is added to the operational amplifier circuit together with the input signal. In the compander compressor circuit, a compander having a gain control circuit for changing the bias current of the operational amplifier circuit to increase the open loop gain when the input signal is weaker than a set value. Compressor circuit. 2. The gain control circuit comprises a comparator circuit for comparing the output of a rectifying / smoothing circuit applied to the input side with a reference voltage, and the output side being connected to the output side of a current source for a bias current of an operational amplifier circuit. The compressor circuit for a compander according to claim 1. 3. The gain control circuit comprises a comparison circuit for comparing an input signal applied to the input side with a reference voltage and having an output side connected to an output side of a bias current source of an operational amplifier circuit. Compressor circuit for compander.