JPS59125115A - Variable time constant circuit - Google Patents

Abstract

PURPOSE:To obtain a variable time constant circuit which is stable to temperatures by supplying the input to two current output circuits having the same temperature characteristics and different threshold levels and connecting the outputs of two time constant circuits which are charged and discharged with the output of each of said two current output circuits. CONSTITUTION:A current mirror circuit 22 contains an input terminal 21 and transistors TR24 and 25 which deliver the same current. While, TR34 and 37 hold points E1 and E2 at a fixed potential respectively by constant current sources 33 and 32 and resistances 35 and 38. In this case, it is possible to set the difference of potential between the points E1 and E2 at an optional value in response to the resistance value. Therefore the potentials are different between points P and Q when a current equal to the input current flows to resistances 31 and 33. For instance, if the potential of the point E1 is higher than that of the point E2, the signal is transmitted only to a terminal 51 of a time constant circuit 50 when the input signal is on a small level. While the signal is transmitted to both terminals 51 and 52 when the level of the input signal is sufficiently high. The time constant can be varied in response to the level of the input signal. In addition, the temperature changes are cancelled for both circuits.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a variable time constant circuit for, for example, a noise reduction circuit, and particularly to a variable time constant circuit that can operate at low voltage.

[Background technology and its problems]

It is desired that noise reduction circuits, which are often used in general audio cassette decks, etc., be incorporated into small portable head-pump monitor type tape recorders, etc., to further improve the sound quality. As a power source for such a portable small tape recorder, etc., for example, dry cell batteries 2
The voltage that can be extracted directly from this battery power source is as low as 3V to 2V. Also,
Electronic circuits such as noise reduction circuits are normally supplied with power supply voltage via a constant voltage circuit, etc., and due to the voltage drop caused by this constant voltage circuit, the actual circuit power supply voltage is as low as, for example, 1.6V. It has become. However, conventional noise reduction circuits are designed to operate at a voltage of about 12V, for example, and it is difficult to operate them at a voltage as low as about 1.6V.

Here, FIG. 1 shows the basic configuration of a general noise reduction circuit. In this FIG. 1, an input terminal 1 includes a first signal path 2 having no frequency characteristics, and a variable bypass filter 3. and a second signal path consisting of a series circuit of the limiter circuit 4, and these first . The outputs from the second signal path are summed by an adder 5 and sent to an output terminal 6.

The signal at the connection point between the variable bypass filter 3 and the limiter circuit 4 is sent to a control circuit 10 consisting of a rectifier circuit 7, a variable time constant circuit 8, and a nonlinear conversion circuit 9. Bypass filter 30 with variable output
The signal is sent to the control terminal f.

As a variable time constant circuit 8 used in such a noise reduction circuit, for example, one shown in FIG. 2 is known. In this FIG. 2, the rectifier circuit 7
A rectified output from the diode 11 corresponding to a part of the circuit is supplied to the input terminal 12 of the variable time constant circuit 80. A parallel circuit consisting of a resistor 14 and a capacitor 5 is inserted and connected between the input terminal 12 and the terminal 13, and a parallel circuit consisting of a resistor 17 and a diode 18 is connected between the input terminal 12 and the output terminal 16. A capacitor 9 is connected between the output terminal 16 and the terminal 13.

In such a variable time constant circuit 8, when the signal supplied to the input terminal 12 rises from a no-signal state (zero level) to a predetermined input level, the voltage between the terminals 12 and 16 due to the input level at this time is The potential difference is the VF of the diode 18
(forward voltage drop), diode 18
The resistance value of the resistor is extremely high, and the resistance value of the resistor 1γ is dominant, and the time constant becomes dog (long attack time). In addition, when the input level at the time of startup is high and the potential difference between the terminals is about VF or 71 or more, the resistance value of the diode 18 is small and the parallel resistance value of the resistor 17 and diode 18 or Since the resistance value is close to zero, the time constant is small (short attack time). FIG. 3 shows the relationship between the input power level and the time at which the signal rises. As is clear from Fig. 3, when the input signal rise level is extremely low at about -40 dB, the attack time is as long as about 100 m5, and as the rise level increases, the attack time becomes shorter; It turns on and the resistance value is close to zero, and the attack time is a small, almost constant value, for example 1.
converges to m's.

As mentioned above, such a conventional variable time constant circuit is designed to operate with a power supply voltage of, for example, about 12V, and the maximum amplitude value of the rectified signal input to the input terminal 12 is about 4V. It has become.

Therefore, in order to apply the above-mentioned 1.6-inch circuit power supply, it is necessary to enable operation at a maximum input amplitude value of about 1V to the variable time constant circuit. Also, the V of the diode 18
F has a characteristic value that is determined depending on the semiconductor material, and is approximately 0.7 cm in commonly used silicon diodes, but it is necessary to reduce this value to approximately 175 mV in appearance. At this time, it is necessary to reduce the temperature characteristics as well. Here, instead of the above silicon diode,
Although it is possible to use a germanium diode or a Schottky barrier diode, it is difficult to realize this because the voltage-current characteristics of the diodes are quite different and the control voltage is limited.

[Purpose of the invention]

The present invention has been made in view of the above-mentioned circumstances, and can be used in a noise reduction circuit that operates at low voltage, and can be used in a noise reduction circuit that operates at low voltage without deteriorating the performance even if the amplitude value of the input signal is small. The purpose of the present invention is to provide a variable time constant circuit which can realize the standard variable time constant circuit specifications as is (except for reducing the operating level).

[Summary of the Invention] In order to achieve such an object, a variable time constant circuit according to the present invention provides first... a current mirror circuit that outputs a second current output signal; a series circuit of a first resistor and a first reference voltage source to which the first current output signal from the current mirror circuit is supplied;
a series circuit of a second resistor and a second reference voltage source to which the second current output signal from the current mirror circuit is supplied; The first resistor is supplied with a voltage at each terminal of the second resistor. a second PN junction element and these first
．． Each output from the second PN junction element causes the first... a second time constant circuit;
and an output terminal from which the output from the second time constant circuit is taken out. By making the voltage values of the second reference voltage sources different from each other, the first... The second time constant circuit is operated.

〔Example〕

First, the variable time constant circuit according to the present invention includes a part of the rectifier circuit 7 in the noise reduction circuit shown in FIG. 1 described above, and converts the output from the p1 variable bypass filter 3 into at least a current signal. and is supplied to the variable time constant circuit of the present invention.

That is, in one embodiment of the present invention shown in FIG. 4, the input terminal 21 is supplied with a current input signal obtained by, for example, double-wave rectification of the output from the variable bypass filter 3.

The current mirror circuit 22 outputs first and second current output signals having a current value proportional to the current input signal supplied to the input terminal 21, for example, having mutually equal current values. This current mirror circuit 22I/'i, for example, the input terminal 21
A diode 23. whose cathode is connected to the diode 23. Each has a PNP transistor 24.25 connected to its base, a resistor 26 on the anode side of two diodes and a resistor 27.28 on the emitter side of each transistor 24.25.
By setting the respective resistance values of the first .
A second current output signal is obtained from each collector of each transistor 24,25. These transistors 2
4.25 each collector has a first .4.25 collector. Second resistance 31.32
are connected to one end of each. Let these connection points be P and Q, respectively. A voltage source that generates a first reference voltage E is connected to the other end of the first resistor 31, and a voltage source that generates a second reference voltage E2 is connected to the other end of the second resistor 32. ing. The first reference voltage source is, for example, a constant current source 33. Transistor 34. and a resistor 35, and the second reference voltage source is similarly configured, for example, a constant current source 36. Transistor 37. and a resistor 38. When setting the current values of the constant current sources 33.36 to be equal to each other, for example, about 50 pA, each resistor 35.38 is set to 2.8 kQ, 50 pA, respectively.
．． 4 and select Ω as the first reference voltage E.

is set to 550 mV, and the second reference voltage E2 is set to 430 mV.

Next, the connection point P between the collector of the transistor 24 and one end of the resistor 31 is connected to the base of the transistor 41, which is the second PN junction element, and the connection point Q between the collector of the transistor 25 and one end of the resistor 32 is , is connected to the base of the transistor 42, which is the second PN junction element. I
・The emitter of the transistor 41 is connected to the first input terminal 51 of the time constant circuit 5o via the resistor 43.
The emitters of are respectively connected to a second input terminal 52 of a time constant circuit 50 via a resistor 44. Time constant circuit 50
Vi, a resistor 54 is connected between the first input terminal 51 and the terminal 53.
and a capacitor 55 are inserted and connected, a resistor 57 is connected between the first input terminal 51 and the second input terminal 52, and a capacitor 58 is connected between the second input terminal 52 and the terminal 53. are connected, and the second input terminal 52 is used as the output terminal 56 of the time constant circuit 5o.

In the variable time constant circuit having the above configuration, a current output signal equal to the current value of the input signal supplied to the input terminal 21 is supplied to each resistor 31.32, but at the other end of each resistor 31.32. Voltage E.

Since E2 is set differently from each other, for example, 550 mV and 430 mV, each connection point P.

The voltages on Q appear different from each other. In other words, when the input signal rises within a small level range,
Only the first transistor 41 operates, and the time constant circuit 50
A signal is supplied only to the first input terminal 51 of the output terminal 5
When the output signal from 6 rises, the moon/A appears. On the other hand, if the rising level of the input signal is too high and a voltage is generated at the connection point Q that can operate the transistor 42, for example,
1st. The second transistors 41 , 42 operate together and the first . By supplying signals to both the second input terminals 51 and 52, the rise time constant (attack time) of the output signal from the output terminal 56 appears small (short).

Therefore, when the input level at the time of the signal rise is low, a small first rise time constant (attack time) is obtained by supplying the signal to the first input terminal 51 of the time constant circuit 50, and the signal rises. When the input level of the time constant circuit 50 is high, the first input level of the time constant circuit 50 is high. A large second rise time constant is obtained by supplying signals to both second inputs 51,52.

Here, the resistors 43 and 44 also act as capacitor charging resistors when the signal rises, so they become elements that determine the first and second rising time constants and the slope of the non-linear portion, and the time constant circuit. The non-linear threshold voltage (corresponding to the VF of the diode 18 in the conventional example) corresponding to the control voltage that changes the time constant of a variable time constant circuit like this one, which is considered as a part of the circuit, is as a result of the first and second This appears as a difference between the reference voltages E and -E2, which in this embodiment is approximately 120 mV, for example. Here, each voltage E1. Since E2 is a function of the VBE (base-emitter voltage) of the transistors 34 and 37, it is possible to compensate for the temperature characteristics of the transistors 41 and 42. For example, if the current flowing through the transistors 34 and 37 is constant, the temperature characteristics can be almost completely compensated for. Furthermore, since the voltage-current characteristics closely approximate those of a silicon diode, the characteristics of the conventional circuit can be well approximated over the entire input level range.

For example, if we consider the case where a diode such as an 18VC germanium diode or a Schottky barrier diode is used in the conventional variable time constant circuit shown in FIG. However, it is limited to the voltage value specific to the semiconductor element, and the temperature characteristic is about the same as silicon, for example 2 mV/'c, so as a result, the temperature characteristic with respect to the signal voltage will deteriorate by 3 to 7 times. Furthermore, since the voltage-current characteristics are different from those of silicon diodes, there are parts that do not meet the standards of standard variable time constant circuits for noise reduction circuits.

According to the embodiments of the present invention, all of these drawbacks can be eliminated, and a variable time constant circuit with a low voltage and small input level can be realized with high performance.

〔Effect of the invention〕

As is clear from the above description, according to the variable time constant circuit according to the present invention, not only low voltage operation is possible, but also the above-mentioned first. The control voltage threshold (corresponding to the VF of a conventional diode) for variable time constant can be freely set according to the setting of the second reference voltage, and it has good temperature characteristics and can be used over the entire input level range. Current of silicon diode −
Characteristics that are extremely similar to voltage characteristics can be achieved.

[Brief explanation of the drawing]

Fig. 1 is a block circuit diagram showing an example of a noise reduction circuit in which the variable time constant circuit of the present invention is used, Fig. 2 is a circuit diagram showing an example of a conventional variable time constant circuit, and Fig. 3 is a block circuit diagram showing an example of a conventional variable time constant circuit. FIG. 4 is a graph showing the relationship between the active time and the input level of the circuit. FIG. 4 is a circuit diagram showing an embodiment of the present invention. 21... Input terminal 22... Current mirror circuit 31... First resistor 32... Second resistor El, First reference voltage E2... Second reference voltage 41.42...・Transistor 50 ・・Time constant circuit 56 ・・Output terminal Patent applicant Akira Koike, patent attorney representing Sony Corporation

Claims (1)

[Claims] A first proportional current value is applied to the current input signal supplied to the input terminal. a current mirror circuit that outputs a second current output signal; a series circuit of a first resistor and a first reference voltage source to which the first current output signal from the current mirror circuit is supplied; a second resistor and a second resistor to which said second current output signal from the circuit is supplied;
a series circuit of reference voltage sources, and the first reference voltage source. The first resistor is supplied with a voltage at each terminal of the second resistor. a second PN junction element; The first and second time constant circuits are activated by respective outputs from the second PN junction element, and the first and second time constant circuits are activated by respective outputs from the second PN junction element. and an output terminal from which an output from the second time constant circuit is taken out.