JPH0619217Y2 - Variable impedance circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to improvement of a variable impedance circuit, and particularly to a variable impedance circuit suitable for forming in an IC (integrated circuit) using a low voltage power supply. Regarding

(B) Prior Art As shown in Japanese Patent Laid-Open No. 59-97215, there is known a variable impedance circuit formed by connecting a plurality of diodes in a ring. In the case of the variable impedance circuit, a normal PN junction diode is used as the diode, and a bias current is caused to flow through the diode to make it conductive so that an impedance corresponding to the value of the bias current is exhibited. Therefore, the impedance can be changed by changing the value of the bias current according to the control signal, and the variable impedance circuit can be used for a variable attenuation circuit or the like.

(C) Problems to be Solved by the Invention However, when the variable impedance circuit is formed in an IC of a low voltage power source, it has a drawback that it is easily saturated and the distortion rate deteriorates. That is, in the case of the variable impedance circuit, a diode, a constant current source that supplies a bias current to the diode, and a current source that draws a bias current from the diode are connected in series between a power source and ground. An operating voltage of at least 0.8 V (saturation voltage of current source transistor: 0.1 V, forward voltage of diode: 0.6 V) is required. Therefore, when the power supply voltage is 1.5V, considering the voltage reduction characteristic (1V), only an amplitude of up to 0.2V can be allowed, and when an input voltage higher than that is applied, the distortion factor due to clipping occurs. Getting worse. In particular, when the variable impedance circuit is used as the low-frequency component attenuation circuit of the automatic Uradness control circuit, when the input signal becomes large while the output signal level of the power amplifier is small and the control signal becomes large, the distortion factor Was significantly worse and was a problem.

(D) Means for Solving the Problems The present invention has been made in view of the above points, and is characterized in that a Schottky diode is used as the variable impedance element forming the variable impedance circuit.

(E) Function According to the present invention, since the Schottky diode is used as the variable impedance element, it can be operated sufficiently with a voltage of at least 0.5V, the dynamic range is expanded, and the distortion factor is increased. Can be prevented from worsening. Moreover, even if the bias current of the Schottky diode is reduced,
Since a sufficient impedance can be obtained, the consumption current can be reduced.

(F) Embodiment FIG. 1 shows an embodiment of the present invention. (1) and (2)
Is a first and second Schottky diode whose anodes are commonly connected, (3) and (4) are third and fourth diodes whose anodes are commonly connected, and (5) is a variable current source for generating a control current, ( 6 ) is a first current inverting circuit for inverting the control current, which comprises first to fourth transistors (7) to (10), ( 1
1 ) is composed of fifth to seventh transistors (12) to (14),
A second current inverting circuit for inverting the collector current of the second transistor (8), and (15) are signal terminals connected to the common anode of the first and second Schottky diodes (1) and (2). is there. The seventh transistor (14) is a first current source that supplies a bias current to the common anode of the first and second Schottky diodes (1) and (2), and the sixth transistor (13) is the third and third transistors. 4 Schottky diode
As a second current source for supplying a bias current to the common anodes of (3) and (4), the fourth transistor (10) has first and fourth
As a third current source that draws a bias current from the common cathode of the Schottky diodes (1) and (4), the third transistor (9) is a common cathode of the second and fourth Schottky diodes (2) and (3). Fourth to draw bias current from
Each operates as a current source.

The control current generated from the variable current source (5) is the first and second
The current is inverted by the current inversion circuits ( 6 ) and ( 11 ), and becomes the bias currents of the first to fourth Schottky diodes (1) to (4). Therefore, the first to fourth Schottky diodes
The impedance presented by (1) to (4) has a value corresponding to the control current, and is large when the control current is small and small when the control current is large. Therefore, the impedance seen from the signal terminal (15) is Rx, where Rx is the impedance of each Schottky diode.

The forward voltage of the Schottky diode is smaller than that of a normal PN junction diode and is about 0.3V. Therefore, the minimum operating voltage in the variable impedance circuit of FIG. 1 is about 0.5V, and a sufficient dynamic range can be secured even if a constant voltage power supply such as a 1.5V power supply is used. Also, the Schottky diode is
Since the impedance equal to that of a normal PN junction diode can be obtained with a small bias current, the current consumption of the circuit can be reduced.

FIG. 2 shows an example in which the present invention is applied to a low frequency component attenuation circuit of an automatic loudness control circuit. When a large-amplitude negative signal is generated from the left and right power amplifiers (16) and (17), a current according to the level of the negative signal flows through the detection resistors (18) and (19), and the output of the comparison amplification circuit (20). Becomes "L", the charging transistor (21) is turned on, and the capacitor (22) is charged. Therefore, the output signal of the control circuit (23) becomes large and the impedance of the variable impedance circuit (24) becomes small. Therefore, the input signal applied to the input terminal (25) is divided by the input resistance (26) and the impedance of the variable impedance circuit (24), and the output signal of the boost amplifier (27) becomes small. Also, the left and right power amplifiers (16) and (1
When the output signal level of 7) becomes small, the output of the comparison amplification circuit (20) becomes "H", the charging transistor (21) turns off,
The capacitor (22) is not charged. Therefore, the control signal is not generated from the control circuit (23), and the variable impedance circuit (24) does not operate. Therefore, the input signal applied to the input terminal (25) is not attenuated and the boost amplifier (27)
Amplified by, a high level reduced boost signal is generated at the output terminal (28). Therefore, by using the circuit of FIG. 2, it is possible to provide an automatic loudness control circuit which does not perform the reduction boost when the output signal level is high, but can perform the reduction boost only when the output signal level is low.

The output control signal of the control circuit (23) is the comparison amplification circuit (20).
Since it is fed back to the positive input terminal of, the comparison amplification circuit (20), the charging transistor (21) and the capacitor (22) operate as a peak current detection circuit, and when switching from the high volume listening state to the low volume listening state, A sudden reduction boost can be prevented from being performed, and an improvement in hearing can be achieved.

(G) Effect of the Invention As described above, according to the present invention, it is possible to provide a variable impedance circuit that operates with a low voltage power supply. Moreover, a variable impedance circuit with low current consumption can be provided. For that reason,
A variable impedance circuit suitable for forming in an IC using a low voltage battery as a power source can be obtained, and a variable impedance circuit having a wide range of applications such as an automatic loudness control circuit can be obtained.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing an application circuit example of the present invention. (1) (2) (3) (4) ... Schottky diode, ( 6 ) ( 11 ) ... Current reversing circuit.

Claims (1)

[Scope of utility model registration request] 1. A first and a second diode whose anodes are commonly connected, a third and a fourth diode whose anodes are commonly connected, and a first which supplies a bias current to the anodes of said first and second diodes. A current source and a second for supplying a bias current to the anodes of the third and fourth diodes
A current source, a third current source that draws a current from the common cathode of the first and fourth diodes, a fourth current source that draws a current from the common cathode of the second and third diodes, and a control signal And a control circuit for changing the currents flowing through the first to fourth current sources equally.
The fourth to fourth diodes are Schottky diodes, and any anode or cathode of the first to fourth diodes serves as a signal terminal, and impedance seen from the signal terminal is changed according to the control signal. A variable impedance circuit characterized in that