JPS586610A - Controller for transient characteristic of circuit during power feeding - Google Patents

Abstract

PURPOSE:To make a noise reduction circuit steady rapidly by minimizing the value of the variable impedance of the noise reduction circuit for a prescribed time after the circuit is powered up. CONSTITUTION:When a power source is put to work, the terminal voltage of the resistance R4 of a time constant circuit 7 rises instantaneously to turn on the transistor (TR) Q7. Then, a current flows from the current source I2 to diodes D3 and D4 through a TRQ9. At time time, the impedance of the signal line 5 of a variable impedance circuit 2 becomes less than the forward impedance of the diodes D3 and D4. Therefore, the coupling capacitor C1 connected in series to the signal line 5 is discharged quickly to obtain a steady state. A prescribed time later, the terminal voltage of the resistance R4 drops to turn off the TRQ7, thus ending transient operation.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for controlling transient characteristics of a circuit device when power is turned on, and more particularly to a control device suitable for controlling transient characteristics of a tape recorder device having a tape noise reduction system when power is turned on.

There is a Dolby noise reduction system as a noise reduction system for tape recorders, and there are both B and C types depending on the system characteristics. Both of these methods compress and record the level of the mid-high range of the signal, and expand this when playing back to make the playback signal.
The difference between the two is the level control characteristics of this mid-high range. In a tape recorder that allows selection of both of these systems and also selection of the Dolby noise reduction system, it is necessary to control the signal level according to each of the above-mentioned modes (B and C type modes, off mode).

FIG. 1 is a schematic block diagram of such a level control circuit, in which an input signal to be controlled is applied via a coupling capacitor to an amplifier 1 having a high input impedance configured as an operational amplifier.

A variable impedance circuit 2 is provided between the input signal line 5 of this amplifier 1 and the ground, that is, in parallel with this input signal line 5. In order to control the impedance of the variable impedance circuit 2 according to the input signal level, and to determine the initial conditions when there is no input signal, this impedance value is initialized according to each of the above operation modes. For this purpose, an impedance control circuit 3 is provided, which controls the initial state setting of the variable impedance circuit 2 according to the reference control input (zero, E1 volt) selected by the motor switch 4. configured to generate a signal. A bias Vr of a predetermined level is applied to the signal input terminal of the coupling capacitor C1 via a resistor R1.

FIG. 2 is a diagram showing an example of the variable impedance circuit 2, in which a pair of PNP transistors whose bases are connected to the reference potential point VR and the signal line 5, respectively, have emitters as shown in FIG.
They are connected in common through each emitter resistor R2e''8, and are supplied with a current source 1o.The collectors of both transistors Q1 and Q1 are connected to a diode D1.
e are connected to the common potential vH through D2, respectively. In addition, a differential pair of NPN transistors Q3yQ4 is provided, each of which uses the collector outputs of both transistors Q1s and QR as pace inputs, and a differential pair of transistors Q8pQ4 receives a DC current that is changed according to the input signal level. Source l! A current mirror circuit such as transistor Q6*Q6 is provided as a collector load for each transistor. The collector of the transistor Q3 and the pace of the transistor Q2, that is, the signal line 5 are commonly connected.

In such a configuration, the impedance Zi when looking at this circuit 2 from the signal line 5 is Z* = R6e Io/l, ...
It is expressed as (1). child\
, Ro is R,=R2=Ro.

IO is an initially set value, and Il is a current corresponding to an input signal to be controlled, and is approximately zero when there is no signal. Therefore, by controlling the current value of the current source Io, Zi will not change, and the impedance control circuit 3
The initial setting of this current value is done according to each operation mode.

Now, if we consider the case where the power of the circuit device is turned on, the input impedance of the amplifier L can be considered to be almost infinite, so the charging current to the coupling capacitor CI is increased by the resistance R.
1 and a variable impedance circuit 2. Here, if the noise reduction mode is selected as off, the impedance value Z of the variable impedance circuit 2
i is set to be extremely low, for example, several lOΩ. - Therefore, in this case, since the resistance value of the charging path of the coupling capacitor C1 is low, the charging is done quickly and the circuit does not change when the noise reduction mode is selected as B or C type. The impedance Zi of the impedance circuit 2 ranges from several tens of Ω to several hundreds of Ω.
It is initially set to a very high value of KΩ.

Therefore, the resistance value of the charging path of the capacitor CI is extremely high, and charging is performed slowly, resulting in a prolonged transient state, which is inconvenient.

SUMMARY OF THE INVENTION An object of the present invention is to provide a transient characteristic control device which can terminate the transient characteristic control device when the power is turned on and quickly shift to stable steady operation in order to eliminate the above-mentioned drawbacks.

The transient characteristic control device according to the present invention includes a coupling capacitor that is inserted in series with a signal line and has a predetermined bias voltage applied to one end, and a coupling capacitor that is connected in parallel to the signal line at the other end of the capacitor whose impedance is controlled according to the operating mode of the circuit. The present invention is directed to a device including an inserted variable impedance means; its feature is that the impedance of the variable impedance means is automatically reduced during the disconnection period from the time the power is turned on.

The present invention will be explained below with reference to the drawings.

FIG. 3 is a circuit diagram of an embodiment of the present invention, and parts equivalent to those in FIGS. 1 and 2 are designated by the same reference numerals. 1 and 2, a time constant circuit 7 consisting of a capacitor C2 and a resistor R4, which are charged by the circuit power supply +VCC, is provided, and the time constant output of this time constant circuit 7 is a resistor. In order to detect the terminal voltage of R4, an NPN (NPN) transistor Q7 whose base is applied with the voltage and a transistor QIO whose base is connected to a reference voltage vR3 for comparing this input voltage are provided with different emitters. There is. A constant current source I2 is connected between the emitters of these transistors Q7 and QIO and the ground.
A current mirror circuit including PNP transistors Q8 r Q9 is inserted between the collector of the transistor Q7 and the circuit power supply vec, so that the collector current of the transistor Q7 is transferred to the transistor Q via the transistor Q8. This transfer current flows through the diode D3.
Transistor Q of variable impedance circuit 2 via D4
1, Q2.

The other circuit configurations are the same as those in Figures 1 and 2.
When c is turned on, the terminal voltage of resistor R4 of time constant circuit 6 instantly rises to the VCC level, turning on transistor Q7 and turning off transistor QIO. Therefore, the current of current source 2 flows from transistor Q9 to diode D3.
y Flows into D4. At this time, the impedance z, / as seen from the signal line 5 of the variable impedance circuit 2 is ! o112 zi'= (Ro/"e door-111+++・+++,
(21 hours 1. Ro== R1==-, ro is the forward impedance of the diode D3 + D4, and in the following equation, 2 is ma. Therefore, isometrically An impedance of r is inserted in parallel to the variable impedance circuit 2!D (2) If the current value of the current source is selected so that it has approximately the minimum value, then the coupling capacitor C1
The charging time constant during the transition to can be minimized, and therefore, regardless of the selection of the operating mode by the switch 4, it is possible to quickly bring the circuit to the steady state. In other words, the impedance of the variable impedance circuit 2 is automatically controlled to be small for a predetermined period of time, thereby quickly ending the transient state.

After a predetermined time, when the voltage of resistor R4 decreases and becomes lower than vR3, transistor Q7 is turned off and transistor QI
When O is off, the current value! 2 no longer flows into the variable impedance circuit 2, and the impedance r inserted in parallel is deactivated from the active state and returns to the steady state. Therefore, if the time constant of the time constant circuit 6 is appropriately selected, the impedance parallel to the signal line 5 including the variable impedance circuit 2 can be forcibly controlled externally for a period necessary until the circuit becomes stable. The impedance can be set as follows.

As described above, according to the present invention, the parallel impedance including the variable impedance circuit is forcibly controlled to a small value for a certain period of time at the same time as the power is turned on, so that the large capacity coupling capacitor charged by this impedance can be Charging can be accelerated and transient operation of the circuit can be quickly terminated.

The circuit configuration described above is not limited to the illustrated example, nor is it limited to the noise reduction system circuit of a tape recorder.

[Brief explanation of drawings]

Fig. 1 is a partial circuit diagram of a tape recorder having a Dolby noise reduction system, Fig. 2 is a diagram showing a specific example of the variable impedance circuit shown in Fig. 1, and Fig. 3 is a circuit diagram of an embodiment of the present invention. It is. Explanation of symbols of main parts 2... Variable impedance circuit 3... Impedance control circuit 4... Mode switch 5... Signal line 7... Time constant circuit C1... Coupling capacitor Q7... Transistor representative for power-on detection Patent attorney Motohiko Fujimura Maku/Kuchi r L2 diagram

Claims (3)

[Claims] (1) A coupling capacitor that is inserted in series with the signal line and has a predetermined bias voltage applied to one end, and a coupling capacitor whose impedance is initially set according to each operating mode of the circuit and which is inserted in parallel to the signal line at the other end of the capacitor. A device for controlling transient characteristics of a circuit device when the power is turned on, including a variable impedance means, the device being configured such that the impedance of the variable impedance means automatically becomes small for a predetermined period from the time the power is turned on. (2) The apparatus according to claim 1, wherein another impedance means is automatically inserted isometrically in parallel with said variable impedance means during said period. (3) Claims comprising: a time constant circuit that is charged when the power is turned on; and means for activating the other impedance means until the output of the time constant circuit reaches a predetermined level. The device according to item 1 or 2.