JPH0629764Y2 - High frequency compensation circuit - Google Patents

Description

[Detailed Description of the Invention] (a) Field of Industrial Application The present invention relates to a high frequency compensating circuit for changing the amount of high frequency compensation by switching capacitors according to the tape speed of a VTR (video tape recorder). In particular, the present invention relates to a high frequency compensating circuit which prevents generation of shock noise when switching the capacitor.

(B) Conventional technology As a special function of the VTR, a method of recording or recording in a double mode or a quad mode is known. In order to reproduce a recorded image or a recorded tape by this method, it is necessary to switch the capacity of the capacitor for compensating the high frequency component in the output signal of the magnetic head according to the tape speed. "'85 Sanyo Semiconductor Handbook Monolithic Bipolar Integrated Circuit Edition" (Published March 20, 1985), pages 962 to 9
Page 65 has a voice recording / playback amplifier IC (integrated circuit) L
A7090 is described. The ICLA7090 is provided with a switch for performing the switching operation as described above, and as shown in FIG. 2, a transistor is provided by using a control circuit (2) which operates according to a control signal applied to a control pin (1). (3) is turned on and off.

For example, when the control circuit (2) is operated and the transistor (3) is turned on, one end of the first capacitor (4) for high frequency compensating is grounded, and another second capacitor (5) for high frequency compensating (5). ) Is connected in parallel. When the transistor (3) is turned off, one end of the first capacitor is released, and only the second capacitor (5) is used for high frequency compensation. Therefore, the audio signal from the magnetic head (6) is compensated for high frequency components by the switching operation of the first and second capacitors (4) and (5) as described above, and the coupling capacitor is
It is applied to the amplifier circuit (8) via (7). The amplifier circuit (8)
Is generated at the output pin (9) and applied to the speaker (not shown). Therefore, the magnetic head (6) is controlled by the control signal applied to the control pin (1) according to the tape speed.
It is possible to perform appropriate high-frequency compensation of the audio signal from and apply it to the speaker.

(C) Problems to be solved by the device However, if the transistor is switched on and off and the capacitor is switched to adjust the high frequency compensation amount as in the second case, the output terminal of the amplifier circuit is switched when the transistor is switched. There was a problem that a loud shock noise was generated.
When the transistor (3) is in the off state, the magnetic head (6)
Is used in a non-biased state, the DC potential at the connection point of the first and second capacitors (4) and (5) becomes the ground potential, and the collector DC potential of the transistor (3) also becomes the ground potential. When the transistor (3) is turned on in that state, an offset voltage is generated in the collector of the transistor (3) due to the reinjection of electrons, and the collector DC potential of the transistor (3) is abruptly determined from the ground potential. Rises to the potential (corresponding to the offset voltage level). Since the abrupt change of the collector DC potential is differentiated by the first capacitor (4) and the coupling capacitor (7) and applied to the amplifier circuit (8), a large shock noise is generated at the output end of the amplifier circuit (8). There was a problem that occurs.

(D) Means for Solving the Problems The present invention has been made in view of the above-mentioned points, and a first transistor for switching a capacitor for high frequency compensation and a predetermined voltage are applied to the base of the emitter. Is connected to the collector of the first transistor via a resistor, while the second transistor is grounded to keep the second transistor always on.

(E) Action According to the present invention, since the collector of the second transistor which is always on is connected to the collector of the first transistor through the resistor, the collector of the first transistor is turned on and off by turning the first transistor on and off. DC potential does not change,
As a result, the generation of shock noise can be prevented.

(F) Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which (10) is a first transistor via a resistor (11) having a large value with a bias voltage applied to the base. It is a second transistor connected to the collector of (3). In FIG. 1, the same circuit elements as those in FIG. 2 are designated by the same reference numerals and the description thereof will be omitted.

The second transistor (10) whose base is applied with a predetermined voltage and whose emitter is grounded is always kept in the ON state, and its collector DC potential becomes equal to its offset voltage. Now, assuming that the VTR is in the normal speed state, the control pin
Since the control signal of "L" is applied to (1) and the output of the control circuit (2) becomes "L", the first transistor (3) is turned off. Therefore, the potential at the point A becomes equal to the collector potential of the second transistor 10, that is, the offset voltage of the second transistor 10. In that case, since the value of the resistor (11) is large, the point A is in an AC open state, and the second capacitor (4) is not involved in the high frequency compensation operation. Therefore, the output signal of the magnetic head (6) is high-frequency compensated only by the first capacitor (5), is applied to the amplifier circuit (8) via the coupling capacitor (7), and is amplified by the amplifier circuit (8). After being output terminal (9)
Be derived to.

Next, when the VTR is set to the high speed running state, the control signal of "H" is applied to the control pin (1), the output of the control circuit (2) becomes "H", and the first transistor (3) is turned on. . When the first transistor (3) is turned on, the DC potential at the point A becomes equal to the collector DC potential of the first transistor (3), that is, the offset voltage of the first transistor (3). Therefore, if the offset voltages of the first and second transistors (3) and (10) are set to be equal, the DC potential at the point A does not change when the first transistor (3) is turned on and off,
As a result, no shock noise is generated. When the first transistor (3) is turned on, the point A is grounded in an alternating current, so that the second capacitor (4) becomes the first capacitor (5).
The output signal of the magnetic head (6) is high-frequency compensated by the first and second capacitors (4) and (5). Therefore, if the circuit of FIG. 1 is used, it is possible to perform high frequency compensation according to the tape speed, and even if the first transistor is turned on and off to switch the compensation amount, no shock noise is generated.

(G) Effect of the Invention As described above, according to the present invention, it is possible to provide a high frequency compensating circuit capable of performing high frequency compensation according to the tape speed.
Further, it is possible to provide a high frequency compensating circuit which does not generate a shock noise even if the high frequency compensation amount is switched. Further, according to the present invention, since the above effect can be obtained only by adding the transistor and the resistor, it is possible to provide a high frequency compensating circuit suitable for an IC.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional high frequency compensation circuit. (3) …… First transistor, (4) …… First capacitor,
(5) …… Second capacitor, (6) …… Magnetic head, (10) ……
Second transistor, (11) ... Resistance.

Claims (1)

[Scope of utility model registration request] 1. A high-frequency compensating first capacitor arranged between an output end of a magnetic head and ground, a second capacitor connected in parallel with the first capacitor, and between the second capacitor and ground. A first transistor having a collector-emitter path inserted therein, and a predetermined voltage is applied to the base in a high-frequency compensation circuit in which the high-frequency compensation amount is changed by turning on and off the first transistor. By providing a normally-on second transistor whose emitter is grounded and connecting the collector of the second transistor to the collector of the first transistor via a resistor, a shock noise due to the on / off of the first transistor is generated. A high-frequency compensating circuit, which prevents the occurrence of