JPH0152801B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an amplifier changeover switch circuit for switching between a recording amplifier and a reproduction amplifier connected to a recording/reproduction head of a video tape recorder, for example.

For example, in a conventional video tape recorder, a recording amplifier that supplies a recording signal to a recording/playback head and a playback amplifier (so-called head amplifier) to which a playback signal from the head is supplied are connected via a transformer 3 as shown in FIG. and is connected to the recording/reproducing head 1. That is, the recording/reproducing head 1
Since it is provided on the rotating body of the rotary head device, it is connected to the primary coil 4 of the transformer 3 via the rotary transformer 2, and a switch is connected to the two secondary coils 5 and 6 of the transformer 3, respectively. The input terminal of the head amplifier 9 and the output terminal of the recording amplifier 10 are connected via 7 and 8.

In the configuration shown in Fig. 1, the transformer 3 is a so-called step-up transformer, and it is necessary to use two transformers or provide two secondary coils as shown in Fig. 1 for recording and reproduction. There is. Furthermore, switches 7 and 8 as individual parts
recording amplifier 9 and playback amplifier 1.
When integrating 0 etc., the number of external parts is large,
Wiring is troublesome and expensive.

Next, FIG. 2 shows a partially integrated head amplifier circuit, in which the recording/reproducing head 11 is connected to the primary side of a transformer such as the rotary transformer or input transformer, and the secondary coil 12 of this transformer is connected. One end is grounded via a capacitor 13,
The other end is the input transistor 14 of the head amplifier circuit.
is connected to the base of This transistor 14
The collector output of is amplified by an amplifier 15, which is the main part of the head amplifier circuit, and taken out from an output terminal 16. In this case, in order to increase the SN ratio, the emitter of the transistor 14 is grounded, the output of the amplifier 15 is fed back to the base of the transistor 14 via the resistor 17, and this base is grounded via the variable resistor 18. This serves as both a feedback resistor and a damping volume.

However, if a recording amplifier is connected to the configuration shown in FIG. 2, the amplitude of the signal from the secondary coil 12 will be clamped by the base-emitter voltage V _BE of the input transistor 14, which will cause an adverse effect. Even if the head amplifier and recording amplifier are integrated, the connection terminal with the head (external connection pin)
cannot be a common pin, and requires a separate external switch component. Although it is possible to insert and connect a semiconductor switching element or the like between the emitter of the input transistor 14 and the ground, it is impossible to reduce the resistance value between the emitter and the ground to near zero during reproducing operation. Yes, this is not preferable in terms of improving the signal-to-noise ratio.

The present invention has been made in view of these conventional problems, and it is possible to use a common pin for the recording output terminal and the reproduction input terminal when a recording amplifier and a head amplifier (reproducing amplifier) are integrated. can be converted into
The purpose of the present invention is to provide an amplifier changeover switch circuit that can perform switching between a recording amplifier and a head amplifier without deteriorating the signal-to-noise ratio, requires fewer external parts, and is suitable for integration.

Preferred embodiments of the present invention will be described below with reference to the drawings.

FIG. 3 is a circuit diagram showing one embodiment of the present invention. In FIG. 3, the recording/reproducing head 21
is connected to the primary side of the aforementioned rotary transformer or input transformer, one end of the secondary coil 22 of this transformer is connected to the voltage source 23, and the other end is
It is connected to a common terminal 25 for recording signal output and reproduction signal input of the integrated circuit. This input/output common terminal 25 is connected to the base of the input transistor 31 of the reproduction amplifier (head amplifier) and the collector of the output transistor 41 of the recording amplifier. The output stage of this recording amplifier consists of transistor 4
1, 42, 43, etc., and the current of the transistor 41 is controlled according to the current flowing through the transistor 42 from the current source 44 which is a recording signal source. A current is outputted via the input/output common terminal 25, resulting in a current-driven recording signal output. Transistor 42 of this current mirror circuit
A first switch 45 is connected to the collector of the switch. This switch 45 can be easily constructed using a semiconductor switching element.

Next, the emitter of the input transistor 31 of the regenerative amplifier is connected to the collector of a transistor 32, which serves as a constant current source, and is in a direct current floating state. It is connected to a bias voltage source 35 via a second switch 34. This switch 34 can also be easily constructed within an integrated circuit using a semiconductor switching element. Further, the base of the transistor 32 is grounded via a diode 36 and a resistor 37. Here, a voltage source 39 is connected to the emitter of the input transistor 31 via a resistor 38, and when the switch 34 is turned off to cut off the current of the transistor 32 which serves as a current source, the input transistor 31 is completely turned off. It is preferable to do so. Further, the emitter of the input transistor 31 is grounded via the capacitor 26 to short-circuit the input transistor 31 in an alternating current manner, thereby improving the signal-to-noise ratio. The collector output of this input transistor 31 is connected to the output terminal 28 via the transistor 27.
taken from.

In the amplifier changeover switch circuit having the above configuration, during recording, the first switch 45, the second switch 45,
34 are both turned off. At this time, transistor 4
The current mirror circuit consisting of transistors 1, 42, and 43 operates normally, and the collector current of transistor 41 is supplied to input/output common terminal 25. Further, since the transistor 32 serving as a current source is turned off, the input transistor 31 is turned off. Here, when the voltage of the voltage source 39 is V ₁ and the voltage of the voltage source 23 connected to one end of the coil 22 is V ₂ , V ₁ >
By setting the respective voltages V ₁ and V ₂ so as to satisfy the condition of V ₂ , the input transistor 31 is reliably cut off and mutual interference between the recording amplifier system and the reproduction amplifier system is prevented. Furthermore, the voltage of the voltage source 39
As V ₁ , output transistor 4 of the reproduction amplifier
From the maximum voltage of the collector output of transistor 3
It is preferable to set the level higher than the voltage level obtained by subtracting the base-emitter voltage V _BE of 1.

Next, during playback, the first and second switches 45,
34 are both turned on. At this time, since the collector of the transistor 42 of the current mirror circuit is grounded, all current from the current source 44, which is the recording signal source, flows through the switch 45, and no output appears on the collector of the output transistor 41. do not have. Further, a bias voltage from a bias voltage source 35 is supplied to the base of the transistor 32,
This transistor 32 operates as a constant current source. Therefore, the emitter of the input transistor 31 is biased by being connected to a constant current source, and is shot by the capacitor 26 in terms of alternating current, thereby improving the SN ratio.

As is clear from the above explanation, according to the amplifier changeover switch circuit according to the present invention, the output transistor of the recording amplifier is set as the collector output, and the emitter of the input transistor of the playback amplifier is connected to a current source, so that the DC Since the terminal is floating and short-circuited in an alternating current manner by a capacitor, the reproduction input and recording output can be shared by one input/output terminal, and deterioration of the SN ratio can also be prevented. Further, the first switching means for controlling on/off the output transistor of the recording amplifier and the second switching means for controlling the on/off of the current source connected to the emitter of the input transistor of the reproduction amplifier are integrated circuits. It can be easily constructed using semiconductor switching elements, and not only is the circuit configuration suitable for integration, but the number of external parts is minimized, and manufacturing is easy and costs can be reduced. Furthermore, by connecting the emitter of the input transistor of the regeneration amplifier to a voltage source, the input transistor is completely shut off when switching to the regeneration mode.

Here, Figure 4 shows an amplifier changeover switch circuit with the above configuration as an actual rotary two-head type VTR.
FIG. 2 is a block diagram showing a specific example applied to a recording/reproducing amplifier system of a video tape recorder.

In this Fig. 4, the same reference numbers are used for parts corresponding to those in Fig. 3 described above, respectively.
Subscripts A and B are attached to each signal system of the two recording and reproducing heads 21A and 21B, respectively. The two recording and reproducing heads 21A and 21B are connected to the VTR.
Rotating drum (not shown) of a rotating head device, etc.
They are installed at angles of 180° and are connected to the primary coils of the rotary transformer, input transformer, etc., respectively. At one end of each of the secondary coils 22A, 22B of each of these transformers, amplifier changeover switch circuits 50A, 5 as shown in FIG.
The input/output common terminals 25A and 25B of 0B are connected to each other, and the other ends of the secondary coils 22A and 22B are commonly connected to the voltage source 23. The amplifier changeover switch circuits 50A and 50B are
Because they have similar configurations and perform similar operations,
To explain only one circuit 50A, the input/output common terminal 25A is connected to the reproduction head amplifier section 30A and the recording amplifier section 40A via a virtual changeover switch _SA . The reproduction head amplifier section 30A includes the transistors 31 and 27 shown in FIG.
The constant current source connected to the emitter of the transistor 31 turns on and off the switch 34, thereby controlling the operation of the amplifier itself. The recording amplifier section 40A includes the transistors 14 to 4 shown in FIG.
When the switch 45 connected between the recording signal input terminal and the ground is off, the operation of the amplifier itself is on;
When is on, the amplifier itself is turned off. These switches 45, 34 correspond to the virtual changeover switch S _A , for example, switches 45, 34.
When both are off, this corresponds to the fact that the selector switch S _A is switched to the selected terminal R. At this time, the recording signal from the input terminal 47 is transmitted to the recording amplifier section 4.
It is amplified at 0A and output from terminal _25A via changeover switch SA. Also switch 45, 34
When both are on, it corresponds to the changeover switch S _A being switched to the selected terminal P, and the terminal 25 is connected to the selected terminal P.
The playback signal from A is input to the playback head amplifier section 30A via the changeover switch S _A , and output to the output terminal 2.
8A. The reproduced output signal from the output terminal 28A is an RF amplifier (high frequency amplifier).
54A and is supplied to the selected terminal a of the changeover switch 55. Here, the switching control signal of the switching switch 55 supplied to the terminal 56 is a switching control pulse for switching to the heads 21A and 21B that come into sliding contact with the magnetic tape as the rotating drum or the like rotates. While in sliding contact with the tape, the changeover switch 55 is controlled to be connected to the selected terminal a side. The output signal from the changeover switch 55 is sent to the buffer amplifier 57.
It is taken out from the output terminal 58 via.

Here, when the VTR is switched to the recording mode, the signal to be recorded (recording signal 5) is supplied to the input terminal 47, and the recording mode command signal Rec is sent to the switching signal generation circuit 5 via the terminal 52.
1 and is not in pause mode, that is, when pause mode command signal Pause is “0”,
A recording side switching control signal is generated from the switching signal generating circuit 51, and the switching control signal controls switching of the switching switch S _A (and S _B ) to the terminal R side.
This corresponds to each switch 45,
This corresponds to turning off all 34. At this time, the recording amplifier section 40A (and 40B) is connected to the terminal 47.
generates sufficient current for magnetic recording by amplifying the recording signal supplied to the secondary coil 22A (and 22B) via the changeover switch S _A (and S _B ).
send to As a result, a current is generated in the corresponding primary coil, a recording current flows through the coil of the head 21A (and 21B), a magnetic field is generated, and a recording signal is recorded on the magnetic tape.

Next, when the VTR is switched to the playback mode, the recording mode command signal Rec at the terminal 52 is "0", and the pause mode command signal
When Pause is “0”, the switching signal generation circuit 51
A reproduction side switching control signal is generated from. This switching control signal controls the switching switch S _A (and S _B ).
is controlled to switch to the terminal P side. This corresponds to turning on both switches 45 and 34 in the original circuit of FIG. At this time, the signals recorded on the magnetic tape are picked up by the head 21A (and 21B), supplied to the input/output terminal 25A (and 25B) via the transformer, and then sent to the input/output terminal 25A (and 25B) via the changeover switch S _A (and S _B ). and is input to the reproduction head amplifier section 30A (and 30B).

This reproduction head amplifier section 30A (and 30B)
Since it is the first stage of the VTR's playback circuit, it has a large effect on image quality. i.e. frequency characteristics, S.N.
In order to improve the electrical characteristics such as ratio and distortion rate, the circuit configuration of the reproduction head amplifier section 30A (and 30B) should be designed to reduce noise generation as much as possible, set the current value of the first stage amplifier transistor, and adjust the input impedance of the amplifier. It is important to set the resonance point of the resonant circuit made up of the total impedance of the head amplifier as high as possible in the frequency range.

Such a reproduction head amplifier section 30A (and 3
The reproduction signal taken out from terminal 28A (and 28B) from 0B) is amplified by high frequency amplifier 54A (and 54B), supplied to buffer amplifier 57 via changeover switch 55, amplified, and reproduced from output terminal 58. I get a signal.

As described above, the amplifier changeover switch circuit shown in FIG. 3 is preferably applied to the circuit configuration for switching between the recording mode and the playback mode in the recording and playback circuit system of a VTR as shown in FIG. It is.

Note that the present invention is not limited to the above embodiments, and various changes can be made without departing from the gist of the present invention.

[Brief explanation of drawings]

FIG. 1 and FIG. 2 are circuit diagrams showing different conventional examples. FIG. 3 is a circuit diagram showing one embodiment of the present invention, and FIG. 4 is a block circuit diagram showing a specific example in which the embodiment is applied to a recording/reproducing circuit system of a VTR. 21... Recording/reproducing head, 23, 39... Voltage source, 25... Input/output common terminal, 26... Capacitor, 31... Input transistor of playback amplifier, 3
2...Transistor serving as a current source, 34...Second
switch, 35... bias voltage source, 41...
Recording amplifier output transistor 45...first switch.

Claims (1)

[Scope of Claims] 1. In an amplifier changeover switch circuit that switches between a recording amplifier and a reproduction amplifier connected to a signal input/output terminal connected to one end of a secondary coil of a transformer that inputs and outputs signals to and from a recording and reproduction head. , a first reference voltage source for supplying a first reference voltage to the other end of the secondary coil of the transformer; and a first reference voltage source having an output terminal connected to the signal input/output terminal and between the input terminal and the reference potential. a current-driven recording amplifier to which a first switching means is connected and which is controlled on and off in accordance with the off-on operation of the first switching means; an input terminal is connected to the signal input/output terminal; a reproduction amplifier from which a reproduction signal is extracted from an output terminal and a constant current source connected to a second output terminal; and an AC grounding capacitor connected between the second output terminal of the reproduction amplifier and a reference potential. and a second reference voltage source that supplies a second reference voltage to the second output terminal of the reproduction amplifier via a resistor, and the constant current connected to the second output terminal of the reproduction amplifier. a second switching means for controlling the source on and off; and a second switching means supplied with a recording control signal for turning off the first and second switching means in order to supply a recording signal to the head via the recording amplifier during recording. , switching signal generating means for turning on the first and second switching means in order to supply the reproduction signal from the head to the reproduction amplifier during reproduction; and switching from the switching signal generating means. By the signal
An amplifier changeover switch circuit characterized in that the recording amplifier and the reproduction amplifier are switched by interlocking the first and second switching means to perform a switching operation.