JPH0247124B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an input switching amplifier circuit that selects and extracts a specific input signal from a plurality of input signals.
For example, the present invention relates to an amplifier circuit suitable for signal selection in a head amplifier or the like when amplifying and extracting the reproduced output of a magnetic head.

[Conventional technology]

FIG. 1 shows the configuration of a conventional preamplifier section for an auto-reverse deck. An auto-reverse deck that automatically switches the playback track by reversing the running direction of the recording tape has two magnetic heads, 2A and 2A, for each left and right channel for stereo playback.
2B is installed. Each magnetic head 2A, 2B
The reproduction output of is selected by the operation of relay 4 according to the reproduction track and applied to amplifier 6. In this case, the excitation of the excitation coil 8 of the relay 4 is switched based on the detection of the reversal state by the auto-reverse switch, and its contact 10 is switched to either side of the magnetic heads 2A, 2B. The reproduction input signal selected by switching the relay 4 is amplified by the amplifier 6 and then output from the output terminal 12. Note that an equalizer element 14 is installed in the feedback circuit of the amplifier 6.

[Problem that the invention seeks to solve]

By the way, when a relay is used to select a specific input signal from multiple input signals, it is more expensive than an electronic switch, and because the selection operation is mechanical, the operation is less reliable and unstable. There is something
A separate power supply is required to drive the relay.
As a result, the wiring in the surrounding area becomes complicated, and in addition to the installation of relays, the complexity of the wiring impedes the miniaturization of the device.The magnetism generated when switching operations causes noise, and the timing of switching is delayed. There are disadvantages such as difficulty in adjusting the timing, noise generated by chattering during switching, and the noise causing auditory discomfort.

Therefore, the present invention provides an input switching amplifier circuit which electronically controls the selection of a plurality of input signals to simplify the circuit configuration, reduce the size, and improve reliability, and which also prevents noise from occurring during switching. For the purpose of providing.

[Means for solving problems]

As shown in FIGS. 2 and 3, the input switching amplifier circuit of the present invention includes a plurality of amplifiers (input stage amplifier 22A , 22B) and transistors 76, 78 installed for each of these amplifiers.
an operation switching circuit 24 that configures a differential switch circuit by making the emitters of the amplifiers common through resistors 81 and 82, and selectively switches the operating current of each amplifier to switch its operation; a bias circuit 32 that sets a constant bias voltage to the base of one of the transistors 76 forming the differential switch circuit, and sets a reference potential to each of the signal sources; A potential balancing circuit 30 connects the reference potential point and the feedback point of each of the amplifiers via a resistor 93 and sets the input point of each of the amplifiers and the feedback point to the same potential; a switching control circuit 60 that generates a control voltage for switching the operation of the differential switch circuit; A time constant circuit 59 is added to the base of the other transistor 78 forming the differential switch circuit.

[Effect]

In the input switching amplifier circuit of the present invention, the control voltage generated by the switching control circuit is applied to the time constant circuit to convert it into a control voltage with a gradual level change, and then applied to the operation switching circuit. The operation switching circuit is composed of a differential switch circuit consisting of a transistor installed for each amplifier. When a control voltage with a gradual level change is applied to the base of the other transistor, each transistor of the differential switch circuit is slowly switched into a conductive state and a non-conductive state. In accordance with this switching operation, the operating current for each amplifier also increases and decreases with gradual changes, and as a result of this change in operating current, the operating state of each amplifier is gradually switched, so that the input signal is selected continuously and gradually. The amplified output can be extracted from the active amplifier.

In this case, with a gradual change in the control voltage,
The differential switch circuit that makes up the operation switching circuit uses a resistor to share the emitters of the transistors provided for each amplifier, so the switching operation is slow and the synergistic effect is reduced in response to gradual changes in the control voltage. Since the operation of the differential switch circuit itself changes slowly, there is no potential fluctuation when the operation is changed, and it is possible to reliably prevent the occurrence of shock noise during switching.

In addition, regarding operation switching of the differential switch circuit, operation switching can be achieved with only a single control voltage from the switching control circuit in opposition to the bias voltage from the bias circuit for the differential switch circuit.
The switching control circuit and the time constant circuit can be simple ones, and the number of terminals can be reduced by unifying the control input terminal on the input switching amplifier side, so the configuration can be simplified and it can be easily integrated into an IC.

Then, the output voltage of the bias circuit is set as a reference potential at the input point of each amplifier to which the input signal is applied, and the input point and feedback point of each amplifier are connected via a resistor. By setting the feedback points at the same DC potential, the influence of disturbance noise is avoided, and the generation of a transient potential difference when the power is turned on is suppressed, thereby preventing abnormal noise from occurring.

〔Example〕

Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.

FIG. 2 shows an embodiment of the input switching amplifier circuit of the present invention, and this embodiment shows the configuration of a preamplifier section for an auto-reverse deck.

In FIG. 2, an input switching amplifier 20 selects a particular input signal from a plurality of signal sources.
is an input stage amplifier 22A, 2 for each plurality of input signals.
2B, and an operation switching circuit 24 that controls the operation of these amplifiers 22A and 22B by switching the operating current.

The output of the amplifier 22A or 22B switched to the active state by the operation switching circuit 24 is applied to the middle stage amplifier 26 and amplified, and its amplified output is applied to the output stage amplifier 28 and amplified.

A potential balancing circuit 30 is installed on the input side of the amplifiers 22A and 22B to suppress potential fluctuations when the power is turned on and prevent the generation of pop sounds. An additional bias circuit 32 is installed.

The circuit portion surrounded by the dashed line constituting the input switching amplifier 20 is composed of one semiconductor integrated circuit, and the input terminals 34A and 34B are connected to one terminal of the magnetic heads 36A and 36B as signal sources. are individually connected, and these magnetic heads 36
The other terminals of A and 36B are collectively connected to the bias output terminal 38 of the bias power supply circuit 32.

That is, the DC voltage generated at the bias output terminal 38 is applied to the magnetic heads 36A, 3 as a signal source.
6B is set as a reference potential.

Further, a gain determining element 42 is connected between the feedback terminal 40 provided on the input side of the amplifiers 22A, 22B and the bias output terminal 38, and the feedback terminal 4
0 and the output terminal 44, an equalizer element 4
6 is connected. The gain determining element 42 is composed of a series circuit of a resistor 48 and a capacitor 50,
Further, the equalizer element 46 is composed of resistors 52 and 54 and a capacitor 56. Further, the output signal taken out from the output terminal 44 is applied to a tone control circuit 57.

The control input terminal 58 of the operation switching circuit 24
, a time constant circuit 5 converts the level change of the control signal from the switching control circuit 60 into a gradual control signal.
9 is connected, and a control signal converted into a voltage with a gradual level change by this time constant circuit 59 is applied to the control input terminal 58.

With the above configuration, the switching control circuit 60
When a control signal is given to the time constant circuit 59 from
This control signal is converted by a time constant circuit 59 into a ramp function voltage or the like whose level changes slowly and smoothly. Further, this control signal is applied to the control input terminal 58, and based on this, the operating current of each amplifier 22A, 22B is changed to the operation switching circuit 24.
As a result, either one of the amplifiers 22A or 22B is controlled to be in an operating state. For example, this control signal may cause the amplifier 22
When the operation is switched from A to amplifier 22B, the input signal from magnetic head 36B is transferred to amplifier 22B.
After being amplified by 2B, it is amplified by the middle stage amplifier 26, and the amplified output is taken out from the output terminal 44 via the output stage amplifier 28 and applied to the tone control circuit 57. These amplification operations are the same even when the operation is switched from amplifier 22B to amplifier 22A.

In this way, the plurality of input stage amplifiers 22A and 22B that amplify the plurality of input signals are electronically switched by the selection operation of the operation switching circuit 24 based on the control signal applied to the single control input terminal 58.

The control signal generated by the switching control circuit 60 is converted into a control signal with a gradual level change by the time constant circuit 59 and is applied to the control input terminal 58.
The switching operation between A and 22B is performed smoothly, and it is possible to reliably prevent the occurrence of shock noise or the like due to sudden switching of the operation.

FIG. 3 shows a specific circuit configuration example in which the input switching amplifier 20 of the input switching amplifier circuit shown in FIG. 2 is constructed from a semiconductor integrated circuit.

In FIG. 3, input stage amplifiers 22A, 22B
includes in common a current mirror circuit consisting of transistors 64 and 66, and a constant current source consisting of a transistor 68 and a resistor 70, and a transistor 72
It is composed of a differential amplifier consisting of A, 74A or 72B, 74B.

The operation switching circuit 24 includes each input stage amplifier 22A,
Transistors 76 and 78 are installed for each 22B, and the emitters of each transistor 76 and 78 are coupled via resistors 81 and 82 installed for each transistor 76 and 78, and a resistor 83 is installed on the base side of transistor 78. A differential switch circuit is constructed. Each of the resistors 81 and 82 reduces the amplification gain of the differential circuit of the transistors 76 and 78, making the switching operation gentle.

The middle stage amplifier 26 includes a transistor 84 and a resistor 8.
6 and a capacitor 88 for high-frequency phase correction, and the output stage amplifier 28 is an emitter follower circuit consisting of a transistor 90 and a resistor 92, and is configured as a buffer circuit.

The potential balancing circuit 30 is composed of a resistor 93 that connects the bias output terminal 38 and the feedback terminal 40. That is, the potential balance circuit 30
By connecting the reference potential point for the magnetic heads 36A, 36B as a signal source and the feedback point of each amplifier 22A, 22B via a resistor 93,
It is configured as a circuit that sets the input point and feedback point of each amplifier 22A, 22B to the same potential to maintain a balanced state. In addition, the bias circuit 32
is a constant current source consisting of transistors 94, 96, a diode 98, and resistors 100, 102, and constant voltage diodes 104, 106, 108, 110,
It consists of a constant voltage source made up of 112 and 114, and an output circuit made of a transistor 116 and resistors 118 and 120.

Then, the time constant circuit 59 has a control input terminal 58
It consists of a resistor 122 and a capacitor 124 connected to. In this embodiment, the switching control circuit 60 is composed of an auto-reverse switch 126 provided on the deck.
DC voltage Vcc is applied to terminal 128 by the conduction of .

Further, a DC voltage Vcc is applied to the power supply terminal 130, and a grounding terminal 132 is connected to the grounding terminal 132 (GND).
connected to a common line. Other terminals 34
A, 34B, 38, 40, and 44 are connected to the same circuits and elements as in the previous embodiment.

With the above configuration, the input terminal 34A receives the input signal shown in FIG. 4A from the magnetic head 36A, and the other input terminal 34B receives the input signal shown in FIG. 4B from the magnetic head 36B. shall be given. The switch 126 detects the running direction of the tape and is turned on or off.

Therefore, when the switch 126 shifts from the off state to the on state, the DC input as a control voltage applied to the terminal 128 instantaneously rises due to the application of the power supply voltage Vcc, but this potential change is caused by the time constant circuit 59. Therefore, it is converted into a voltage with a gradual level change with a constant time constant, and is applied from the control input terminal 58 to the base of the transistor 78 as a control voltage with a level change shown in C in FIG.

By the way, the transistor 7 of the operation switching circuit 24
8, the operating current flowing through the transistor 76 of the switching circuit 24 to the other input stage amplifier 22B is I ₁ , and the operating current flowing through the transistor 76 of the switching circuit 24 to the other input stage amplifier 22B is I ₂ , the transistor shown in C in FIG. Corresponding to the base potential of 78, I ₁ and I ₂ change as shown in FIG. 4D. In this case, at C in FIG. 4, V ₁ is the diode 110, 112,
From the addition value 3V _F of the forward voltage V _F of 114, the current is
The voltage after subtracting the voltage drop across the resistor 81 (resistance value R ₈₁ ) due to conduction of I ₁ and I ₂ {3V _F −R ₈₁ (I ₁ + I ₂ )},
V ₂ is 3V _F , V ₃ is 3V _F plus the voltage drop across resistor 82 (resistance value ₈₂ ) due to the flow of currents I ₁ and I ₂ {3V _F + R ₈₂ (I ₁ + I ₂ )}, Also, V ₄ is the control input terminal 5
8 is the saturation voltage of the capacitor 124 applied to the capacitor 124.

In this way, the base potential of the transistor 78 is
It can be seen that the control is performed in response to control inputs applied by resistors 81 and 82 connected in series to the emitters of transistors 76 and 78. Therefore, in controlling the base potential of the transistor 78, the base potential changes from V=0 (GND) through V ₁ and V ₂ to V ₃
By reaching , the current I ₁ gradually increases and the current I ₂ gradually decreases.

When the current I ₂ is cut off, the input stage amplifier 22B stops operating, and when the current I ₁ flows, the input stage amplifier 22A becomes active. As a result, the output terminal 44 has a current as shown in FIG. 4E.
Input signal A changes from input signal B to input signal A as I ₁ and I ₂ increase or decrease.
The amplified output is taken out by being gently switched to .

Further, when the switch 126 shifts from the on state to the off state, the base potential of the transistor 78 changes to the capacitor 12 through the control input terminal 58.
Due to the discharge at step 4, the potential shifts to zero with the level change shown at C in FIG. Therefore, the currents I ₁ , I ₂ change in an opposite relationship to the changes in the currents I 1 , I ₂ shown in FIG.
I ₂ is increased or decreased, the input signal A is gently switched to the input signal B, and its amplified output is taken out.

In this way, the control voltage applied to the terminal 128 upon switching of the switch 126 is controlled by the time constant circuit 59.
Since the voltage is converted into a smooth level change and applied to the base of the transistor 78, the operating currents I ₁ and I ₂ of each input stage amplifier 22A and 22B increase and decrease smoothly, suppressing potential fluctuations during switching. As a result, the desired input signal A or B can be selected, amplified, and output without generating abnormal sounds. In particular, when increasing or decreasing the currents I ₁ and I ₂ , the composite value of the currents (I ₁ + I ₂ ) is always constant and there is no increase or decrease in the overall current, so it is possible to reliably prevent abnormal sounds such as pops. .

Note that the output of the bias circuit 32 is supplied from the bias output terminal 38 to each magnetic head 36A, 36B and the feedback terminal 40, so that both the input system and the feedback system are placed in a floating state at the same mode potential.
Moreover, since the input signal is amplified differentially, the influence of disturbance noise is effectively prevented.

〔Effect of the invention〕

As explained above, according to the present invention, the operation of the installed amplifiers is selectively and gradually increased/decreased for each input signal, and the operation of each amplifier is gradually switched, thereby eliminating the inconvenience caused when using relays. At the same time, it is possible to reliably prevent the occurrence of noise due to potential fluctuations during switching, and operation switching is performed by using a control voltage that opposes the bias voltage of the bias circuit.
Therefore, the switching control circuit and time constant circuit need only be simple, and the configuration can be simplified by reducing the number of terminals by unifying the control input terminal.
It is easy to integrate ICs, making it possible to downsize the device, and since the input point and feedback point of each amplifier are set to the same potential by a potential balancing circuit, disturbance noise and transients at power-on can be avoided. It is possible to prevent abnormal noise from occurring due to potential changes.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing the configuration of a conventional preamplifier for an auto-reverse deck, FIG. 2 is a block diagram showing an embodiment of the input switching amplifier circuit of the present invention, and FIG.
This figure is a circuit diagram showing a specific circuit configuration example of the input switching amplifier circuit shown in FIG. 2, and FIG. 4 is a diagram showing the operation of the input switching amplifier circuit shown in FIG. 3. 20... Input switching amplifier, 22A, 22B... Input stage amplifier, 24... Operation switching circuit, 59... Time constant circuit.

Claims (1)

[Scope of Claims] 1. A plurality of amplifiers installed for each of a plurality of signal sources to individually amplify each input signal from each signal source, and a common emitter of a transistor installed for each of these amplifiers via a resistor. an operation switching circuit that configures a differential switch circuit by selectively switching the operating current of each amplifier; and one of the transistors forming the differential switch circuit of the operation switching circuit. A bias circuit that sets a constant bias voltage to the base and a reference potential to each of the signal sources; and a resistor is connected between the reference potential point of this bias circuit for each of the signal sources and the feedback point of each of the amplifiers. a potential balancing circuit that connects through the amplifier and sets the input point and the feedback point of each of the amplifiers to the same potential; and a switching circuit that generates a control voltage for switching the operation of the differential switch circuit in opposition to the bias voltage. a control circuit; a time constant circuit that converts the control voltage generated by the switching control circuit into a control voltage with a gradual level change and applies it to the base of the other transistor forming the differential switch circuit of the operation switching circuit; An input switching amplifier circuit equipped with , .