JPH0247123B2 - - 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 for each left and right channel for stereo playback.
A and 2B are installed. Each magnetic head 2A,
The reproduction output of 2B is selected by the operation of relay 4 according to the reproduction track and is applied to amplifier 6. In this case, the excitation of the excitation coil 8 of the relay 4 is controlled based on the detection of the reversal state by the auto-reverse switch, and the contact 10 is connected to the magnetic head 2.
It can be switched to either side A or 2B. The reproduction input signal selected by switching relay 4 is
After being amplified by the amplifier 6, it is taken out 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 relays are used to select a specific input signal from multiple input signals, relays are more expensive than electronic switches, and the selection operation is mechanical, resulting in low operational reliability and failure. It must be stable, a separate power source is required to drive the relay, which complicates the wiring in the surrounding area, and the complexity of the wiring in addition to the installation of the relay hinders miniaturization of the device. Magnetism generated by switching operations causes noise, and
The disadvantages include that it is difficult to optimize the switching timing, and that noise is generated due to chattering during switching, and the noise causes audible discomfort.

SUMMARY OF THE INVENTION An object of the present invention is to provide an input switching amplifier circuit that electronically controls the selection of a plurality of input signals to simplify the circuit configuration, reduce the size, and improve reliability.

[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 sharing the emitters of the amplifiers through resistors 80 and 82, and switches the operation by selectively switching the operating current of each amplifier; 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 (pop noise prevention circuit 30) that connects a reference potential point and a 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 is provided for applying a control voltage to the base of the other transistor 78 constituting the differential switch circuit in opposition to the bias voltage to switch the operation of the differential switch circuit.

[Function] In the input switching amplifier circuit of the present invention, the operation switching circuit is constituted by a differential switch circuit in which the emitters of transistors installed for each amplifier are shared via a resistor, and a differential switch circuit is constituted. By applying a bias voltage from a bias circuit to the base of one transistor, and applying a control voltage from a switching control circuit to the base of the other transistor in opposition to this bias voltage, the operation of the transistors in the differential switch circuit is switched and the amplifier is An operating current is selectively applied to any one of them to operate the amplifier, and an amplified output of an arbitrary input signal is extracted by the operated amplifier.

The differential switch circuit that makes up the operation switching circuit connects the emitters of the transistors provided for each amplifier through a resistor, which slows down the switching operation and allows gradual operation switching. , it is possible to suppress rapid potential fluctuations associated with switching the operation, and to prevent switching noise from occurring.

In addition, regarding operation switching of a differential switch circuit, a constant bias voltage is applied to the differential switch circuit from a bias circuit, and a single control voltage is applied from a switching control circuit in opposition to this bias voltage. Therefore, the switching control circuit can be simple, and the control input terminal for applying the control voltage can be unified, and by reducing the number of terminals, it is easy to integrate the input switching amplifier side into an IC.

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. The input point and the feedback point of the device are at the same DC potential, suppressing the potential difference, avoiding the influence of disturbance noise, and preventing pop noises from occurring when the power is turned on.

〔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 pop noise prevention circuit 30 is installed on the input side of the amplifiers 22A and 22B to prevent pop noises by suppressing potential fluctuations when the power is turned on. An additional bias circuit 32 is installed.

And input switching amplifier 2 surrounded by a dashed line
The circuit section 0 is composed of one semiconductor integrated circuit using a common substrate, and has input terminals 34A, 3
4B has magnetic heads 36A and 36 as signal sources.
One terminal of the magnetic heads 36A and 36B are connected individually, and the other terminals of the magnetic heads 36A and 36B are collectively connected to the bias output terminal 38 of the bias circuit 32. That is, the output DC voltage generated at the bias output terminal 38 is set as a reference potential with respect to the magnetic heads 36A and 36B as signal sources.

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 40
An equalizer element 46 is connected between the output terminal 44 and the output terminal 44 . The gain determining element 42 is composed of a series circuit of a resistor 48 and a capacitor 50, and the equalizer element 46 is composed of a series-parallel circuit of resistors 52, 54 and a capacitor 56.

The control input terminal 58 of the operation switching circuit 24
A switching control signal is applied from the switching control circuit 60 , and an output signal taken out from the output terminal 44 is applied to the tone control circuit 62 .

With the above configuration, the control input terminal 58
When a control input signal is applied from the switching control circuit 60, the operation switching circuit 24 controls the drive current of each amplifier 22A, 22B based on this control input level, and either one of the amplifiers 22A or 22B is put into an operating state. Can be switched. For example, when the switching control circuit 60 is constituted by an auto-reverse switch in the deck section, an inverted signal is generated by turning this switch on and off, and the operation switching circuit 24 is controlled by the DC level of this inverted signal. Therefore,
For example, when the operation is switched from the amplifier 22A to the amplifier 22B using this control input, the reproduction output of the magnetic head 36B is amplified by the amplifier 22B and then
It is further amplified by the middle stage amplifier 26, and this amplified output is output from the amplifier 28 to the output terminal 44 and input to the tone control circuit 62. The same applies 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 dedicated control input terminal 58. This eliminates the need for a relay, reduces the number of external components, reduces costs, simplifies the circuit configuration, improves the reliability of switching operation, and reduces noise caused by magnetic leakage during switching when using a relay. The occurrence of such problems can be prevented.

Furthermore, since the output of the bias circuit 32 is given to the magnetic heads 36A, 36B and the feedback terminal 40, the input system and the feedback system are placed in a floating state at the same potential, and as a result, noise mixing can be prevented. At the same time, a coupling capacitor becomes unnecessary, and the reduction time constant of the input switching amplifier 20 can be improved.

Next, FIG. 3 shows a specific example of the circuit configuration when the input switching amplifier 20 is configured with an IC.

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

The operation switching circuit 24 includes transistors 76 and 78 for each of these input stage amplifiers 22A and 22B.
The emitter of each transistor 76, 78 is connected to a resistor 8 installed for each transistor 76, 78.
0 and 82, and a resistor 83 is placed on the base side of the transistor 78 to form a differential switch circuit. Each resistor 80, 82 lowers the amplification gain of the differential circuit of transistors 76, 78, making the switching operation gradual.

The middle stage amplifier 26 includes a transistor 84 and a resistor 86.
and a capacitor 88 for high-frequency phase correction, and the output stage amplifier 28 includes a transistor 9
0 and a resistor 92, the emitter follower circuit is configured as a buffer circuit.

The pop noise prevention circuit 30 has a bias output terminal 3
8 and a resistor 93 connected to the feedback terminal 40 constitute a potential balancing circuit. That is, the pop sound prevention circuit 30 uses the magnetic head 3 as a signal source.
Reference potential points for 6A and 36B and each amplifier 22
By connecting the feedback points of amplifiers A and 22B via a resistor 93, a circuit is formed in which the input point and feedback point of each amplifier 22A and 22B are set to the same potential. The bias circuit 32 also includes transistors 94, 96, a diode 98, and a resistor 100.
A constant current source consisting of 102 and a constant voltage diode 1
04,106,108,110,112,114
The output circuit includes a constant voltage source consisting of a transistor 116 and resistors 118 and 120.

Then, the collector supply DC voltage Vcc is applied to the power supply terminal 122, and the ground terminal 124
is connected to the grounding (GND) common line. Other terminals 34A, 34B, 38, 40, 44, 5
8 is connected to the same element or circuit as in the previous embodiment.

With the above configuration, when the power is turned on, the DC voltage Vcc is applied to the power supply terminal 122, and the fourth
An input signal as shown in A in the figure is applied, and a fourth
Assume that an input signal as shown in B in the figure is given.

It is also assumed that the switching control circuit 60 is constituted by an auto-reverse switch 126 provided in the deck section, and that a DC voltage Vcc is applied to the base of the transistor 78 via this switch 126.

When the switch 126 is open, the base potential of the transistor 78 will be maintained at zero potential, as shown at _d1 in FIG. 4D. At this time, the operation of the transistor 78 is stopped, and the base of the transistor 76 is connected to the diodes 110, 112, 11 of the bias circuit 32.
Since a constant bias potential is set by the sum of the forward voltages _VF of 4 and _3VF , the transistor 76 is in an operating state. Therefore, transistors 72A and 74A stop operating, and
Transistors 72B and 74B are transistors 76
Since the drive current flows through the magnetic head 36B, the input signal from the magnetic head 36B is input to the amplifier 2.
2B and is amplified, and after this amplified output is amplified by the output stage amplifier 28, the amplified output is taken out from the output terminal 44. In C of FIG. 4, C ₁ represents this output state.

Further, when the switch 126 detects the reversal of the tape and turns on, the base potential of the transistor 78 increases in accordance with the DC voltage Vcc. In this case, if it rises like a ramp function as time t passes, as shown by d ₂ in D in FIG. 4, then since the base potential of transistor 76 is at _3V is 3
When V ₁ approaches V _F , current flows through transistor 78 and both transistors 76 and 78 are activated.

The base potential of the transistor 78 is V ₂ (=3V _F )
When , the currents of both transistors 76 and 78 become the same. Therefore, as shown at _C2 in FIG. 4, a composite value of both input signals appears at the output terminal 44.

Then, when the base potential of the transistor 78 shifts to a value V ₃ higher than the base potential of the transistor 76, only the transistor 78 becomes in the operating state, and the transistors 72A and 74A are in the operating state because the drive current flows instead of the transistors 72B and 74B. The input signal A in FIG. 4 is selected and amplified as shown in C ₃ in FIG. 4.

In this way, the drive currents of the input stage amplifiers 22A and 22B are controlled by the switching operation of the operation switching circuit 24 based on the DC level of the switching control input, and a highly reliable switching operation is obtained, and a predetermined input signal output is amplified. can be output. In particular, since the operation switching circuit 24 is configured as an internal circuit of the IC together with the amplifiers 22A and 22B, rather than as an external circuit such as a relay, it is extremely small and does not require the trouble of wiring unlike a relay circuit. , can be configured simultaneously with each amplifier under the same conditions without magnetic generation or the like. For this reason,
Excellent effects such as cost reduction by reducing the number of parts, prevention of magnetic noise, elimination of the need for power supply for relays, and speeding up of switching can be obtained.

Further, 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, and the input system and the feedback system are placed in a floating state at the same phase and the same potential, and each input Since the stage amplifiers 22A and 22B are constituted by differential amplifiers, the influence of disturbance noise is effectively prevented. In particular, the resistor 93 prevents the generation of a potential difference in the gain determining element 42 when the power is turned on, and effectively prevents the generation of pop sounds.

〔Effect of the invention〕

As explained above, according to the present invention, the operation of the installed amplifier is selectively and gently switched for each input signal to extract an arbitrary input signal, which reduces the reliability of operation switching when using a relay. Eliminates inconveniences such as noise caused by magnetic leakage,
In addition to simplifying the circuit configuration and downsizing the device, the amplifier operation can be easily switched using a single control input, making it easier to simplify the control input side and use an IC. And, moreover,
Since the operation of the amplifier is switched gradually, it is possible to suppress potential fluctuations during operation switching, prevent noise generation during switching, and achieve highly reliable signal switching.In addition, the bias circuit for the signal source can be used as a reference. In addition to setting the potential, the input point and feedback point of each amplifier are set to the same potential by the output voltage of the bias circuit through a potential balancing circuit, which eliminates abnormal noise due to disturbance noise and transient potential changes when the power is turned on. can be prevented from occurring.

[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, 76, 78... Transistor, 80, 82... Resistor.

Claims (1)

[Claims] 1. A plurality of amplifiers installed for each of a plurality of signal sources to individually amplify input signals from each signal source, and emitters of transistors installed for each of these amplifiers being shared via a resistor. an operation switching circuit that configures a differential switch circuit by selectively switching the operating current of each of the amplifiers, and a base of one of the transistors forming the differential switch circuit of the operation switching circuit; a bias circuit that sets a constant bias voltage for each of the signal sources and a reference potential for 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 the input point and the feedback point of each of the amplifiers to the same potential; and a potential balancing circuit that connects the input point of each of the amplifiers and the feedback point to the same potential; An input switching amplifier circuit comprising: a switching control circuit that applies a control voltage to switch the operation of a switch circuit; and an input switching amplifier circuit.