JPH0149046B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention relates to an amplifier circuit used in a tape player or the like to switch reproduction equalizer characteristics depending on the type of tape.

(B) Prior Art Since the high-frequency compensation during recording is different between so-called normal tape and metal tape, it is necessary to switch the high-frequency range of the equalizer characteristics depending on the tape during playback. For example, NAB (National
Association of Broadcasters) characteristics,
The low frequency time constant for both tapes is 3180 μsec, but the high frequency constant is set at 120 μsec for the normal tape and 70 μsec for the metal tape.

FIG. 1 is a circuit diagram schematically showing an example of a conventional amplifier circuit.

In the figure, 1 is an amplifier, 2 is a magnetic head, and resistors R1, R2, R3 and a capacitor C1 form a feedback loop. R4 is the reference resistance, C
2 and C3 are DC blocking capacitors C1.

The reproduction equalizer characteristics are changed by varying the time constant of the feedback loop by operating a mechanical changeover switch SW provided in the feedback loop.

However, for example, in the case of a stereo device, since two of the above-mentioned amplifier circuits are used, the changeover switch SW needs to be equipped with two circuits, which has the disadvantage that the device becomes expensive.

On the other hand, the changeover switch SW is attached to the operation panel of a stereo device, etc., but in order to suppress noise induction and oscillation, it is necessary to shorten the wiring length of the switch SW. Therefore, it also has the disadvantage that there are many restrictions on the installation position of the amplifier circuit.

Furthermore, the stereo device requires four wires to connect to this switch, which is not suitable for miniaturization and cost reduction.

(c) Purpose This invention enables the reproduction equalizer characteristics to be changed at low cost, and there are few restrictions on the installation position.
The main purpose is to provide an amplifier circuit suitable for downsizing devices.

(D) Configuration The amplifier circuit according to the present invention has a plurality of feedback loops determined according to reproduction equalizer characteristics, a buffer amplifier provided in at least one of the feedback loops, and a mechanical changeover switch that operates the buffer amplifier. The main feature is that it is equipped with a control circuit that selects an arbitrary feedback loop by switching with a constant current.

(E) Embodiment FIG. 2 is a block diagram schematically showing the configuration common to the amplifier circuits according to the first invention and the second invention.
The same parts as in FIG. 1 are indicated by the same reference numerals. 2A
and 2B are buffer amplifiers (amplifiers with an amplification factor of 1) to which the output of the amplifier 1 is applied. The operation of the buffer amplifiers 2A and 2B is controlled to be ON/OFF by, for example, switching a constant current for driving each amplifier by a control circuit 3 linked to a tape changeover switch SW. Therefore, when only the buffer amplifier 2A is ON, the output of the amplifier 1 is fed back to the negative terminal of the amplifier 1 via the feedback loop including the feedback resistor R3, so the high frequency time constant of this loop is (R2+R3)・C1. Become. Therefore,
The equalizer characteristics of the amplifier circuit at this time are, for example,
It becomes suitable for normal tape.

On the other hand, when only buffer amplifier 2B is ON,
Since the output of the amplifier 1 is fed back to the negative terminal through a feedback loop that does not include the feedback resistor R3, the high frequency time constant in this case is R2·C1.
Therefore, the equalizer characteristics of the amplifier circuit become suitable for, for example, metal tape.

Next, a more specific embodiment of the first invention will be described.

FIG. 3 is a circuit diagram showing an embodiment of the amplifier circuit according to the first invention. In this figure, the same parts as in FIG. 2 are designated by the same reference numerals. Batsuhua amp 2
A and 2B are equipped with transistors Q1 to Q8 and constitute a so-called current mirror circuit, in which the output signal of amplifier 1 is applied as an input to the base of each transistor Q1, and an inverted signal is applied as a feedback signal to the base of transistor Q2. It will be done.

Control circuit 3 includes a current switch including transistors Q9 and Q10, and a constant current source 31.
There is a changeover switch at the base of transistor Q9.
An H or L level control signal is input in relation to SW. On the other hand, a constant internal bias VB (L level < VB < H level) is applied to the base of the transistor Q10.

R5 and R6 are voltage dividing resistors connected to the changeover switch SW.

Next, the operation of the embodiment having the above-described configuration will be explained.

For example, when using a normal tape, the changeover switch SW is set to ON. Then,
Since the base of transistor Q9 becomes H level, transistor Q9 is turned on and transistor Q1 is turned on.
0 is OFF. Therefore, buffer amplifier 2A
As a result of being driven with a constant current, a feedback loop including feedback resistor R3 is selected.

Therefore, the high frequency time constant of this amplifier circuit becomes a constant (R2+R3)·C1 corresponding to the normal tape.

When switching to metal tape, the changeover switch SW is set to OFF. As a result, transistor Q10 turns on, so
Buffer amplifier 2B is driven with a constant current, and a loop that does not include feedback resistor R3 is selected. Therefore, the high-frequency time constant of the amplifier circuit at this time is R2·C1, and equalizer characteristics suitable for the metal tape can be obtained.

FIG. 4 is a circuit diagram showing an embodiment of the amplifier circuit according to the second invention.

In this figure, the same parts as in FIG. 3 are indicated by the same reference numerals. The amplifier circuit according to the second invention has the same main parts as the first invention, but in particular includes a charging circuit in relation to the control circuit in order to slowly switch the constant current for driving the buffer amplifier. It is characterized by having the following.

That is, a charging circuit including resistors R5 and R6 and a capacitor C4 is connected to the base of the transistor Q9 of the control circuit 3. Since this charging circuit performs charging or discharging in conjunction with switching of the changeover switch SW, switching of the control circuit 3 becomes slow. As a result, the switching of the constant current for driving the buffer amplifiers 2A and 2B becomes gradual, so that the switching between the buffer amplifiers 2A and 2B is performed gradually.

FIG. 5 is a diagram showing changes in the amplification factors of each buffer amplifier when the changeover switch SW is turned on. As is clear from the figure, the buffer amplifier 2A is gradually switched to 2B.

Slow switching between the buffer amplifier 2A and the buffer amplifier 2B in this way means that the impedance (the impedance of the feedback loop seen from both ends of the feedback resistor R3 in the figure) changes continuously as the feedback loop is switched. is equivalent to

In the above embodiment, an amplifier circuit with two feedback loops has been described, but the present invention is not limited to this, and can be modified by adding a control circuit, a buffer amplifier, etc. according to the number of feedback loops. It can be applied to amplifier circuits with any number of feedback loops.

Furthermore, in the case where there are two feedback loops as in the embodiment, one of the buffer amplifiers 2A can be omitted. That is, even if one end of the feedback resistor R3 is directly connected to the amplifier 1, the feedback loop can be selected in the same manner as described above by switching the changeover switch SW.

(F) Effect This invention controls a buffer amplifier by a control circuit that interlocks a mechanical changeover switch, selects an appropriate feedback loop, and switches the reproduction equalizer characteristics, so it is suitable for stereo equipment, etc. Furthermore, by providing two amplifier circuits according to the present invention, the control circuit of each amplifier circuit can be connected in conjunction with one circuit changeover switch. Therefore, unlike when a conventional amplifier circuit is used in a stereo device, there is no need to use two circuit changeover switches, so that reproduction equalizer characteristics can be changed at low cost. Furthermore, since less wiring is required for the mechanical switch, the device can be made smaller.

Furthermore, in the amplifier circuit according to the present invention, since the control circuit is interposed between the feedback loop and the changeover switch, even if the wiring between the control circuit and the changeover switch becomes long, the influence of noise induction caused by this, etc. I never receive it. Therefore, since the installation position of the amplifier circuit is not limited by the installation position of the changeover switch, the degree of freedom in the installation position of the amplifier circuit and further the reliability of the circuit can be improved.

Furthermore, since this invention switches the buffer amplifier using a constant current, oscillations are less likely to occur compared to, for example, selecting an arbitrary feedback loop by switching the voltage of an emitter follower installed in the feedback loop. , it also has the unique effect of stable operation.

Further, according to the present invention, since the components constituting the feedback loop are only resistors and capacitors and do not include mechanical switches, it is easy to integrate the feedback loop into an integrated circuit.

Furthermore, in the second aspect of the invention, since the switching of the buffer amplifiers is performed slowly, there is an effect that no so-called pop sound is generated when switching the feedback loops.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram schematically showing a conventional amplifier circuit, FIG. 2 is a block diagram schematically showing a configuration common to amplifier circuits according to the first invention and the second invention, and FIG. A circuit diagram showing an embodiment of the amplifier circuit according to the invention, FIG. 4 is a circuit diagram showing an embodiment of the amplifier circuit according to the second invention, and FIG.
FIG. 6 is a diagram showing changes in amplification factors of each buffer amplifier when turned on. 1...Amplifier, 2A, 2B...Buffer amplifier, 3
...Control circuit, R1-R3...Feedback resistor, R4...Reference resistor, C1...Compensation capacitor, C2, C3...DC blocking capacitor, C4...Charging capacitor, To
...Output terminal.

Claims (1)

[Claims] 1. A plurality of feedback loops determined according to reproduction equalizer characteristics, a buffer amplifier provided in at least one of the feedback loops, and an arbitrary feedback loop selected by switching the buffer amplifier with a constant current. An amplifier circuit characterized by comprising a control circuit. 2. A plurality of feedback loops determined according to reproduction equalizer characteristics, a buffer amplifier provided in at least one of the feedback loops, and an arbitrary feedback loop by switching the buffer amplifier with a constant current in conjunction with a mechanical changeover switch. An amplifier circuit comprising: a control circuit for selecting the current; and a charging circuit provided in association with the control circuit for slowly switching the constant current.