JPH0470104A - Wide-band amplifier - Google Patents

Abstract

PURPOSE:To shorten time for setting operations with simple circuit configuration by connecting a feedback resistor between the gate terminal of the input FET of an initial-step amplifier and the input terminal of a connected amplifier. CONSTITUTION:A feedback resistor R1 is connected between the base terminal of an input transistor Q5 of a second-step amplifier 32 and the gate terminal of a J-FET Q1. Therefore, the R1 executes a damping operation, and the gate terminal bias voltage of the J-FET Q1 is supplied by V1 and R1 as the base terminal bias circuit of the input transistor Q5 of the second-step amplifier 32. Since the gate terminal bias current of the J-FET hardly flows, there is no potential drop caused by an R10, and the regular bias voltage is supplied to the base terminal of the Q5. Accordingly, the bias voltage of the gate terminal of the Q1 is fixed regardless of the state of an operation control switch, and a coupling capacitor C1 is not charged/discharged. Thus, time for setting the operations of a reproducing amplifier can be shortened with simple circuit configuration.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a wideband amplifier, and more particularly to a wideband amplifier that amplifies a reproduced signal from a magnetic head whose operation is switched on and off at high speed.

[Conventional technology]

As shown in FIG. 3, the recording/reproducing circuit of the DAT magnetic recording/reproducing device includes a magnetic head 1. Rotary transformer 2. Reproduction signal amplifier 3. Waveform equalizer 4. Recording signal amplifier5. It is composed of a digital interface 6. The reproduced signal amplifier 3 includes a first stage amplifier 31. Second stage amplifier 32.
Head changeover switch 33. and third stage amplifier 34
It is made up of. Reproduction signal amplifier 3. Waveform equalizer 4. Recording signal amplifier5. Digital interface 6
is built into the recording/reproducing circuit C7.

The rotary transformer 2 is coupled to the input of the reproduced signal amplifier 3 through a coupling capacitor C1. The first stage amplifier 31 and the second stage amplifier 32 are coupled with a coupling capacitor C.
Combined by 2.

Since the recording/reproducing circuit handles minute signals from the magnetic head, the circuit area is usually reduced and housed in a shield case in order to block unnecessary radiation noise from other sources.

The transmission system from the magnetic head 1 to the output of the reproduced signal amplifier 3, as shown in FIG. The stray capacitance C37 and the feedback resistor R1 exhibit a resonance characteristic, and by actively utilizing this resonance characteristic, the reproduced signal can be amplified with a good S/N ratio. This means that the noise generated in the reproduced signal amplifier 3 is
This is because the resonance is equivalently reduced.

Further, since an excessively large recording signal is input to the reproduction signal amplifier during recording, in order to protect this, an operation SSO switch is provided to turn off the first stage amplifier during recording.

Here, the track format of DAT is shown in Figure 5(a).
The recording track of the DAT shown in FIG. 5A is divided into a plurality of recording areas, and various signals are recorded in each divided area.

A PCM signal area (PCM) is arranged in the center of the recording track, and ATF areas (ATFl, ATF
-2), and a subcode area (SUB
-1,5UB-2) are arranged.

In the PCM signal area, digital data obtained by adding an error correction code to PCM data obtained by converting a musical tone into PCM is recorded.

The subcode area contains information for searching programs and specific positions within programs, such as the start ID indicating the start position of a program (song, etc.), the program number indicating the number of the program, and the time from the start position of the program. Search information etc. are recorded.

A tracking control signal for causing the magnetic head to correctly scan the recording track during reproduction is recorded in the ATF area.

In this way, various signals are recorded in divided areas, so it is possible to perform a so-called post-recording operation in which the PCM signal or subcode is rewritten individually, as shown in FIG. 5(b). .

During dubbing, the ATF
Tracking control of the magnetic head is performed by reproducing the area tracking control signal. That is, while the magnetic head traces the -th recording track, the operation of the reproduction signal amplification circuit is switched on and off at high speed by the operation control switch.

Here, the operation control switch performs its operation by cutting off the idle current of the input transistor of the first stage amplifier. When the idle current of the input transistor is cut off, the bias voltage at each node of the first stage amplifier changes, so
During control from OFF to ON, the terminal voltage of the coupling capacitor connected to the rotary transformer also changes, and a transient response accompanying charging and discharging of the coupling capacitor appears in the output of the reproduction signal amplifier as an operation settling time.

Therefore, if the settling time of the playback signal amplifier from off to on is longer than the time from when the magnetic head finishes tracing the recording area until it traces the beginning of the playback area, the magnetic head will Even though the correct area is being traced, the playback signal amplifier has not finished settling its operation, so a correct playback signal cannot be obtained.

In order to shorten this operation settling time, it is possible to use a coupling capacitor with a low capacitance value, but this increases the low cutoff frequency of the reproduced signal amplification circuit, which causes problems in transmitting the reproduced signal. becomes. Therefore, JP-A-64
As disclosed in Japanese Patent No. 43866, a constant voltage bias circuit was added to the first stage amplifier to prevent the bias voltage of each node from changing.

[Problem to be solved by the invention]

Here, as data recording and reproducing speed increases, it is necessary to widen the band of the recording and reproducing circuit. In order to transmit the reproduced signal without distortion, the above-mentioned resonance frequency must be outside the signal frequency band. Therefore, in order to widen the bandwidth of the regenerative transmission system, it is necessary to reduce the input capacitance of the first stage amplifier of the regenerative signal amplifier.

Therefore, it is conceivable to use an amplifier externally attached to the conventional recording/reproducing circuit IC and having a low input capacitance device at the input stage as the first stage amplifier of the reproduced signal amplifier. This results in
It is possible to reduce the input capacity of the reproduced signal amplifier without impairing its function as a conventional recording/reproducing circuit IC. If an FET is used as this input stage device, it is possible to relatively easily achieve a reduction in the amount of input input.

However, in order to implement the constant voltage bias circuit for shortening the operation settling time using an external circuit as described above, the circuit scale becomes large.

An object of the present invention is to provide a wideband amplifier with a simple circuit configuration, short operation settling time, and low input capacitance.

[Means to solve the problem]

In order to achieve the above object, the present invention provides an FET input first stage amplifier connected to a signal source by a first coupling capacitor and having a switch for controlling on/off of amplification operation, and a second coupling capacitor connected to the first stage amplifier. In a multi-stage broadband amplifier consisting of amplifiers with input bias circuits cascaded by ring capacitors, a feedback resistor was connected between the gate terminal of the input FET of the first stage amplifier and the input terminal of the subsequent amplifier.

[Effect]

Since the feedback resistor was connected between the gate terminal of the input FET and the input terminal of the subsequent amplifier, the input FET of the first stage amplifier
The gate terminal bias voltage of the input FET is supplied from the input bias of the subsequent amplifier via the feedback resistor, and even when the first stage amplifier is off, the gate terminal bias voltage of the input FET is constant at the same potential as when the first stage amplifier is on, and from off to on. Since charging and discharging of the first coupling capacitor does not occur when the transition occurs, the settling time of the reproduction signal amplifier from off to on can be shortened.

〔Example〕

Embodiments of the present invention will be described below with reference to FIG.

In FIG. 1, 1 is a magnetic head, 2 is a rotary transformer, Ql is a J-FET, Q2 to Q6 are transistors,
R1-R11 are resistors, C1-C5 are capacitors, and 7 is a recording/reproducing circuit IC (Hitachi recording/reproducing circuit ICHA1213).
3) and 32 are second-stage amplifiers of the reproduction signal amplifier 3 built in the recording and reproduction circuit IC7.

Q1-Q4. Di, R1-R7, C3, C4 makes J-
It constitutes a first stage amplifier 30 which receives FETQI as an input.

Also, Q5. Q6. Il, R8-R11, Vl.

V2 is the input stage of the second stage amplifier 32.

A gate terminal of J-FET QI of the first stage amplifier 30 is coupled to the rotary transformer 2 through a coupling capacitor C1. The output of the first stage amplifier 30 is input to the base terminal of the input transistor Q5 of the second stage amplifier 32 via the coupling capacitor C2.

Q2 and R4 of the first stage amplifier 30 are operation control switches that control the operation of the first stage amplifier 30. That is, Q
The operation of the first stage amplifier 30 is controlled by cutting off the drain-source current of l.

As described above, the feedback resistor R1 damps the resonance characteristics caused by the inductance of the rotary transformer 2 seen from the first-stage amplifier 30, the input capacitance (not shown) of the first-stage amplifier 30, and the stray capacitance (not shown) of wiring, etc. It is for. This feedback resistor R1 is connected to the second stage amplifier 32.
The base terminal of the input transistor Q5 and the J-FET QI
is connected between the gate terminals of the Therefore, R1 is
While performing the damping operation, the gate terminal bias voltage of the J-FET QI is applied to the base terminal bias circuit Vl of the input transistor Q5 of the second stage amplifier 32.

Supplied by R1. Since almost no bias current flows through the gate terminal of the J-FET, there is no voltage drop due to RIO, and a normal bias voltage is supplied to the base terminal of Q5.

According to this circuit configuration, the bias voltage at the gate terminal of Ql is constant regardless of the state of the operation control switch.
There is no charging or discharging of the coupling capacitor C1. Therefore,
This has the effect of shortening the operation settling time of the regenerative amplifier.

FIG. 2 is a diagram showing a recording/reproducing circuit according to the present invention.

The first stage amplifier 30 is externally attached instead of using the first stage amplifier 31 built into the recording/reproducing circuit IC7. Thereby, it is possible to reduce the input capacitance of the reproduced signal amplifier and shorten the operation settling time without losing the function of the recording/reproducing circuit TC7.

〔Effect of the invention〕

According to the present invention, the feedback resistor is connected to the input FET of the first stage amplifier.
The gate terminal of the input FET of the first stage amplifier is connected between the gate terminal of the first stage amplifier and the input terminal of the succeeding amplifier. Also input I”E
The bias voltage at the gate terminal of T is constant at the same potential as when the first stage amplifier is on, and the coupling capacitor connected to the rotary transformer does not charge or discharge, so the bias voltage is constant from OFF to 1. It is possible to shorten the time required for the operation to settle.

In this case, the capacitance value of the coupling capacitor can be arbitrarily selected, and the reproduced signal can be transmitted without increasing the low cutoff frequency of the reproduced signal amplification circuit. Furthermore, there is no need to add a constant voltage bias circuit so that the bias voltage at the coupling capacitor terminal does not change.

[Brief explanation of the drawing]

Figure 1 is a reproduction signal amplification circuit diagram showing an embodiment of the present invention, Figure 2 is a recording/reproduction circuit diagram according to the present invention, and Figure 3 is:
A conventional recording/reproducing circuit diagram, FIG. 4 is a frequency characteristic diagram of a transmission system from a magnetic head to a reproduced signal amplifier output, and FIG. 5 is a DAT track format and an operation mode diagram of a recording/reproducing circuit. 1...Magnetic head 2...Rotary transformer 3
...Reproducing signal amplifier 5...Recording signal amplifier 7...
・Recording/playback circuit diagram C/2

Claims (1)

[Claims] 1. F having a switch for controlling on/off of amplification operation connected to the signal source by a first coupling capacitor;
In a multi-stage wideband amplifier consisting of an ET input first-stage amplifier and an amplifier having an input bias circuit cascade-connected to the first-stage amplifier by a second coupling capacitor, the feedback resistor is connected to the input FET of the first-stage amplifier. A wideband amplifier, characterized in that it is connected between a gate terminal and an input terminal of the subsequent amplifier.