JP2001043504A - Head amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control a generation factor of distortion and a decrease in an S/N even if characteristic compensation at a subsequent stage is not necessitated, by offsetting amplifier input capacitance according to negative feedback and positive feedback by a balanced amplification means constructed by symmetrically arranging a 1st amplifier for inverse-amplifying one end output of a head and a 2nd amplifier for non-inverse-amplifying the other end output of the head. SOLUTION: A head amplifier 100 is provided with an inverse-amplifying part 10 and a non-inverse-amplifying part 20. The inverse-amplifying and non- inverse-amplifying parts 10, 20 negatively feed back the reproduced outputs vas feedback elements CNF1, RNF1, CNF2, and RNF2 while controlling the output impedances of one end side and the other end side of the head 1 via bias elements Rb1, Rb2, while the amplifiers positively feed back the reproduced outputs via the feedback elements CPF1, CPF2. And, when the positive feedback level is made lower than the negative feedback level by the feedback element CPF2, the input capacitance of the inverse- and non-inverse-amplifying parts 10, 20 can be offset, it is possible to avoid the decreasing of S/N by using high resistance for the bias elements Rb1, Rb2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a head amplifier suitable for use in a magnetic recording / reproducing apparatus such as a VTR.

[0002]

2. Description of the Related Art A magnetic recording / reproducing apparatus such as a VTR utilizes resonance caused by an inductive component L of a head and an input capacitance C of a preamplifier stage including a stray capacitance between lines (hereinafter referred to as head resonance). Then, a technique of equalizing a reproduced signal induced by the head to a desired frequency characteristic in a transmission band is used.

A reproduced signal output from a head amplifier composed of a head and a preamplifier stage has a characteristic compensation in a stage subsequent to the head amplifier unless the frequency amplitude characteristic FL in the required band is monotonous and the group delay characteristic GD is not flat. And it is likely to cause signal distortion. Therefore, in the conventional head amplifier, the circuit constant is determined so that the head resonance frequency does not enter the required band.

(A) First conventional example FIG. 5 is a circuit diagram showing a configuration of such a head amplifier (first conventional example). In this figure, the signal generator SG
Generates a constant magnetic flux by being 1T-coupled through a load resistor Rg. The winding head 1 (hereinafter, abbreviated as head 1) generates an induced electromotive force (reproduction signal) corresponding to a constant magnetic flux generated by 1T coupling. The FET 2 whose source is grounded amplifies the current of the reproduction signal output from the head 1.

[0005] R1 and C1 are gate bias elements.
Transistor 3, which converts the output of FET 2 to a voltage,
The base is grounded to reduce the feedback capacitance from the drain terminal of ET2. R2, R3 and C2 are bias elements, and R4 and R5 are output resistors. Reference numeral 4 denotes an output transistor for reducing the impedance of the amplified signal voltage.

According to the above arrangement, the input capacitance C is C ^SD + C
^DG + | A | · C ^DS , and assuming that the inductive component of the head 1 is L ^K , the head resonance frequency f ⁰ is ππ (L ^K · C)
^1/2 . As a specific example of the characteristic, as shown in FIG. 7, the frequency amplitude characteristic FL monotonically increases / decreases with the peak at the head resonance frequency f ⁰ , while the SN ratio and the group delay characteristic GD are almost equal to the head resonance frequency. made to the characteristics to be a peak at f ^0.

In the first conventional example, a high SN ratio is obtained by utilizing head resonance. Therefore, an fQ compensation circuit for damping head resonance characteristics is provided at a subsequent stage.
Adjustment for flattening the group delay characteristic GD is required. Therefore, in recent years, in order to make such adjustment unnecessary, the second and third damping head resonance characteristics in the head amplifier have been proposed.
Conventional examples have been devised.

(B) Second Conventional Example Next, a second conventional example will be described with reference to FIGS. FIG. 8 is a circuit diagram showing a configuration of a head amplifier according to a second conventional example formed of a cascode type, similarly to the first conventional example. In this figure, circuit elements common to the above-mentioned first conventional example are denoted by the same reference numerals, and description thereof will be omitted.

The head amplifier shown in FIG. 8 differs from the first conventional example shown in FIG. 5 in that a resistor Rd (about 500Ω) is inserted in parallel with the head 1 to dampen head resonance. is there. According to such a configuration, FIG.
As is clear from the characteristics shown in FIG. 0, the group delay characteristics GD are flattened by the damping and the frequency amplitude characteristics FL are also monotonous characteristics as compared with the characteristics of the first conventional example (see FIG. 7). This eliminates the need for characteristic compensation in the subsequent stage.

(C) Third Conventional Example Next, a third conventional example will be described with reference to FIGS. 9 and 11. FIG. FIG. 9 is a circuit diagram showing a configuration of a head amplifier according to a third conventional example formed of a cascode type, similarly to the first conventional example. In this figure, circuit elements common to the above-mentioned first conventional example are denoted by the same reference numerals, and description thereof will be omitted.

The difference between the head amplifier shown in FIG. 9 and the first conventional example shown in FIG. 5 is that the feedback amplifiers C ^NF and R ^NF and the bias resistor Rb are provided, and negative feedback is performed via these components. The purpose is to dampen head resonance. Here, if the feedback capacitance C ^NF is increased, the feedback resistance R ^NF is sufficiently reduced, and the negative feedback level is increased to amplify the electromotive current of the head 1 equivalently, as is apparent from the characteristics shown in FIG. As compared with the characteristics of the first conventional example (see FIG. 7), the group delay characteristic GD becomes flatter and the frequency amplitude characteristic FL becomes monotonous. Therefore, in this case, the characteristic compensation at the subsequent stage is unnecessary. Become.

[0012]

As described above, in the head amplifier according to the first conventional example in which the head resonance frequency does not enter the required band, a mode in which a preamplifier is provided outside the head drum. Then, FIG.
As shown in (a), the configuration is such that the head 1 and the preamplifier stage are coupled by using a rotary transformer RT. In such a configuration, as shown in (b) of FIG. Mutual inductance M and leakage inductance cause the head resonance frequency to decrease. To prevent this, the number of turns of the head 1 must be reduced, and as a result, the number of turns of the head, the turn ratio of the rotary transformer, and the There is a disadvantage that the combination of the input capacitance of the preamplifier stage and the like is limited, and the degree of freedom in design is lost.

In the case where the number of FETs used in the preamplifier stage is switched according to the reproduction speed (for example, double speed reproduction), in such an embodiment, the figure of merit M (mutual conductance gm / gate) of the FETs used in parallel is used. Capacity Cg)
For example, as shown in FIG. 12, if four FETs 2-1 to 2-4 are used in parallel, as shown in FIG. 12, it becomes equivalent to using a 1: 2 step-up transformer ST. As in the above, the head resonance frequency is reduced,
Also in this case, the degree of freedom in design is lost.

On the other hand, the head amplifier according to the second conventional example, in which the head resonance is damped by interposing a resistor Rd in parallel with the head 1, has a problem that the SN ratio is reduced. In the head amplifier according to the third conventional example for damping, the feedback amount changes according to the signal frequency. Therefore, if the feedback level cannot be increased within the required band, the group delay characteristic GD and the frequency amplitude characteristic FL vary. There is a possibility that this may be a factor for generating distortion.

In other words, in other words, the conventional head amplifier can achieve a high S / N ratio if the head resonance frequency does not fall within the required band, but requires characteristic compensation at a subsequent stage. However, there is an adverse effect that the degree of freedom in design is lost. On the other hand, if the characteristic compensation in the subsequent stage is not required, there is a conflicting problem that the SN ratio may be reduced or a distortion may be caused. become.

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and a head amplifier capable of suppressing distortion generation factors and a decrease in SN ratio and maintaining design flexibility while eliminating the need for characteristic compensation in a subsequent stage. It is intended to provide.

[0017]

According to a first aspect of the present invention, there is provided a head amplifier for improving output characteristics using head resonance caused by a head induction component and an amplifier input capacitance. The first in which the output of one end of the head is positively fed back while negatively fed back and amplified in reverse.
And a second amplifier for non-inverting amplification by negatively feeding back the output of the other end of the head while negatively feeding back the output of the head are formed symmetrically. And canceling out the amplifier input capacitance according to positive feedback.

According to a second aspect of the present invention, in a head amplifier in which output characteristics are improved by using head resonance caused by a head induction component and an amplifier input capacitance, one end of the head limited by a high resistance element is provided. A first amplifying unit that inverts and amplifies the output of the head by negatively feeding back the output of the head, and a second amplifying unit that performs non-inverting amplification by feeding back the output of the other end of the head limited by the high resistance element while negatively feeding back the output. And a symmetrical arrangement of the amplifying section, and the balanced amplifying means cancels the amplifier input capacitance according to a positive feedback amount smaller than a negative feedback amount.

According to the present invention, the head output is positively fed back while the head output is negatively fed back, and balanced amplification is performed. Therefore, the amplifier input capacitance is canceled in accordance with the negative feedback and the positive feedback to drive the head resonance frequency out of the required band. It becomes possible. As a result, the fluctuations of the group delay characteristic GD and the frequency amplitude characteristic FL in the required band are reduced, so that it is possible to suppress the cause of distortion. Further, since the head resonance is damped by the negative feedback, the characteristic compensation at the subsequent stage is not required.
Further, since the head output is limited by the high-resistance element, a reduction in the SN ratio can be avoided in a mode in which the head resonance is damped by feedback. In addition, since the head resonance frequency rises due to the cancellation of the amplifier input capacitance, the mode in which the head and the preamplifier stage are coupled using the rotary transformer RT or the FET used in the preamplifier stage depending on the reproduction speed are used. It is possible to flexibly cope with a mode in which the number is switched, and it is possible to freely combine the number of head turns, the turn ratio of the rotary transformer, and the input capacity of the preamplifier stage, and the degree of design freedom is maintained.

[0020]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing a configuration of a head amplifier 100 according to one embodiment. The head amplifier 100 forms a balanced amplifier including an inverting amplifier 10 that inverts and amplifies the output of one end of the head 1 and a non-inverting amplifier 20 that amplifies the output of the other end of the head 1 in a non-inverting manner.

In the inverting amplifier 10, the FET 11 amplifies the current on one end of the head 1, the gate bias resistor R11 of the FET 11, the transistor 12 for converting the output of the FET 11 into a voltage, the bias elements R12, R13, C11, output resistors R14, R15
And the output transistor 13. While the output impedance at one end of the head 1 is limited by the bias resistor Rb1, the reproduction output is negatively fed back through the feedback elements C ^NF1 and R ^{N F1} , while the output impedance is fed through the feedback element C ^PF1. The playback output is fed back positively.

On the other hand, in the non-inverting amplifier 20,
FET 21 that amplifies the current of the other end side output, this FET 11
Gate bias resistor R21, the output of FET21
The transistor 22 for converting to
Bias elements R12, R13, C11, output resistance R2
2, a head including R23 and an output transistor 23
1 with the bias resistance Rb2
While limiting, the feedback element C ^NF2, R^NF2Playback output via
The feedback element C^PF2Playback output via
Positive feedback.

As described above, in the head amplifier 100 in which the inverting amplifier 10 and the non-inverting amplifier 20 are symmetrically arranged to form a balanced amplifier, if the feedback element ^CPF2 has a positive feedback level smaller than the negative feedback level, Since the input capacitance C in the amplifiers 10 and 20 is offset, the head resonance frequency can be driven out of the required band, and if the bias resistors Rb1 and Rb2 are set to high resistance (for example, 10 KΩ or more), the head is returned by feedback. Despite the mode of damping the resonance, it is possible to avoid a decrease in the SN ratio.

FIG. 2 shows the frequency characteristics of such a head amplifier 100. As can be seen from FIG. 2, the SN ratio characteristics are almost the same as those of the first conventional example (see FIG. 7), and the group delay is also shown. The characteristic GD and the frequency amplitude characteristic FL have their peaks shifted to higher frequencies as the head resonance frequency increases as compared with the second conventional example (see FIG. 10) and the third conventional example (see FIG. 11). Therefore, the fluctuation in the required band is reduced, so that the distortion generation factor can be removed.

Further, since the head resonance frequency rises due to the cancellation of the input capacitance C, the head 1 and the preamplifier are coupled using the rotary transformer RT, or used in the preamplifier according to the reproduction speed. Therefore, the number of head turns, the turn ratio of the rotary transformer, and the input capacitance of the preamplifier stage can be freely combined, and the degree of design freedom is maintained.

In the embodiment described above, the FET
Although the head amplifier based on the input has been described, the gist of the present invention is not limited to this, and the characteristics obtained by the configuration using the operational amplifier IC as shown in FIG. 3 are the same. However, if a bias resistor having a small resistance value is used for the input, the improvement of the SN ratio is impaired, so that it is necessary to use an input form via a transformer or an inductor. Also, when using a wideband preamplifier stage, the phase delay cannot be ignored and positive feedback cannot be applied. In such a case, as shown in FIG. Then, feedback may be applied to obtain equivalent characteristics.

[0027]

According to the first aspect of the present invention, the first amplifying unit for inverting and amplifying the output of one end of the head by negatively feeding back the output of the head and negatively feeding back the output of the other end of the head. A balanced amplifying unit is formed by symmetrically arranging a second amplifying unit that performs non-inverting amplification by performing positive feedback while the balanced amplifying unit cancels the amplifier input capacitance according to negative feedback and positive feedback. The head resonance frequency can be driven out of the required band, whereby the group delay characteristic GD in the required band is obtained.
In addition, the fluctuation of the frequency amplitude characteristic FL is reduced, and the cause of distortion can be suppressed. In addition, since the head resonance is damped by negative feedback, it is not necessary to perform characteristic compensation at a subsequent stage. According to the second aspect of the present invention, the first amplifying section that inverts and amplifies the output of one end of the head limited by the high-resistance element while performing negative feedback and negative feedback, and is limited by the high-resistance element. And a second amplifying unit that non-inverts and amplifies the output of the other end of the head by performing a negative feedback while providing a negative feedback, to form a balanced amplifying unit. Since the amplifier input capacitance is canceled in accordance with the amount of feedback, it is possible to avoid a reduction in the SN ratio while damping the head resonance by feedback. In addition, since the head resonance frequency increases due to the cancellation of the amplifier input capacitance,
It is also possible to flexibly cope with a mode in which the head and the preamplifier are coupled using the rotary transformer RT and a mode in which the number of FETs used in the preamplifier is switched according to the reproduction speed.
The number of turns of the head, the ratio of the number of turns of the rotary transformer, and the input capacity of the preamplifier stage can be freely combined, and the degree of design freedom can be maintained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of a head amplifier according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of a frequency characteristic according to the embodiment;

FIG. 3 is a circuit diagram showing a configuration example of a head amplifier using an operational amplifier IC.

FIG. 4 is a diagram illustrating an example of a feedback circuit when a wideband preamplifier stage is used.

FIG. 5 is a diagram showing a first conventional example.

FIG. 6 is a diagram showing an equivalent circuit in a case where the head 1 and a preamplifier stage are coupled using a rotary transformer RT.

FIG. 7 is a diagram showing frequency characteristics of a first conventional example.

FIG. 8 is a diagram showing a second conventional example.

FIG. 9 is a diagram showing a third conventional example.

FIG. 10 is a diagram illustrating frequency characteristics of a second conventional example.

FIG. 11 is a diagram showing frequency characteristics of a third conventional example.

FIG. 12 is a diagram showing an equivalent circuit when FETs are used in parallel.

[Explanation of symbols]

10: inverting amplifier (first amplifier), 11: FET, 1
2, 13 ... transistor, Rb1, R11, R12, R
13, C11: bias element, R14, R15: output resistance, ^CNF1 , ^RNF1 , ^CPF1 : feedback element, 20: non-inverting amplifier (second amplifier), 21: FET, 22, 23: transistor, Rb2, R21 ... bias element, R22,
R23: output resistance, ^CNF2 , ^RNF2 , ^CPF2 : feedback element,
100 head amplifier (balanced amplification means)

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5D030 AA02 BA17 BA21 BA22 5J090 AA01 AA12 AA13 AA22 CA21 CA41 CA64 CA72 DN01 FA09 FA17 HA02 HA09 HA18 HA25 HA29 HA35 HN16 KA02 KA12 KA13 KA47 MA04 MA08 MA12 MA13 MA17 MA21 NN05 SA10 TA01 TA03

Claims (2)

[Claims] 1. A head amplifier in which output characteristics are improved by using head resonance caused by a head induction component and an amplifier input capacitance, wherein the one end of the head is positively fed back while being negatively fed back, and the first output is inverted and amplified. An amplifying unit and a second amplifying unit that non-invertingly amplifies by non-inverting and amplifying the output of the other end of the head by performing a negative feedback are formed in a balanced amplifying unit. A head amplifier for canceling the amplifier input capacitance according to positive feedback. 2. A head amplifier in which an output characteristic is improved by using head resonance caused by a head induction component and an amplifier input capacitance. The first amplifying unit for inverting and amplifying and the second amplifying unit for non-inverting and amplifying the output of the other end of the head, which is limited by the high-resistance element, is negatively fed back while being positively fed back. A head amplifier, comprising: a balanced amplifying means, wherein the balanced amplifying means cancels the amplifier input capacitance according to a positive feedback amount smaller than a negative feedback amount.