JPH0574123B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a magnetic recording/reproducing device. [Prior art] Regarding preamplifiers in conventional magnetic recording and reproducing devices such as VTRs, "Home VTR" Corona Corporation, Vol.
Discussed on pages 158-159. As shown in Figures 14 and 15 of the above-mentioned document, there is a conventional technique in which the resonance caused by the input capacitance between the magnetic head and the input terminal is adjusted by using a resistor and capacitor at the input terminal of the amplifier circuit included in the preamplifier. Ta.
On the other hand, in recent years there has been an increasing demand for higher image quality for home VTRs, and technologically speaking, wider bandwidth and lower noise preamplifiers have become essential conditions. For this reason, the above-mentioned conventional techniques are no longer able to adequately cope with this problem. [Problems to be Solved by the Invention] The above conventional technology adds a capacitor to the input terminal of the amplifier circuit included in the preamplifier, and selects the frequency at which resonance between the magnetic head and the preamplifier input capacitance occurs to be an arbitrary frequency within the band. It is. In this method, the bandwidth of the reproduced signal is determined by the inductance of the magnetic head, the preamplifier input capacitance, and the variable capacitance. For this reason, the only way to realize a wideband magnetic recording/reproducing device is to reduce the inductance of the magnetic head, even if the output voltage of the magnetic head is lowered. Further, the above-mentioned prior art is a method of adjusting the resonance sharpness Q by connecting a resistor in parallel to the input terminal of the amplifying circuit included in the preamplifier. This method not only causes loss of the reproduced signal obtained from the magnetic head due to the adjustment resistor, but also causes thermal noise caused by the adjustment resistor in the VTR preamplifier, which amplifies the signal whose reproduction signal level is as small as several hundred μV from the magnetic head. There was a problem that the signal-to-noise ratio could no longer be ignored and the S/N decreased. SUMMARY OF THE INVENTION An object of the present invention is to realize a magnetic recording/reproducing apparatus that can perform amplification in a wide band without reducing the inductance of a magnetic head and can obtain an output signal with a good signal-to-noise ratio. [Means for solving the problem] The above purpose is to apply positive feedback to the input terminal using a capacitive element from the positive phase output terminal of the amplifier circuit into which the signal from the magnetic head of the magnetic recording/reproducing device is input, and at the same time This is achieved by applying negative feedback from the inverting output terminal of the amplifier circuit to the input terminal using a resistor element, and by equivalently reducing the input capacitance by the effects of the two positive and negative feedbacks and optimizing the input resistance. [Operation] The capacitive element connected from the positive-phase output terminal to the input terminal of the amplifier circuit in the preamplifier causes a current in phase with the input signal to flow into the input capacitance of the preamplifier, so that it functions to equivalently reduce the input capacitance. Further, a current having a phase opposite to that of the input signal flows through a resistive element connected from the inverting output terminal of the amplifier circuit in the preamplifier to the input terminal. In other words, since the current of the input signal is sucked out to the output terminal side, it works to equivalently reduce the input resistance of the preamplifier compared to when the resistor element is not provided. The two positive and negative feedback circuits operate as described above. Therefore, the resonance occurring between the magnetic head inductance and the preamplifier input capacitance has a higher frequency than when a positive feedback circuit using a capacitive element is not used. At the same time, the sharpness Q of the resonance is suppressed by the negative feedback circuit using the resistive element, and a flat gain frequency characteristic can be obtained in a wide band. [Example] An example of the present invention will be described below. First, the overall operation of the circuit of the present invention will be explained using FIG. In Fig. 1, 1 is the output voltage Vh of the magnetic head, 2 is the inductance Lh of the magnetic head, and 3 is the inductance Lh of the magnetic head.
is the preamplifier input terminal of the magnetic recording/reproducing device, 4 is the preamplifier input capacitor Cin, 5 is the positive feedback capacitor Cf, and 6 is the preamplifier input terminal of the magnetic recording/reproducing device.
is a negative feedback resistor Rf, 7 is a positive phase signal output terminal, 8 is an inverted signal output terminal, 9 is a preamplifier output terminal, 1
7 is the amplifier circuit in the preamplifier, V ₀ is the preamplifier output voltage, if is the positive feedback current, i _io is the current flowing into the input capacitor 4, and Gup is when looking at the positive phase output terminal 7 from the input terminal 3. Gun is the voltage gain when looking at the inverted output terminal 8 from the input terminal 3. Now, there is no negative feedback resistor Rf and the capacitance Cf
The case where only positive feedback is applied will be explained below. Assuming that the input resistance of the amplifier circuit 17 is relatively large and the current flowing into the amplifier circuit 17 can be ignored, the only current flowing from the input terminal 3 is the current _iio flowing into the capacitor 4. At this time, if equation (1) holds true in the circuit shown in Figure 1, i _io is obtained entirely by the positive feedback current if, and Cf/Cin + CfGup・Vi=Vi − (1) Current supplied from head output signal Vh becomes 0. Therefore, the equivalent input capacitance Cin' viewed from the input terminal 3 to the amplifier circuit 17 can also be set to zero. However, in equation (1), Gup・(Cf/(Cin+Cf)
is the positive feedback loop gain, and when this value becomes 1, the circuit of FIG. 1 becomes unstable. Therefore, although equation (1) does not hold completely, increasing Gup little by little within a range where the positive feedback loop gain is less than 1 can have the effect of reducing the input capacitance Cin. Next, when a negative feedback resistor Rf is connected to the input terminal 3, the input resistance Rin when looking at the amplifier circuit 17 from the input terminal 3 is approximately given by equation (2) due to the well-known Miller effect. Rin=Rf/Gun+1 −(2) According to equation (2), any input impedance can be achieved by varying Rf. Therefore, in order to keep the voltage Vi at the input terminal 3 constant regardless of the frequency, Rf must be determined so that the relationship shown in equation (3) holds, taking into account the equivalent input capacitance Cin′ obtained by positive feedback. Bye.

〔Effect of the invention〕

According to the present invention, the equivalent input capacitance of the preamplifier can be reduced, and the resonance frequency between the magnetic head and the input capacitance can be moved out of the band. In addition, since the equivalent input resistance of the preamplifier can be adjusted appropriately, the sharpness of the resonance between the magnetic head and the preamplifier input capacitance can be appropriately selected, and a magnetic recording/reproducing device having a preamplifier with extremely flat gain characteristics even outside the band is realized. It has the effect of being able to Furthermore, in a system that does not require a wide band, the same bandwidth as before can be achieved even if a magnetic head having a larger inductance is used than when the present invention is not used. Generally, the output voltage of a magnetic head is proportional to its inductance, so it is possible to use a magnetic head that can output a larger output than conventional ones, which has the effect of realizing a system such as a VTR with a very good signal-to-noise ratio.

[Brief explanation of the drawing]

Figure 1 is a circuit configuration diagram showing the operation of the present invention, Figure 2 is a circuit configuration diagram showing the operation of the present invention.
The figure is a circuit diagram of one embodiment of the present invention, and FIG. 3 is a diagram showing characteristics of one embodiment of the present invention. FIG. 4 is a circuit diagram of another embodiment of the present invention. 4... Capacitance of the input terminal of the preamplifier, 5... Capacitance for feeding back the positive phase amplified signal to the input terminal, 6... Resistor for feeding back the inverted amplified signal to the input terminal, 16... Wideband and flat gain frequency characteristics due to positive and negative feedback.

Claims (1)

[Scope of Claims] 1. A magnetic recording/reproducing device comprising a magnetic head for reading signals from a recording medium and an amplifier circuit for amplifying the signals to an appropriate level, the output terminal of the magnetic head circuit being connected to the input terminal of the amplifier circuit. In the device, the above amplifier circuit connects two output terminals: a positive phase output terminal that outputs a signal obtained by amplifying the input signal applied to the above input terminal in the same phase, and an inverting output terminal outputting a signal obtained by amplifying the above input signal in opposite phase. The positive phase output terminal is connected to the input terminal via a capacitive impedance element, the inverted output terminal is connected to the input terminal via a resistive impedance element, and the two impedance elements provide positive feedback. A magnetic recording/reproducing device comprising a circuit and a negative feedback circuit. 2 When the value of the capacitive impedance element is Cf, the value of the resistive impedance element is Rf, the voltage gain when looking from the input terminal of the preamplifier to the positive phase output terminal is Gvp, and the input capacitance of the preamplifier input terminal is Cin. The value Cf of the capacitive impedance element above is set to a predetermined value not exceeding Cin/(Gvp-1), the voltage gain when looking at the inverted output terminal from the input terminal is Gvn, the inductance of the magnetic head is Lh, and the magnetic Claim 1 characterized in that the value Rf of the resistive impedance element is approximately (Gvn+1)·√′ when the sum of the head capacitance and the equivalent input capacitance of the preamplifier input terminal is Cin′. The magnetic recording/reproducing device described in Section 1.