JPS6122405A - Bias current circuit - Google Patents

Abstract

PURPOSE:To set and hold a bias current flowing a magnetic head to an optimum value by detecting the bias current flowing to a magnetic head, comparing an output voltage obtained by rectifying a detection signal with a reference voltage and controlling the oscillating level of a bias oscillator. CONSTITUTION:A voltage VB generated across a resistor 5 and proportional to the bias currnet iB is amplified by an amplifier 10 and rectified by a rectifier circuit 11, then fed to an input terminal of a control circuit 12. A difference between the voltage level of the signal fed to the circuit 12 and a level Vref of a preset reference voltage source 13 is detected and a control signal to eliminate the voltage level difference is fed to the bias oscillator. Then the output signal level of the oscillator 1 is adjusted by the control signal and the current iB is set to an optimum value designated by the voltage source 13. Thus, the bias current fed to the magnetic head 4 is set and held to the optimum value.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a bias current circuit suitable for use in an audio signal recording system of a magnetic recording/reproducing device such as a VTR or tape recorder.

[Background of the invention]

In magnetic recording and reproducing devices such as VTRs and tape recorders, a bias current circuit is installed in the audio signal recording system, and the audio signal is recorded by superimposing it on the bias current to suppress waveform distortion of the reproduced audio signal. However, if the value of the bias current supplied to the magnetic head is not appropriate, the frequency characteristics of the reproduced audio signal will deteriorate, the level will drop, and waveform distortion will increase. Generally, the bias current circuit is provided with means for adjusting the level of the bias current.

FIG. 3 is a circuit diagram showing an example of a conventional binary current circuit, in which 1 is a bias oscillator, 2 and 9 are capacitors, 3 is a variable resistor, 4 is a magnetic head, 5 is a resistor, and 6 is a A magnetic tape, 7 is an input terminal, and 8 is an amplifier.

In the figure, a bias signal output from a bias oscillator 1 passes through a DC blocking capacitor 2, is converted into a current by a variable resistor 3, becomes a bias current tB, and flows through a winding of a magnetic head 4 and a resistor 5. flows through to the ground terminal.

On the other hand, the audio signal to be recorded is transmitted from the input terminal 7 to the amplifier 8.
The signal is supplied to the amplifier 8, where it is amplified to a predetermined level and given a desired frequency characteristic. The audio signal output from the amplifier 8 is superimposed on the bias current iB via the DC blocking capacitor 9, and is recorded on the magnetic tape 6 by the magnetic head 4.

A circuit using an AC bias recording method having such a configuration is widely used because the waveform distortion of the recording signal is small.

FIG. 4 is a characteristic diagram showing the bias characteristics of the magnetic head.

In the figure, the horizontal axis represents the bias current, the left vertical axis represents the reproduction output level, and the right vertical axis represents the distortion rate.

The characteristic curves α to e represent 2,
Characteristic curve 1 represents the fluctuation of the reproduction output level with respect to the change of the bias current, and characteristic curve 1 represents the fluctuation of the reproduction output level with respect to the change of the bias current.
It represents the change in distortion rate with respect to the change in bias current.

As you can see from the figure, if the bias current is not appropriate,
The reproduction output level fluctuates, and the frequency characteristics and distortion authenticity deteriorate.

By the way, the causes of the bias current varying from the optimum value include variations in the level of the bias signal output from the bias oscillator 1 and variations in the impedance of the magnetic head 4.

Therefore, conventionally, the bias current value is detected by measuring the voltage IB across the resistor 5 shown in FIG.
was set to a value that would give the optimum bias current.

However, it takes a lot of effort to make such adjustments for each mass-produced magnetic recording/reproducing device, and in order to make the adjustments easy, it is necessary to adjust the size of the variable resistor 3. It is necessary to design with placement in mind.
The degree of freedom in design will be lost.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a bias current circuit that eliminates the drawbacks of the prior art and eliminates the need for manual adjustment of the bias current of a magnetic head.

[Summary of the invention]

In order to achieve this object, the present invention is characterized in that the bias current flowing through the magnetic head is detected and the output signal level of the bias oscillator is controlled so that the bias current reaches an optimum value.

[Embodiments of the invention]

It is a block diagram showing an example, where 1o is an amplifier, 11 is a rectifier circuit, 12 is a control circuit, and 13 is a reference voltage source.
The parts corresponding to those in FIG. 3 are given the same reference numerals.

In the figure, a voltage uB generated in a resistor 5 and proportional to a bias current IB is amplified to an appropriate level by an amplifier 1o, rectified by a rectifier circuit 11, and then supplied to one input terminal of a control circuit 12. .

In the control circuit 12, the voltage level of this supplied signal and the voltage level of the reference voltage source 13 set in advance ref
A control signal for eliminating the difference in voltage level is supplied to the bias oscillator l.

Then, the output signal level of the bias oscillator 1 is adjusted by the cough control signal, and the bias current value B is set to the optimal value specified by the reference voltage source 13.

Since the optimal bias current LB is automatically set in this way, the process of adjusting the bias current in the production line of magnetic recording/reproducing devices is no longer necessary, and the adjustment process can be simplified. This eliminates the need for a design that takes into account the placement of adjustment parts, increasing the degree of freedom in design.

FIG. 2 is a circuit diagram showing a specific example of the bias current circuit of FIG. 1.

In the figure, capacitors 14, 20 . . . correspond to the amplifier 1o in FIG. Amplifier “15, resistance 17.18,
19. It also includes a reference voltage source 17 and the like.

The rectifier circuit 11 in FIG. 1 is composed of an NPN transistor 21, a resistor 23, a capacitor 24, and the like.

A differential amplifier 25 corresponds to the control circuit 12 in FIG.
, resistors 26, 27, 29, 31, and 32 degrees, NPN type transistors 28 and 30, a voltage source 33, and the like.

First, the voltage υB generated across the resistor 5 is supplied to the non-inverting input terminal of the amplifier 15 via the DC blocking capacitor 14. A voltage from a reference voltage source 16 is applied to the non-inverting input terminal via a resistor 17. The gain of the amplifier 15 is determined by the resistance 18.19 and the DC total feedback capacitor 26.
is set.

In this way, voltage 1)B is amplified to a predetermined level,
The signal is supplied to the transistor 21.

A power supply 22 is connected to the collector terminal of the transistor 21, and a resistor 23 and a capacitor 24 are connected in parallel to the emitter terminal, forming an emitter follower detection circuit.

The signal supplied from the amplifier 15 is rectified here and supplied to the non-inverting input terminal of the differential amplifier 25.

The differential amplifier 25 is provided with gain setting resistors 26 and 27, and has an inverting input terminal connected to the reference voltage source 13 via the resistor 27. Then, the signal level supplied to the non-inverting input terminal and the voltage of the reference voltage source 13
The difference with R6f is detected, amplified to a predetermined level, and then supplied to transistor 28.

The transistor 28 has an emitter terminal connected to a ground terminal via a resistor 29, and a collector terminal connected to the transistor 30.
The base of the terminal is connected.

The base terminal of the transistor 300 is connected to a ground terminal via a resistor 31, and is also connected to the collector terminal of the transistor 30 and a voltage source 33 via a resistor 32.

Therefore, depending on the level of the signal supplied from the differential amplifier 25, the transistor 28
Controls pace current.

Here, the collector current of the transistor 28 is expressed as () the voltage of the o1 voltage source 33 is Vcc, the emitter terminal voltage of the transistor 30 is vc'c, the resistance value of the resistor 32 is R1, the voltage between the forward pace and emitter of the transistor 30 is V , then Vc'c=Vac (IO@R+Vy)""(1). Note that the voltage VtJc is used as a power source for the bias oscillator 1, and the output signal level of the bias oscillator 1 changes in proportion to this voltage vc'c.

Therefore, if the impedance of the magnetic head 4 used is larger than the specified value and the bias current jB is reduced accordingly, the voltage vB generated across the resistor 5 becomes lower than the specified value. Therefore, the signal level supplied to the differential amplifier 25 becomes lower than the voltage of the reference voltage source 13, and the collector current of the transistor 28 becomes small. Therefore, from the above equation (1), the voltage of the resistor 32 is Since the drop becomes smaller, the voltage vc'c becomes higher and the output signal level of the bias oscillator 1 increases.For this reason, the bias current jB is maintained at the optimal value specified by the reference voltage source 13. Become.

Conversely, when a recording head 4 with a small impedance is used, the bias current iB increases and the voltage νB generated across the resistor 5 increases. A signal of a higher voltage than the reference voltage source 13 is supplied to the differential amplifier 25 . Therefore, the output signal level of differential amplifier 25 increases and the collector current of transistor 28 1. will also increase. Therefore, from the above equation (1), the voltage vc'c decreases, the output signal level of the IAS oscillator 1 decreases, and the IASMI current iB is maintained at a predetermined value.

〔Effect of the invention〕

As explained above, according to the present invention, since the bias current is controlled so that the bias current is maintained at an optimum value, manual adjustment of the bias current is completely unnecessary, and magnetic recording and reproduction When designing a device, it is no longer necessary to consider the placement of the bias current adjustment means as in the past, which increases the degree of freedom in design, and eliminates the drawbacks of the prior art described above. can be provided.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of a bias current circuit according to the present invention, FIG. 2 is a circuit diagram showing a specific example of the bias current circuit shown in FIG. 1, and FIG. 3 is a block diagram showing an example of a conventional bias current circuit. , M4 is a characteristic diagram showing the bias characteristics of the magnetic head. 1...Bias oscillator, 5...resistance, 1o
...Amplifier, 11... Rectifier circuit, 12
...Differential amplifier, 13...Reference voltage source. . Figure 1 Figure 2-6 Figure 3

Claims (1)

[Claims] A bias current circuit configured to superimpose a bias current from a bias oscillator on a recording signal and supply it to a magnetic head, comprising: a first means for detecting the bias current flowing through the magnetic head; and an output of the first means. second means for rectifying the signal; third means for comparing the output voltage of the second means with a reference voltage; and controlling the oscillation level of the bias oscillator by the output signal of the third means. 4. A bias current circuit characterized in that the bias current circuit is provided with a fourth means for controlling the magnetic head, and is configured such that the bias current flowing through the magnetic head can be set to a predetermined magnitude.