JPH05275947A - Ac bias control circuit - Google Patents

Abstract

PURPOSE:To attain the automatic control of an AC bias current by controlling the output level of an AC bias generating circuit in accordance with the detection output of the AC bias current flowing to a magnetic head coil. CONSTITUTION:An AC bias control circuit consists of a filter 11 which extracts an AC bias out of the signal produced at a resistance 10, a level detection circuit 12 which detects the output signal level of the filter 11, and a variable DC power supply 13 which changes its DC level in response to the detection output of the circuit 12. In a recording state, the signal produced at the resistance 10 is applied to the filter 11 and only the AC bias component is extracted out of the produced signal. Then the extracted bias component undergoes the level detection through the circuit 12. Then the output DC voltage of the power supply 13 changes in accordance with the detection level of the circuit 12. That is, the value of the power supply 13 is set at an extremely low level in an initial state and the operating current of a transistor 14 is oscillated at a high level. Then the oscillation output signal of an AC bias circuit 5 is set at an optimum level in response to the high level of oscillation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AC bias adjusting circuit required for recording a signal on a recording medium such as a magnetic tape using a magnetic head coil.

[0002]

2. Description of the Related Art When a signal is recorded on a magnetic tape using a magnetic head coil, the recording signal is superimposed on an AC bias. At that time, the value of the AC bias had to be constant and had to be adjusted. Figure 2
Shows such an AC bias adjusting circuit. At the time of recording, the recording signal from the recording amplifier circuit (1) is applied to one end of the magnetic head coil (4) via the capacitor (2) and the resistor (3). To be done. Also, from 70k [Hz] to 8
The output signal of the AC bias generation circuit (5) which oscillates at 0 kHz is also applied to the magnetic head coil (4) via the variable resistor (6). Therefore, the recording signal superimposed on the AC bias is supplied to the magnetic head coil (4), and the signal can be recorded on the magnetic tape or the like. The first and second switches (7) and (8) are switched to the REC side during recording and to the PB side during reproduction. The terminal (9) is for detecting the magnitude of the AC bias during recording, and has a small resistance R
₁ (10) gives a voltage output. Therefore, in the manufacturing line, an operator adjusts the value of the variable resistor (6) while measuring the level of the terminal (9) to determine the optimum magnitude of the AC bias.

[0003]

However, manual adjustment is not preferable because it complicates the manufacturing process. or,
The adjustment by the variable resistance is a problem because it causes an increase in external parts when integrated into an IC.

[0004]

The present invention has been made in view of the above points, and includes a recording amplifier circuit for amplifying a recording signal,
An AC bias generating circuit for generating an AC bias, a magnetic head coil to which a recording signal from the recording amplifying circuit is superimposed on the AC bias of the AC bias generating circuit and applied to one end, and the other end of the magnetic head coil A resistor for measuring the connected recording current, a filter for extracting an AC bias from a signal generated in the resistor, a level detection circuit for level-detecting the output signal of the filter, and a detection output of the level detection circuit And a control circuit for controlling the output signal level of the AC bias generating circuit.

[0005]

According to the present invention, the AC bias current flowing through the magnetic head coil is detected and its level is detected. Then, the output level of the AC bias generation circuit is adjusted according to the level detection output.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a circuit diagram showing an embodiment of the present invention.
(11) is a filter for extracting an AC bias from the signal generated in the resistor (10), (12) is the filter (11)
Level detection circuit for detecting the output signal level of the (13)
Is a variable DC power supply whose DC level changes according to the detection output of the level detection circuit (12).

In FIG. 1, during recording, the signal generated in the resistor (10) is applied to the filter (11), and only the AC bias component is extracted. The extracted AC bias component is level-detected by the level detection circuit (12). Then, the output DC voltage of the variable DC power supply (13) changes according to the detection level. AC bias generator (5)
Is the Hartley oscillator. As the load of the transistor (14), the coil L ₁ and the resonance circuit ( 15 ) exhibit a frequency selection characteristic, and a signal of a certain selected frequency is inverted and generated in the coil L ₂ . Therefore, the level of the oscillation output signal can be changed by the operating current of the transistor (14). In the circuit of FIG. 1, the value of the variable DC power supply (13) is set to be extremely low in the initial state, and the operating current of the transistor (14) is made large to cause oscillation at a large level. Then, in response to the large level, the filter (1
1), level detection circuit (12) and variable DC power supply (1
3) operates to control the oscillation output signal level of the AC bias generation circuit (5) to an optimum value.

FIG. 3 shows the level detection circuit (12) of FIG.
The specific circuit example of a variable DC power supply (13) and a switch (16) is shown, The output signal of a filter (11) is applied to an input signal (17). Transistor Q ₂ has a resistor R ₄
The reference voltage V ₂ is applied by R ₅ and R ₅ , and the reference voltage V ₂ is set higher than the DC level of the input terminal (17). That is, the differential amplifier circuit (18) including the transistors Q ₁ and Q ₂ has an offset. For that reason,
In the no signal state (for example, the reproduction mode), the transistor Q
₁ is off, the current mirror circuit (19) is off, transistors Q ₃ and Q ₄ are off, and the current mirror circuit (2
No current is generated from 0). The resistor R ₆ and the current mirror circuit (21) constitute a constant current source, and a constant current flows through the resistor R ₇
Flow to. Therefore, the reference voltage V ₄ is generated at the point A.
On the other hand, the transistors Q ₅ and Q ₆ form a negative feedback type differential amplifier (22) whose output signal is a current mirror circuit (2
3), and is negatively fed back to the point B via the transistors Q ₇ and Q ₈ . Therefore, when the reference voltage V ₄ is applied to the point A, the voltage V ₅ at the point B has a value equal to V ₄ . In this state, a relatively small voltage V ₆ is generated at the output terminal (24). Therefore, the value of the variable DC power supply (13) in FIG. 1 becomes small, and the oscillation output signal level of the AC bias generation circuit (5) becomes maximum. Then, the input terminal (17) of FIG.
A large level of AC bias signal is applied to the reference voltage V ₂ and exceeds the reference voltage V ₂ . Then, the transistor Q ₁ is turned on and the capacitor C ₁ is charged accordingly, so that a DC signal corresponding to the level of the AC bias is applied to the base of the transistor Q ₄ . As a result, if the current I ₁ is generated from the current mirror circuit (20), the current flowing through the resistor R ₇ is reduced by that amount, and the voltage at the point A is increased. Then, the voltages V ₅ and V ₆ increase accordingly. This rise lowers the oscillation output signal level.

Next, in the reproducing mode, an "H" level signal is applied to the terminal (25) to turn on the transistor Q ₉ and turn off the transistors Q ₇ and Q ₈ . Then, the voltage V ₆ becomes the power supply voltage level, and the oscillation of the AC bias generation circuit (5) is stopped. Therefore, according to the circuit of FIG. 3, the oscillation output signal level of the AC bias generating circuit (5) can be automatically adjusted.

FIG. 4 is a circuit diagram showing another embodiment of the present invention, in which an AC bias is applied to one end of the magnetic head coil (4) and a recording signal is applied to the other end. According to this configuration, the impedance adjustment resistor (2
6) becomes unnecessary, and the circuit can be simplified. Although the detection signal is obtained from the connection point between the resistor R ₃ and the capacitor C _{0 in} FIG. 4, it may be obtained from the connection point between the resistor R ₄ and the capacitor C ₀ . The operation of the other circuits is similar to that of the circuit of FIG.

[0011]

As described above, according to the present invention, it is possible to provide an AC bias adjusting circuit capable of automatically adjusting the magnitude of the AC bias current. In particular, according to the present invention, since the oscillation level of the AC bias generating circuit is adjusted, it is easy to control and is also advantageous in IC implementation.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an AC bias adjustment circuit of the present invention.

FIG. 2 is a circuit diagram showing a conventional AC bias adjustment circuit.

FIG. 3 is a circuit diagram showing a specific circuit example of a level detection circuit (12) and a variable DC power supply (13) in FIG.

FIG. 4 is a circuit diagram showing another embodiment of the present invention.

[Explanation of symbols]

 (4) Magnetic head coil (5) AC bias generation circuit (11) Filter (12) Level detection circuit (13) Variable DC power supply

Claims (3)

[Claims] 1. A recording / amplifying circuit for amplifying a recording signal, an AC bias generating circuit for generating an AC bias, and a recording signal from the recording / amplifying circuit is superimposed on an AC bias of the AC bias generating circuit and applied to one end. Magnetic head coil, a resistance for measuring a recording current connected to the other end of the magnetic head coil, a filter for extracting an AC bias from a signal generated in the resistance, and a level detection of an output signal of the filter An AC bias adjustment circuit comprising: a level detection circuit; and a control circuit that controls the output signal level of the AC bias generation circuit according to the detection output of the level detection circuit. 2. The AC bias adjustment circuit according to claim 1, wherein the level detection circuit includes a comparator having an offset and a smoothing circuit for smoothing a comparison output of the comparator. 3. A recording amplification circuit for amplifying a recording signal, an AC bias generation circuit for generating an AC bias, and an AC bias from the AC bias generation circuit at one end,
A magnetic head coil to which the recording signal from the recording amplifier circuit is applied to the other end through a coupling capacitor, a filter for extracting an AC bias from the signal generated in the coupling capacitor, and a level for level detection of the output signal of the filter An AC bias adjustment circuit comprising: a detection circuit; and a control circuit that controls the output signal level of the AC bias generation circuit according to the detection output of the level detection circuit.