JPS6177406A - Automatic level control circuit - Google Patents

Abstract

PURPOSE:To prevent deterioration in the distortion factor at power supply reduction by changing an offeset voltage of a level detection circuit of an ALC circuit in response to the decrease in the power supply voltage. CONSTITUTION:An output signal level of a sound recording amplifier 9 is detected by a level detection circuit 17. The offset voltage of the level detection circuit 17 is set by an offset voltage set circuit 20. On the other hand, an operation current supply circuit 34 whose output current is decreased in response to the reduction in the power supply voltage is provided. This output current is fed to the offset voltage set circuit 20. Thus, the offset voltage of the level detection circuit of the ALC circuit, that is, the ALC level is varied in response to the reduction in the power supply voltage. Thus, the deterioration in the distortion factor at the power supply voltage drop is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Industrial Application Field The present invention relates to an improvement in automatic level control (ALC) circuits, and particularly to improvements in the distortion rate when the power supply voltage drops (voltage reduction). This invention relates to an ALC circuit that can be prevented.

(b) Conventional technology Japanese Patent Application Laid-open No. 57-44312, Japanese Patent Application Laid-open No. 57-1355
As seen in Publication No. 11, etc., the recording circuit pressure of a tape recorder is provided with an ALC circuit. , that is, the output signal level of the recording amplifier, is made constant, and has a configuration as shown in FIG. 2, for example. In Figure 2, the recorded signal applied to the input terminal (1) is connected to the amplifier (2).
After being amplified by the output terminal (31), it is applied to a magnetic head (not shown) and recorded on a magnetic tape.At that time, the output signal obtained at the output terminal (3) is
The signal is rectified by a capacitor (5), smoothed by a capacitor (5), and converted into a DC signal corresponding to the level of the output signal. and,
When the level of the DC signal reaches a predetermined value, the first and second transistors (6) and (7) become conductive, and the input signal of the amplifier (2) is transferred to the collector-emitter path of the second transistor (7). 3) at the output terminal because it is bypassed by
The level of the output signal obtained is maintained at a predetermined value (ALC level) K. Therefore, in the case of FIG. 2, the ALC operation starting level ■. ■. :2V,,+V,...
Mark... Mountain... Set in (1), ■
□=■. If the voltage is 0.6V, the ALC operation is started when the output voltage of the amplifier (2) reaches approximately 1.8V, and the output bottom pressure is approximately V. is kept at the value of Therefore, even if an excessive input signal is applied to the input terminal (1), the amplifier (
The signal applied to the magnetic head from 2) does not become large, allowing recording without distortion.

Problem to be Solved by the Invention H However, in the case of the circuit shown in FIG. ) The starting level shown by the formula ■. is no longer reached, and ALC operation is no longer performed. As a result, the distortion rate deteriorated significantly, making it impossible to record properly.

B) Means for Solving the Problems The present invention has been made to solve the above problems, and includes a level detection means for detecting the output signal level of an amplifier, and an offset for determining an offset voltage of the level detection means. A voltage setting means, and an operating current supply means for supplying an operating current according to the power supply voltage to the offset voltage setting means? It is characterized by the following:

(E) Function According to the present invention, the offset voltage setting means 9 supplies a current corresponding to the power supply voltage that has dropped during voltage reduction, and the offset voltage setting means 9 supplies the current corresponding to the power supply voltage that has decreased during the voltage reduction. Since the pressure can be changed automatically, the ALC level can be automatically varied.

(v) Embodiment FIG. 1 shows an embodiment of the present invention, in which (8) is a signal source that generates a recorded signal;
8) is connected to the input transistor, a differential amplifier circuit including a pair of transistors 0D and (12), and a pair of push-connected transistors 0D and (151).
5EPP (single-ended gino/nibble) including
O is a level detection circuit consisting of differentially connected first and second transistors α seconds and ■, ■ is the third, fourth and fifth transistors (2fl, (221 and (c) and resistance &41, (25
1 and +2bl and a diode (3)
) voltage setting circuit, c! The IC is the level detection circuit (L!
A rectifier transistor that rectifies the output signal of +, C33 is a smoothing circuit consisting of a resistor C30+ and a capacitor 3+1 that smoothes the output signal of the rectifier transistor, and C33 is a drive transistor that operates according to the output voltage of the smoothing circuit.
(Oki is the 6th, 7th and 8th transistor field, L8 and U37 ), a resistor (to), and a diode (II and (41)), and is an operating current supply circuit that supplies the operating current of the offset voltage setting circuit.The operation will be explained next. .When the power (+vcc) is turned on,
Since the base of the sixth transistor (to) of the operating current supply circuit is biased by the diodes F40 and +411 and the resistor, the sixth transistor becomes conductive, and its collector current 11 becomes 11=-...・・・・・・・・・・・・・・・・・・・・・
......+2>. The collector current of the sixth transistor (to) is the seventh and eighth transistor.
It is inverted by a current inverting circuit consisting of a transistor (to) and C371, and is supplied as an operating current to an offset voltage setting circuit (2).

The current flowing through the eighth transistor (3η) is supplied from the emitter of the third transistor (211) at the beginning of the offset pressure setting circuit through the resistor □□□.Therefore, the base voltage of the third transistor 211 EE V + is V
,=2V□+R7・I, ・・・・・・・・・・・・
・(3) becomes. In addition, the pace voltage ■2 of the level detection circuit cut-off second transistor ■ is as follows: V2=V, +It・R1...... River...f
4). By the way, the electric current 2 flowing through the resistor 1241
is the resistance value of the resistor C24, where R4 is the resistance value of the resistor C24. Therefore, Equation 14) can be expressed as follows using Equation 5. - The pace voltage of one transistor 0υ of the recording amplifier (9) is determined according to the emitter voltage (V□) of the input transistor (10), and the pace voltage of the other transistor u4 also changes due to the negative feedback effect. .

Therefore, the output DC voltage v0 of the recording amplifier (9) is: (2). =■3. ．． +R5・I, river...
River・(6)'K at'L 6e'. Now, the fourth and fifth offset voltage setting circuits
The transistors @ and @ are matched, and their area ratio is 1:
1, the current flowing through the fifth transistor (c) is the current flowing through the fourth transistor @, that is, the current I flowing through the resistor (24).

is equal to Also, the value R3 of the resistance (c) and the negative feedback resistance (
43 values R, are set equal, equation (6) becomes v
, =v,, +Rs-j, :V,.

10-and-(V..-L)...(6)R, +R
.

It can be expressed as

Now consider the offset voltage when the level detection circuit is turned off. The offset y) voltage ΔV is expressed by the equation (4) and the equation (6).
) From the formula, ΔV ” V2 VO” Vlm +N VD
Therefore, when the output signal obtained by being amplified at the recording amplifier (side) exceeds the above ΔV, the output signal is applied from No. 18 # 1 transistor al1g conducts, ALC
The operation will begin.

By the way, if the electric fA voltage is extremely high, the diode +
The forward voltages of 41 and (4υ) are kept approximately constant.

Therefore, the offset voltage shown by equation (7), that is, the output voltage of the recording amplifier (Yuu) at which ALC operation is started, will also remain constant. However, if the ttg pressure decreases due to voltage reduction, the diode (The forward voltage of 4IJ and (41) also decreases, and the offset voltage Δ
■ becomes smaller. Generally, diodes (4G and (
The forward voltage VD of the diode (41) becomes
. It is known that when the forward width pressure V0 decreases, the forward width pressure V0 also decreases. Therefore, by substituting the equation (8) into the equation (7), the following equation is obtained, and the offset voltage ΔV also becomes small.

Therefore, the circuit shown in FIG. 1 provides an ALC circuit that can reduce the offset voltage of the level detection circuit when the voltage is reduced and set the ALC level according to the output signal level of the recording amplifier (9). I can do it.

In addition, in the case of a normal power supply voltage, the input signal from the signal source (8) is amplified by a recording amplifier (a small amount), subjected to a predetermined attenuation by an ALC circuit, and output as a first constant level output signal to an output terminal ( 43, and when the voltage is reduced, the input signal from the signal source (8) is amplified by the recording amplifier (9), subjected to a predetermined attenuation by the ALC circuit, and then output to a second constant signal according to the power supply voltage. Output terminal (4) as level output signal
31, the amplification operation by the recording amplifier (Isamu) and the ALC operation by the ALC circuit are the same as those of the conventional one, and the details thereof will be omitted.

(G) Effects of the Invention As described above, the ALC circuit according to the present invention includes an operating current supply circuit whose output current value decreases as the power supply voltage decreases, and supplies the output current to the offset voltage setting circuit. As a result, the offset voltage of the level detection circuit of the ALC circuit, that is, the ALC level, can be varied according to the drop in the power supply voltage, and as a result, it has the excellent advantage of preventing deterioration of the distortion factor of the recording amplifier. have

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional ALC circuit. Explanation of main drawing numbers (marked...recording amplifier, off...level detection circuit, (
Low...Offset section pressure setting circuit, Immediate)...Operating current supply circuit.

Claims (1)

[Claims] (1) Level detection means for detecting the output signal level of the amplifier in an automatic level control circuit that detects the output signal level of the amplifier and attenuates the input signal of the amplifier to keep the output signal level constant; offset voltage setting means for determining an offset voltage of the level detection means;
an operating current supply means for supplying an operating current to the offset voltage setting means, and the ALC level at the time of voltage reduction is adjusted by supplying an operating current corresponding to the power supply voltage from the operating current supply means to the offset voltage setting means. An automatic level control circuit characterized by being variable.