JPH0543534Y2 - - Google Patents

Description

[Detailed description of the invention] (a) Industrial application field The present invention relates to automatic level control (ALC) circuits used in portable tape recorders, etc., and particularly relates to improving the attack time at the start of ALC operation. be. (B) Prior Art Page 343 of ``'85 Sanyo Semiconductor Handbook Monolithic Bipolar Integrated Circuits,'' published on March 20, 1985, describes an ALC circuit as shown in FIG. 2. In Figure 2, input terminal 1
The input signal (signal to be recorded) applied to the recording head is amplified by an amplifier 2, then led out to an output terminal 4 via an output coupling capacitor 3, and is applied from the output terminal 4 to a magnetic head (not shown). Recording of the input signal is performed. On the other hand, the output signal obtained at the output terminal 4 is applied to the anode of the first diode 7 via the first capacitor 5 and the first resistor 6, and is applied to the anode of the first diode 7 and the second diode 8.
The voltage is doubled and rectified. A second diode inserted between the cathode of the first diode 7 and the ground
Since the capacitor 9 is charged by the voltage doubler rectified output signal obtained at the cathode of the first diode 7, when the terminal voltage of the second capacitor 9 reaches a predetermined value, the drive transistor 10 and the ALC transistor 11 become conductive. ALC operation is performed. In the case of the ALC circuit shown in Fig. 2, if the peak voltage of the output signal obtained at the output terminal 4 is V ₀ , then in the negative half cycle of the output signal, the ground, the second diode 8, the first resistor 6, the 1 capacitor 5
A current flows through the path, and the first capacitor 5
is charged to the polarity shown. If the voltage across the first capacitor 5 at that time is V _C , then V _C =V ₀ −V _F2 −I ₁・R ...(1) [However, V _F1 is the voltage drop across the second diode 8, and R is The resistance value I ₁ of the first resistor 6 is the current flowing through the first resistor 6. On the other hand, in the positive half cycle of the output signal, the first capacitor 5, the first resistor 6, the first
A current flows through the diode 7 path, and the second capacitor 9 is charged. If the terminal voltage of the second capacitor 9 at that time is V _DC , then V _DC =V ₀ +V _C -V _F1 -I ₂・R ...(2) [However, V _F1 is the voltage drop I of the first diode 7 ₂ is the current flowing through the first resistor 6]. In the above equations (1) and (2), if the voltage drop at the first resistor 6 is ignored and the voltage drops at the first and second diodes 7 and 8 are equal to V _F , then V ₀ =1/ 2(V _DC +2V _F ) ...(3). In the above equation (3), V _F =0.45V, and the terminal voltage V _DC (=2V _BE ) of the second capacitor 9 determined by the base-emitter voltage of the drive transistor 10 and the ALC transistor 11 is 2×0.45V. Then, it is possible to provide an ALC circuit that starts ALC operation when the peak voltage V ₀ of the output signal obtained at the output terminal 4 reaches 0.9V, that is, when the average voltage of the output signal reaches 0.636V. (c) Problems to be solved by the invention By the way, in order to suppress fluctuations in the terminal voltage of the second capacitor 9, if the value of the second resistor 12 that discharges the second capacitor 9 is made relatively large, the output The time (attack time) T from when the signal is generated until the terminal voltage of the second capacitor 9 rises to the predetermined value V _DC (=2V _BE ) is T=V _DC・C ₂ /I ₃ ...(4) ) [However, C ₂ is the capacitance I ₃ of the second capacitor 9, and the charging current of the second capacitor 9]. As is clear from the above equation (4), the attack time becomes shorter as the capacitance of the second capacitor 9 is made smaller and the charging current _I3 of the second capacitor 9 is made larger. By doing this, it is possible to measure the reduction in recording distortion. However, in the case of the ALC circuit shown in FIG. 2, if the charging current _I3 is increased, the load on the amplifier 2 becomes heavy, causing a problem that the output voltage obtained at the output terminal 4 decreases. In addition, the second capacitor 9
If the capacitance of is made small, a problem arises in that ALC operation becomes unstable. Furthermore, when the value of the second resistor 12 is increased to substantially increase the value of the charging current _I3 ,
This time, the discharge time (recovery time) of the second capacitor 9 becomes long, which again poses a problem. (d) Means for solving the problems The present invention was made in view of the above points, and includes an amplifier whose gain is controlled, and an output signal of the amplifier that is applied to the base via a capacitor and a resistor. a charging transistor, a diode inserted between the base of the charging transistor and ground, a capacitor charged by the charging transistor, and a control signal generation circuit that generates a control signal according to a terminal voltage of the capacitor; A transistor that attenuates the input signal of the amplifier according to the control signal is provided. (E) Effect According to the present invention, since a charging transistor is provided and the smoothing capacitor is charged by the charging transistor, charging can be performed with a sufficiently large current, and the attack time can be greatly shortened. I can do it. At this time, since the output signal of the amplifier is applied to the base of the charging transistor, the load on the amplifier is not heavy and the output voltage obtained at the output terminal can be prevented from decreasing. (F) Embodiment Figure 1 is a circuit diagram showing an embodiment of the present invention.
13 is an input terminal to which an input signal such as an audio signal is applied; 14 is an amplifier for amplifying the input signal; 15 is an output terminal from which the output signal of the amplifier 14 is derived; and 16 is an output obtained at the output terminal 15. a charging transistor, 19, to whose base a signal is applied via a first capacitor 17 and a first resistor 18;
is a diode whose cathode is connected to the base of the charging transistor 16 and whose anode is connected to the ground, 20 is a second capacitor charged by the charging transistor 16, and 21 is a second resistor connected in parallel to the second capacitor 20. , 22 consists of a transistor 23 whose base is connected to one end of the second capacitor 20, and a current inversion circuit 24 which inverts the collector current of the transistor 23.
a control signal generating circuit that generates a control signal according to the terminal voltage of the second capacitor 20; and a control signal generating circuit 25 that attenuates the input signal according to the control signal.
It is an ALC transistor. Next, the operation will be explained. The input signal applied to the input terminal 13 is amplified by the amplifier 14, then output to the output terminal 15, and applied to a subsequent magnetic head (not shown) for recording. Assuming that the peak voltage of the output signal obtained at the output terminal 15 is _V0 , the first capacitor 17 is charged to the polarity shown in the negative half cycle of the output signal, as in the case of FIG. At that time, the voltage V _C across the first capacitor 17 is the same as equation (1).
In the positive half cycle of the output signal, a voltage obtained by superimposing the voltage V _C across the second capacitor 17 on the peak voltage V ₀ of the output signal is applied to the base of the charging transistor 16 . For that reason,
The charging transistor 16 becomes conductive and charging of the second capacitor 20 is started. When the charging of the second capacitor 20 progresses and its terminal voltage reaches a predetermined value, the transistor 23 becomes conductive, and its collector current is inverted by the current inverting circuit 24 and supplied to the base of the ALC transistor 25. the result,
The ALC transistor 25 becomes conductive, and the input signal is attenuated. In the case of the present invention, the current flowing through the first resistor 18 during the positive half cycle of the output signal is only the base current of the charging transistor 16, which is very small. Therefore, the load on the amplifier 14 becomes lighter, and the output voltage at the output terminal 15 does not drop. In addition, a diode 19 and a first capacitor 17
Since the charging transistor 16 performs voltage double rectification, the ALC level of the output signal is
The voltage is maintained at a low value of 0.477V, making it possible to further improve the voltage reduction characteristics. Furthermore, the charging current of the second capacitor 20 is β (current amplification factor) times as large as the base current of the charging transistor 16, so it is sufficiently large. As a result, the attack time of the ALC operation becomes sufficiently short, and recording distortion during the transition period of the ALC operation can be reduced. In that case,
Since the value of the second resistor 21 can be set to correspond only to the discharging time constant without considering the charging time constant, it does not adversely affect the recovery time. Furthermore, a current inversion circuit 2 in which the control signal generation circuit 22 inverts the output current of the transistor 23
4, the control signal generation circuit 22
This increases the output impedance of the ALC transistor 25, prevents a signal from leaking from the collector to the base of the ALC transistor 25, and improves the distortion factor. (g) Effects of the invention As described above, according to the invention, the smoothing capacitor can be charged with a large current without causing a drop in the output voltage of the amplifier. For that reason,
The attack time of ALC operation can be shortened and the distortion rate can be improved.

[Brief explanation of the drawing]

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. 14...Amplifier, 16...Charging transistor,
17...First capacitor, 18...First resistor, 1
9...Diode, 20...Second capacitor, 2
2...Control signal generation circuit, 25...ALC transistor.

Claims (1)

[Scope of utility model registration request] An automatic level control circuit that detects the level of an output signal of an amplifier and attenuates the input signal of the amplifier to keep the level of the output signal constant,
a charging transistor to which the output signal of the amplifier is applied to the base via a capacitor and a resistor; a diode inserted between the base of the charging transistor and ground and forming a voltage doubler rectifier circuit together with the charging transistor; An automatic level control circuit consisting of a capacitor charged by a charging transistor, a control signal generation circuit that generates a control signal according to the terminal voltage of the capacitor, and a transistor that attenuates the input signal of the amplifier according to the control signal. .