JPH026450B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for automatically controlling the signal level of an analog signal.

Devices that automatically control the signal level of analog signals can be broadly classified into the following two types. One of them is the dynamic level compander (DLC),
The other is a dynamic level expander (DLE). Both are included in automatic level control devices in a broad sense. These DLC/DLE are widely used in noise reduction systems and the like, but DLC is particularly often used in tape recorders. DLC in tape recorders is usually known as ALC (Automatic Level Controller), and is used to record at an appropriate level regardless of the recording input level.

FIG. 1 shows the basic configuration of the above-mentioned automatic level control device. An input signal E _io is input to the variable gain circuit 10 from the input terminal A. Then, the first signal E1 corresponding to the signal _Eio is transmitted from the circuit 10 to the detection circuit 20.
0 is given. Circuit 20 generates a second signal E20 having a DC level corresponding to the magnitude of signal E10. Signal E20 is given to variable impedance element 30 as a control signal. Element 30
is coupled to circuit 10 via coupling line LC. When the ALC shown in FIG. 1 is configured, the average level of the output signal _Eput of the circuit 10 is converged within a certain predetermined range, regardless of the magnitude of the signal level of the input _Eio . The signal _Eput , which has been subjected to automatic level control in this way, is taken out from the output terminal B.

Conventionally, as the detection circuit 20, a circuit that utilizes rectification between the base and emitter of a diode or a transistor has been frequently used. By the way, the detection circuit 20 includes the variable impedance element 3
The second signal E20 must be output at a level greater than the input threshold level of zero. This is because if the level of the signal E20 is lower than the input threshold level, automatic level control will no longer be performed. Therefore, the first signal E10 corresponding to the second signal E20, which is higher than the input threshold level, must be input to the circuit 20. According to the conventional circuit, in order to satisfy this requirement, it was necessary to set the level of the first signal E10 or the output signal E _{put to} a considerably high level. Furthermore, because of the above-mentioned requirements, the conventional circuits were unsuitable for low voltage operating devices on the order of 2 to 3 V. If you would like to know more about why the conventional circuit is unsuitable for low level, low power supply voltage operation, please refer to Japanese Patent Application No. 1985-85439 filed by the same applicant as the present application.

This invention was made in view of the above circumstances,
An object of the present invention is to provide an automatic level control device that can operate stably even at low power supply voltages or low signal levels.

In order to achieve the above object, the automatic level control device according to the present invention is equipped with a specially configured detection circuit. That is, this detection circuit consists of a transistor whose base receives the output signal of the device and which outputs a control signal from its emitter, a capacitor connected to the emitter of this transistor, and a capacitor connected between the emitter and base of this transistor. It is equipped with a feedback element.

According to the above configuration, the charging voltage component of the capacitor is positively fed back to the base side of the transistor via the feedback element. The output signal of the device is superimposed on this positive feedback component. Therefore, even if the output signal level of the device is low, a high level control signal is likely to be generated. Moreover, because of this positive feedback effect, even if the power supply voltage drops to near the lower limit at which the transistor can operate, a relatively large level control signal can be obtained.

Next, embodiments of this invention will be described. In the description, common reference numerals will be used for common parts throughout all the drawings to avoid duplicate description.

In FIG. 2, input terminal A is connected to the input terminal of amplifier 12 via input resistor R10 and input capacitor C10. The output end of amplifier 12 is connected to output end B via output capacitor C12. A first signal E10 is derived from the output of the amplifier 12. The first signal E10 is applied to the base of the NPN transistor 22 via the coupling capacitor C20. The collector of transistor 22 is connected to the positive power supply +V _CC . transistor 2
The emitter No. 2 is grounded via a first resistor (charging resistor) R20 and a charge storage capacitor C22. Connection point P between resistor R20 and capacitor C22
10 is grounded via a second resistor (discharge resistor) R22. At this connection point P10, a capacitor C
A feedback signal EF corresponding to the charging potential of 22 is generated. The signal EF is fed back to the base circuit of the transistor 22 via the anode and cathode of the feedback diode D24. Diode D24 has
A feedback resistor R24 is connected in parallel. Diode D
24 and resistor R24 constitute a feedback element 24 connected between the base circuit and emitter circuit of transistor 22.

The first resistor (charging resistor) R20 connects the positive power supply +V _CC to the capacitor C2 via its collector-emitter path when the transistor 22 is turned on.
It is connected in series with the current path of the charging current flowing into 2. And this resistor R20 is the capacitor C
included in the charging time constant for 22. Also, the second
Resistance (discharge resistance) R22 is included in the discharge time constant for capacitor C22. That is, the attack time of the ALC circuit can be adjusted by the first resistor R20, and the recovery time of the ALC circuit can be adjusted by the second resistor R22. Further, the capacitor C20 is provided to isolate the base circuit of the transistor 22 from the output circuit of the amplifier 12 in terms of direct current. If the capacitor C20 were not present, the transistor 22 would be constantly on (or off) and no ALC operation would occur if the DC potential at the output end of the amplifier 12 deviated significantly from zero. Furthermore, if there is no capacitor C20 and the DC output resistance of the amplifier 12 is small,
Almost no DC positive feedback is applied from the connection point P10 to the base of the transistor 22.

Note that there is no problem in setting R20=0 in order to shorten the attack time of the ALC operation. Also, if the input resistance of element 30 is low or if you want to lengthen the recovery time, R22 = ∞
You can also use it as Usually, as R20, a value of several tens to several hundred ohms is used, and as R22, a value of several hundred to several thousand kilohms is used.

A second signal E20 having a potential corresponding to the first signal E10 is generated at the connection point P10. This signal E20 is applied to the NPN transistor 32
given on the basis of. The collector of transistor 32 is connected to the positive power supply +V _CC through resistor R30, and its emitter is connected to the base of NPN transistor 34. The emitter of transistor 34 is grounded, and its collector is coupled to the connection point between resistor R10 and capacitor C10. Signal E
When the potential of 20 is greater than the sum of the input threshold levels of transistors 32 and 34, 2V _BE , base current flows through transistor 34. The collector-emitter internal impedance of the transistor 34 changes in accordance with its base current. The resistor R10 and the internal impedance of the transistor 34 constitute an attenuator provided on the input side of the variable gain amplifier circuit 10. The amount of attenuation of the input signal E _io by this attenuator changes depending on the second signal E20. That is, the second signal E20 becomes a control signal for the variable impedance element 30. Note that the sum of the input threshold levels of the transistors 32 and 34 is 2V _BE
is the input threshold level of element 30.
This 2V _BE value is approximately 1V at room temperature when using a silicon transistor.

If the base current when the transistor 22 is turned on is very small, the base potential of the transistor 22 will be approximately equal to the potential of the connection point P10. That is, the potential change at the connection point P10 is positively fed back to the base side of the transistor 22 without being significantly reduced by the feedback element 24. Because of this positive feedback, transistor 2
In the range in which the signal E2 performs a non-saturated operation, the change in the signal E20 with respect to the change in the signal E10 becomes large.

FIG. 3 shows the input/output characteristics of the detection circuit 20. Here, the three curves 3A, 3B and 3C are the first
The relationship between the signal E10 and the control signal (second signal) E20 is shown. For curve 3A, when the value of feedback resistor R24 is sufficiently larger than for curve 3B, R24=
∞, and curve 3C shows a case where the resistance R24 is sufficiently smaller than that of curve 3B. Both of these curves show that their slopes (dE20/dE10) are large. That is, the transfer function E20/E10 of the detection circuit 20 is large. Therefore, the loop gain of the ALC loop becomes high, so that the level change of the output signal E _put becomes small.

FIG. 4 shows the input/output characteristics of the variable gain circuit 10. The three curves 4A, 4B and 4C correspond to curves 3A, 3B and 3C in FIG. 3, respectively. For example, regarding curve 4B, when the input level is below L10, transistor 34 is not turned on, and when the input level is above L20, transistor 34 is not turned on.
is saturated. Therefore, the input level range in which the ALC operation is performed is between L10 and L20. According to the circuit configuration shown in FIG. 2, it is possible to suppress the change in output level to within 1 dB or less for a level change of 30 dB or more.

FIG. 3 shows that the minimum input level required to provide the input threshold level 2V _BE of variable impedance element 30 can be varied by feedback resistor R24. In addition, Fig. 4 shows a predetermined output level (curve 4) converged by ALC operation.
This shows that the horizontal portion of A to 4C) can be changed by the feedback resistor R24. In other words, if the resistor R24 is a resistor whose value can be changed,
without changing other parts of the ALC circuit.
The starting level and output level of the ALC circuit can be changed. Therefore, the circuit configuration of the detection circuit 20 is fixed and the circuit 10 and the element 30 are
Even if the configuration of the motor is changed variously, desired ALC characteristics (starting level and other characteristics) can be obtained by changing just one resistor R24. Therefore, according to the present invention, the degree of freedom in designing the ALC circuit increases. This can be said to be a major feature not found in conventional detection circuits.

5 and 6 show modified examples of the detection circuit 20. In FIG. In FIG. 5, a charging resistor R20 is placed on the collector side of the transistor 22. When the transistor 22 is turned on, its collector and emitter are almost in a short state. Therefore, the resistor R20 does not necessarily have to be inserted on the emitter side. In short, the resistor R20 only needs to be connected in series to the charging current path for the capacitor C22. In FIG. 6, the transistor circuit that charges the capacitor C22 is an inverted Darlington circuit. In this case, the charging current for capacitor C22 is supplied primarily through PNP transistor 26. The transfer function E20/E10 in FIG. 6 is the same as the circuit 2 in FIG.
It becomes larger than the transfer function E20/E10 of 0.

7 and 8 show modified examples of the variable impedance element 30. FIG. In FIG. 7, the ALC operation is performed by using a photocoupler 36 and changing the amount of negative feedback to the amplifier 12. In FIG. 8, the amount of negative feedback of the amplifier 12 is changed using the drain-source resistance of the FET 38.
The gate of FET38 is connected via resistor R34.
A control signal 20 having an opposite phase to the second signal E20 is provided. (If we add the second signal E20 here,
It becomes a level expander instead of an ALC. ) The gate of FET38 is connected to the drain via resistor R36. The source of FET38 is grounded, and the drain is connected to amplifier 12 via resistor R18.
is coupled to the inverting input terminal of. Since the change in internal resistance of FET 38 changes the amount of negative feedback, ALC operation becomes possible. Note that due to the presence of resistors R34 and R36, negative feedback is applied to FET38. Since this negative feedback improves the nonlinearity of the internal resistance of the FET 38, an ALC circuit with less distortion can be obtained.

Although not shown, the diode D24
A second feedback resistor may be connected in series with the second feedback resistor. The characteristics shown in FIGS. 3 and 4 change depending on the value of the resistor connected in series/parallel to this diode D24.

The embodiments described in detail in this specification and shown in the drawings are not intended to limit the invention. Various substitutions, additions, changes, etc. can be made within the spirit of the invention and the scope of the claims. For example, instead of the diode D24, the area between the base and emitter of a transistor may be used. In addition, a small amount of
It is also possible to apply a DC bias current.
Further, the present invention can of course be applied to a level expander.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the basic configuration of an automatic level control device; FIG. 2 is a circuit diagram showing an automatic level control device according to an embodiment of the present invention; FIG.
A graph illustrating the input/output characteristics of the detection circuit 20 when the feedback resistor R24 shown in the figure is used as a parameter;
FIG. 4 is a graph illustrating the input/output characteristics of the variable gain circuit 10 when the feedback resistor R24 is used as a parameter; FIGS. 5 and 6 are circuits showing modifications of the detection circuit 20 shown in FIG. 2. FIGS. 7 and 8 are circuit diagrams showing modifications of the variable impedance element 30 shown in FIG. 2. A...Input terminal, 10...Variable gain circuit, E _io ...
...Input signal, 20...Detection circuit, E10...First
Signal, E20...Second signal (control signal), 30...
...variable impedance element, LC...coupling line,
E _put ...Output signal, B...Output end, 12...Amplifier, C20...Coupling capacitor (second capacitor), 22, 32, 34...NPN transistor, +V _CC ...Positive power supply, R20... ...First resistor (charging resistor), C22...Charge storage capacitor, P
10... Connection point, R22... Second resistor (discharge resistance), EF... Feedback signal, D24... Feedback diode, R24... Feedback resistor, 24... Feedback element, 2
6...PNP transistor, 36...photocoupler, 38...FET, 20...control signal.

Claims (1)

[Claims] 1. A variable impedance element whose impedance changes according to a control signal, a variable gain circuit whose gain changes according to the impedance of the variable impedance element, and a variable gain circuit whose gain changes according to the impedance of the variable impedance element, and a variable impedance element whose impedance changes according to a control signal. a detection circuit that generates the control signal, wherein the detection circuit is configured to provide a first signal corresponding to the output signal to a base circuit, and derive a second signal having a potential corresponding to the first signal from an emitter circuit. a feedback element connected between the base circuit and the emitter circuit of the transistor; and a feedback element connected between the emitter circuit of the transistor and a circuit in which the AC potential is substantially zero and corresponds to the second signal. a capacitor for storing a charged electric charge, a feedback signal corresponding to the charging potential of the capacitor is applied to a base circuit of the transistor via the feedback element, and the second signal is used as the control signal. Features an automatic level control device. 2. The transistor is a bipolar transistor, the feedback element includes a PN junction diode, and the transistor is connected between the base and emitter of the transistor.
The automatic level control device according to claim 1, wherein the PN junction and the PN junction of the diode are connected in opposite directions. 3. The automatic level control device according to claim 1 or 2, wherein the feedback element includes a feedback resistor, and the feedback resistor is connected between a base circuit and an emitter circuit of the transistor. 4. The detection circuit includes a charging resistor connected in series with a current path of a charging current flowing into the capacitor via a collector-emitter path of the transistor, and this charging resistor is included in a charging time constant for the capacitor. An automatic level control device according to any one of claims 1 to 3. 5. The detection circuit includes a first resistor connected in series between the emitter circuit of the transistor and the capacitor, and a second resistor connected in parallel between the emitter circuit of the transistor and the AC potential zero circuit. Claims 1 to 4, wherein the first resistor is included in a charging time constant for the capacitor, and the second resistor is included in a discharging time constant for the capacitor. Automatic level control device. 6. The detection circuit includes a second capacitor connected in series between the output circuit of the variable gain circuit and the base circuit of the transistor, and only the alternating current component of the output signal passes through the second capacitor to the second capacitor. An automatic level control device according to any one of claims 1 to 5, which is applied to a base circuit of a transistor. 7. The variable impedance element constitutes an attenuator provided on the input side of the variable gain circuit, and the amount of signal attenuation of the attenuator changes in accordance with the control signal. The automatic level control device described in . 8. Claims 1 to 6, wherein the variable gain circuit has a feedback loop, the variable impedance element is included in the feedback loop, and the closed loop gain of the variable gain circuit changes in accordance with the control signal. The automatic level control device according to any of paragraphs. 9. The automatic level control device according to any one of claims 2 to 8, wherein the detection circuit includes a second feedback resistor connected in series with the diode.