JPS625538B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic gain control circuit, and in particular, an object of the present invention is to provide an automatic gain control circuit that can improve voltage utilization during low voltage operation and shorten attack time.

A conventional general automatic gain control circuit is shown in FIG. In FIG. 1, an input signal applied to an input terminal 1 is input to an amplifier circuit 3 via a resistor 2 and amplified. The output of the amplifier circuit 3 is supplied to an output terminal 5 via a DC blocking capacitor 4, rectified by a diode 6, and supplied to a smoothing circuit 9 composed of a capacitor 7 and a resistor 8. The output voltage of the smoothing circuit 9 is determined by the variable impedance element 1 made of a transistor, FET, etc.
0 control terminal 11. One end of the variable impedance element 10 is grounded, the other end is connected to the input terminal of the amplifier circuit 3, and the control terminal 1
The impedance changes depending on the voltage supplied to 1.

In the above configuration, when the input signal applied to the input terminal 1 exceeds a certain level, a so-called automatic gain operation is performed in which the output signal appearing at the output terminal 5 is limited to a substantially constant level.

However, the conventional automatic gain control circuit as described above has the following drawbacks. That is, since the voltage for driving the variable impedance element 10 is obtained by rectifying the output signal by the diode 6, there is a loss corresponding to the forward voltage of the diode 6. This loss is not a problem when the power supply voltage of the circuit is high and the output amplitude of the amplifier circuit is large, but
This is a consideration when the power supply voltage is low.

Furthermore, the charging capacity of the smoothing circuit 9 depends on the output impedance of the amplifier circuit 3, and this output impedance must be made sufficiently low to shorten the attack time.

The present invention has been made in view of the above-mentioned drawbacks, and it is an object of the present invention to provide an automatic gain control circuit particularly suitable for recorders or radio receivers with low power supply voltages.

FIG. 2 is a circuit diagram showing an embodiment of the automatic gain control circuit according to the present invention.

In FIG. 2, parts having the same functions as those in FIG. 1 are given the same numbers and their explanations will be omitted.

In FIG. 2, 12 is an operational amplifier having an inverting input terminal 13, an output terminal 14, and a non-inverting input terminal 15, 16 has a base connected to the output terminal 14 of the operational amplifier 12, an emitter connected to the power supply terminal V _CC , and a collector connected to the output terminal 14 of the operational amplifier 12. These transistors are each connected to the smoothing circuit 9 via a resistor 17 and operate as a common emitter amplifier circuit. 18 has a smoothing circuit 9 as its base.
The collector is connected to the power supply terminal V _CC , and the emitter is connected to the operational amplifier 1 through a resistor 19.
The transistor is connected to the non-inverting input terminal 15 of the operational amplifier 12 and operates as a common collector amplifier circuit, and forms a negative feedback circuit in cooperation with the resistor 20 connected between the non-inverting input terminal 15 of the operational amplifier 12 and ground. are doing.

Next, the operation of the automatic gain control circuit having the above configuration will be explained.

The input signal applied to the input terminal 1 is amplified by the amplifier circuit 3 and output to the output terminal 5, and is also applied to the inverting input terminal 13 of the operational amplifier 12. When the inverting input terminal 13 becomes positive, the output terminal 1
4 is driven in the negative direction, increasing the emitter-base voltage of transistor 16, causing transistor 16 to
increases the collector current of Most of the collector current of this transistor 16 flows into the smoothing circuit 9 and charges the capacitor 7. This voltage is fed back to the non-inverting input terminal 15 via the transistor 18 and resistor 19. The relationship between the terminal voltage V _O of the smoothing circuit 9 and the peak value V ₁ of the signal applied to the inverting input terminal 13 is as follows: The base-emitter voltage of the transistor 18 is V _BE (0.5V), and the resistance values of the resistors 19 and 20 are respectively Assuming R ₁₉ and R ₂₀ , V ₀ =V ₁ ×(R ₁₉ +R ₂₀ )/R ₂₀ +V _BE .

When the signal applied to the inverting input terminal 13 then decreases, the potential at the non-inverting input terminal 15 remains approximately at its previous value, so the output terminal 14 is driven in the positive direction and the transistor 16 is cut off. That is, when the input terminal 13 changes in the positive direction, the transistor 16 becomes conductive and charges the smoothing circuit 9, but when the input terminal 13 changes in the negative direction, it becomes cut-off, and the transistor 16 becomes substantially similar to the diode 6 in FIG. Performs rectifying operation. Of course, if the input signal is constant and the terminal voltage of the smoothing circuit 9 decreases due to discharge by the resistor 8 included in it, discharge by the current flowing into the control terminal 11 of the variable impedance 10, and discharge by the base current of the transistor 18, then , is fed back to the non-inverting input terminal 15, the output terminal 14 is lowered, the transistor 16 is turned on, and a charging current commensurate with the discharge is supplied to the smoothing circuit 9, and a voltage corresponding to the input signal is applied to the smoothing circuit 9. The terminal voltage will be .

Note that the transistor 18 makes the feedback circuit high impedance with respect to the smoothing circuit 9, and
Even when there is no input signal, the smoothing circuit 9 functions to generate a voltage corresponding to V _BE (0.5V) of the transistor 18. This is because when the terminal voltage of the smoothing circuit 9 is lower than V _BE (0.5V),
Transistor 18 is cut off, the feedback circuit is opened, and the amplification is greatly increased. As a result, even if there is no input signal, the circuit operates due to hum noise etc., and the terminal voltage of the smoothing circuit 9 becomes V _BE (
0.5V) and reach equilibrium. Then, when a signal is applied to the input terminal 13, as described above, a voltage multiplied by the gain determined by the resistance values of the resistors 19 and 20 of the input amplitude is obtained by adding to V _BE .

This is the base-emitter voltage V of a normal transistor as the control voltage of the variable impedance.
It is very convenient to use a Darlington connected transistor as the variable impedance element 10, which requires a voltage around twice _BE (0.5V).

As described above, the transistor 16 has a rectifying function similar to the diode 6 in FIG. 1, and becomes saturated when the input signal suddenly changes in the positive direction. Therefore, if the collector-emitter voltage at saturation is V _CE Sat (0.2V), then the power supply voltage V _CC
- The voltage of V _CE Sat can be used as the charging voltage of the smoothing circuit 9. Furthermore, as shown in FIG. 1, this charging capacity does not depend on the output impedance of the amplifier 3 and can be arbitrarily set by adjusting the resistor 17. Therefore, the attack time of automatic gain control can be arbitrarily and easily set, and the attack time can also be made extremely short.

Further, as described above, the voltage obtained in the smoothing circuit 9 with respect to the signal amplitude applied to the inverting input terminal 13 has a degree of amplification due to the resistors 19 and 20 of the feedback circuit, and the voltage required for the inverting input terminal 13 Signal amplitude can be reduced. Therefore, the dynamic range or dynamic margin of the amplifier circuit 3 can be increased.

FIG. 3 is a circuit diagram showing another embodiment of the automatic gain control circuit according to the present invention.

In FIG. 3, the operational amplifier 12 in FIG. 2 is replaced with a transistor 21 and bias resistors 22 and 23.

Next, the operation of the automatic gain control circuit having the configuration shown in FIG. 3 will be explained.

First, resistors 21 and 22 cause the base of transistor 21 to have a voltage V between its base and emitter.
When biased below _BE (0.5V), transistors 21 and 18 are
and 16 are cut-offs, and the terminal voltage of the smoothing circuit 9 is approximately zero voltage. When an input signal is applied, this is superimposed on the bias voltage and applied to the base of transistor 21, and when its peak value begins to exceed V _BE (0.5V), transistor 21
1 and 16 become active, and the terminal voltage of smoothing circuit 9 rapidly rises to V _BE of transistor 18. When the input signal increases further, the AC gain (R ₁₉
+R ₂₀ )/R ₂₀ times the voltage is V _BE of transistor 18
A considerably raised voltage is obtained at the terminal of the smoothing circuit 9, thereby making it possible to control the impedance of the variable impedance element 10.

Also, if the base of transistor 21 is biased by resistors 22 _and 23 to a voltage V _H higher than its V _{BE (} 0.5V), then (R ₁₉ + _R20 )/
A voltage ₂₀ times higher than R can be obtained by increasing the voltage equivalent to V _BE of the transistor 18.

Therefore, bias resistors 22 and 23, and
By setting the resistors 19 and 20 of the feedback circuit, the signal amplitude can be set to an arbitrary magnitude, and the correspondence between the input signal amplitude and the output voltage can be given an arbitrary degree of amplification. Since the variable impedance element 10 is controlled by this voltage, the signal level appearing at the output terminal 5 can be set arbitrarily, and the output signal increases in response to an increase in the input signal within the range where automatic gain control is operating. In other words, the input/output characteristics can also be adjusted as desired.

In the embodiment, the base bias of the transistor 21 is obtained by the resistors 22 and 23, but it goes without saying that the present invention is not limited to this.

Further, in the embodiment, a common collector amplifier circuit is used to make the feedback circuit high impedance with respect to the smoothing circuit 9, but it goes without saying that other high input resistance amplifier circuits can also be used.

As described above, according to the present invention, there is no voltage loss corresponding to the forward voltage of the diode, and a voltage approximately equal to the power supply voltage can be used for charging the smoothing circuit, and the charging capacity is also higher than that of the preceding stage amplifier. It can be set arbitrarily without depending on the output impedance, and since the variable impedance element is driven by this output voltage, it is possible to perform automatic gain control with extremely fast attack time even in devices with low power supply voltage, and its adjustment and setting are also easy. It's easy.

Furthermore, according to the present invention, since the correspondence between the input voltage and the output voltage of the smoothing circuit can be given an arbitrary degree of amplification, the input/output characteristics can also be set arbitrarily within the range in which automatic gain control is operating. An automatic gain control circuit can be provided that can be adjusted.

Furthermore, according to the present invention, even when there is no input signal, a certain output voltage can be obtained from the smoothing circuit, and a voltage corresponding to the input signal can be added to this output voltage, and this voltage can be used to make the output voltage variable. Since the impedance element is controlled, it is possible to provide an automatic gain control circuit that can arbitrarily set the start level of automatic gain control.

[Brief explanation of the drawing]

FIG. 1 is a circuit configuration diagram of a conventional automatic gain control circuit, FIG. 2 is a circuit configuration diagram showing one embodiment of the automatic gain control circuit according to the present invention, and FIG. 3 is a circuit configuration diagram showing another embodiment. be. 1...Input terminal, 3...Amplification circuit, 5...Output terminal,
9... Smoothing circuit, 10... Variable impedance element,
12... Operational amplifier, 16, 18, 21... Transistor, 19, 20... Resistor forming the feedback circuit.

Claims (1)

[Claims] 1. An automatic gain control circuit that obtains an output signal of a substantially constant level for an input signal of a predetermined level or higher, comprising a variable impedance element connected between an input signal path and ground, and the variable impedance element. a first amplifier circuit into which the signal appearing at both ends of the element is amplified at a certain amplification degree and input; a grounded emitter amplifier circuit connected to the output terminal of the first amplifier circuit; It is characterized by comprising a smoothing circuit connected to the output terminal and a feedback circuit for feeding back the output of the smoothing circuit to the first amplifier circuit, and supplying the output of the smoothing circuit to the control terminal of the variable impedance element. automatic gain control circuit. 2. The automatic gain control circuit according to claim 1, characterized in that the feedback circuit uses an amplifier circuit with a high input resistance. 3. The automatic gain control circuit according to claim 2, characterized in that a common collector amplifier circuit is used as the high input resistance amplifier circuit. 4. The automatic gain control circuit according to claim 1, characterized in that a common emitter amplifier circuit is used as the first amplifier circuit.