JPH0423443B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Industrial Application Field The present invention provides an RF AGC that can automatically control the gain of an RF amplification circuit of an AM radio receiver.
It relates to circuits, and in particular to RF AGC circuits that can cope with sudden changes in the electric field strength of received signals.

(b) Prior art On page 84 of the '85 Sanyo Semiconductor Handbook Monolithic Bipolar Integrated Circuits, published on March 20, 1985, there is a description of how an RF amplifier circuit operates in response to a control signal obtained from a mixing circuit. RF controlling gain
AGC circuit is described. As shown in FIG. 2, the RF AGC circuit includes a FET 2 to which the received signal received by the antenna 1 is applied to the base, an AGC transistor 3 whose emitter is connected to the drain of the FET 2, and a collector of the transistor 3. a selective tuning circuit 4 connected to the tuning circuit 4; a signal circuit 5 that generates an IF signal by mixing the RF signal obtained from the tuning circuit 4 and the local oscillation signal obtained from a local oscillation circuit (not shown); An AGC signal generation circuit 6 detects the level of the IF signal obtained from the mixing circuit 5 and generates a gain control signal, and drives the AGC transistor 3 according to the gain control signal.
The AGC drive circuit 7 is also provided. In the case of the circuit shown in FIG. 2, when the received signal received by the antenna 1 has a strong electric field, the RF signal is amplified by the FET 2 and applied to the mixing circuit 5 via the AGC transistor 3 and the selective tuning circuit 4. The level of the IF signal generated from the mixing circuit 5 increases. Therefore, AGC
The gain control signal generated from the signal generation circuit 6 also increases, and the current flowing through the transistors 8 and 9 forming the AGC drive circuit 7 increases. As a result, the smoothing capacitor 10 is discharged, the base voltage of the AGC transistor 3 decreases, and the gain of the RF amplifier circuit decreases.

Further, when the received signal received by the antenna 1 has a weak electric field, the gain control signal generated from the AGC signal generation circuit 6 becomes small, and the transistors 8 and 9
Since the current flowing through the RF amplifier circuit decreases or becomes zero, the smoothing capacitor 10 is charged by the charging resistor 11, and the gain of the RF amplifier circuit increases. Therefore, by using the circuit shown in FIG. 2, the gain of the RF amplifier circuit can be automatically controlled.

(c) Problems to be Solved by the Invention In order to improve the stability of the RF AGC circuit, the capacitance of the smoothing capacitors 10 and 12 must be increased. However, especially smoothing capacitor 1
When the capacitance of the smoothing capacitor 10 is increased, the time constant determined by the resistance value of the charging resistor 11 and the capacitance of the smoothing capacitor 10 becomes large, and the charging time of the smoothing capacitor 10 becomes longer. Therefore, in the case of a mobile receiver such as a car radio, when the reception state suddenly changes from a reception state of a strong electric field to a reception state of a weak electric field, the charge of the smoothing capacitor 10 cannot keep up, resulting in sound interruptions. A big problem had arisen.

(d) Means for Solving Problems The present invention has been made in view of the above points, and when the electric field strength of a received signal changes from strong to weak, an AGC circuit is configured in accordance with the change. It is characterized by a quick charging circuit that quickly charges the smoothing capacitor.

(E) Effect According to the present invention, when the electric field strength of the received signal changes from strong to weak, the quick charging circuit is activated and the smoothing capacitor is rapidly charged. Even when using a RF amplifier, the terminal voltage can be increased rapidly, and the gain of the RF amplifier circuit can be quickly increased.

(F) Embodiment FIG. 1 shows an embodiment of the present invention.
3 is a quick charging circuit that operates in response to a signal indicating that the received signal obtained from the AGC signal generation circuit 6 has changed from a strong electric field to a weak electric field. In FIG. 1, circuit elements that are the same as those in FIG. 2 are designated by the same reference numerals, and their explanations will be omitted.

When a strong electric field signal is being received, the output signal of the AGC signal generation circuit 6 increases, and the collector current of the transistor 8 increases. Since the transistor 8 is connected to the transistor 9 in a current mirror relationship, the collector current of the transistor 9 also becomes large. Therefore, the smoothing capacitor 10 is discharged, its terminal voltage decreases, and the degree of conductivity of the AGC transistor 3 decreases significantly. Therefore,
The gain of the RF amplifier circuit is controlled, and the level of the output signal of the mixing circuit 5 is kept substantially constant. If the strong electric field reception state continues, the smoothing capacitor 10 continues to discharge, and its terminal voltage decreases significantly.

In this state, if the electric field strength of the received signal changes weakly, the received signal in the weak electric field will become a state with a reduced gain.
The signal is amplified by the RF amplifier circuit, and the output signal level of the mixing circuit 5 decreases. Therefore, AGC
The output signal of the signal generation circuit 6 becomes small, and the collector currents of the transistors 8 and 9 also become small. When the collector current of the transistor 9 becomes small, the smoothing capacitor 10 is charged by the charging resistor 11, but since the charging time constant is large, the terminal voltage of the smoothing capacitor 10 must be suddenly increased. I can't. On the other hand, when the output signal of the AGC signal generation circuit 6 suddenly changes from large to small, the quick charging circuit 13 starts operating in response to the change. The relatively large output current of the quick charging circuit 13 is
Since the smoothing capacitor 10 is rapidly charged, its terminal voltage immediately rises, increasing the conductivity of the AGC transistor 3. Therefore, the gain of the RF amplifier circuit increases, and the weak electric field received signal is sufficiently amplified, and the output signal of the mixing circuit 5 reaches a sufficient level, thereby preventing sound breakage. Quick charging circuit 13
Since it becomes inactive after a predetermined period of time has passed after starting operation, it does not adversely affect normal AGC operation.

FIG. 3 shows a specific circuit example of the present invention.
When an input signal having a level corresponding to the electric field strength is applied from the mixing circuit 5 to the input terminal 14, the level of the input signal is detected by the diodes 15 and 16, and smoothed by the smoothing capacitor 17. A first transistor 18 whose base is connected to the smoothing capacitor 17 generates an emitter current corresponding to the terminal voltage of the smoothing capacitor 17, and causes a detection current corresponding to the input signal to flow through the second transistor 19. A current mirror circuit consisting of third to fifth transistors 20 to 22 includes the second transistor 1
9 collector current is reversed. The sixth and seventh transistors 23 and 24, whose collectors are connected to the collectors of the fourth and fifth transistors 21 and 22, respectively, have bases commonly connected to the emitter of the eighth transistor 25, and have predetermined equal values. The collector current is set to flow.
Therefore, the fourth and sixth transistors 21 and 23
The difference current between the collector currents becomes the control current of the quick charging circuit described later, and the difference current between the collector currents of the fifth and seventh transistors 2
The difference current between the collector currents of 2 and 27 is the 9th and 27th collector current.
A current mirror circuit 28 consisting of 10 transistors 26 and 27 is supplied. Now, when the level of the input signal ignited at the input terminal 14 becomes large and the collector current of the fifth transistor 22 becomes larger than the collector current of the seventh transistor 24, a current flows through the current mirror circuit 28 , and the current flows through the tenth transistor. 27
The smoothing capacitor 10 is discharged by
The conductivity of the AGC transistor 3 decreases, and the gain of the RF amplifier circuit decreases. Conversely, when the level of the input signal applied to the input terminal 14 becomes small and the collector current of the fifth transistor 22 becomes smaller than the collector current of the seventh transistor 24, the current mirror circuit 28 turns off and the charging resistor 11 As a result, the smoothing capacitor 10 is charged, and the gain of the RF amplifier circuit is increased. Therefore, level detection diodes 15 and 16 and smoothing capacitors 17 and 1
0, the first, second, third, fifth, and seventh transistors 18, 19, 20, 22, and 24, the current mirror circuit 28 , and the AGC transistor 3.
An AGC circuit is configured to perform gain control of the RF amplifier circuit.

By the way, while the collector current of the fourth transistor 21 is receiving a signal with a field strength that is larger than the collector current of the sixth transistor 23, the eleventh transistor 30 constituting the quick charging circuit 29
Since the base current does not flow, the quick charging circuit 29 does not operate and does not adversely affect the AGC operation.
In addition, in a weak electric field receiving state where the collector current of the fourth transistor 21 is smaller than the collector current of the sixth transistor 23, the smoothing capacitor 1
0 terminal voltage becomes sufficiently high and the collector voltage of the eleventh transistor 30 also becomes sufficiently high, the eleventh transistor 30 is saturated and turned off,
After all, it does not adversely affect AGC operation.

In strong electric field reception state, smoothing capacitor 1
0 is sufficiently discharged, and the conductivity of the AGC transistor 3 is sufficiently reduced. If the receiving condition suddenly changes to a weak electric field in this state, the input signal applied to the input terminal 14 becomes zero because the RF amplifier circuit has a low gain, and the terminal voltage of the smoothing capacitor 17 decreases due to the differential action of the input capacitor 31. decreases rapidly. Therefore, the first and second transistors 18
and 19 are turned off, and the third to fifth transistors 20 to 22 are also turned off. When the fifth transistor 22 is turned off, the current mirror circuit 28 becomes inactive, and the smoothing capacitor 1
0 charging starts. However, due to the charging time constant, the rate of increase in the terminal voltage of the smoothing capacitor 10 is not fast. On the other hand, the fourth transistor 21
When turned off, the base current flows through the eleventh transistor 30, and since the terminal voltage of the smoothing capacitor 10 is low at that time, the eleventh transistor 30
is turned on, and the smoothing capacitor 10 is rapidly charged by its collector current. Therefore, the terminal voltage of the smoothing capacitor 10 rises rapidly,
The degree of conductivity of the AGC transistor 3 increases, and the gain of the RF amplifier circuit increases. Therefore, the weak electric field received signal is sufficiently amplified by the RFB amplification circuit, and the signal level decreases instantaneously, thereby preventing sound interruptions.

When the terminal voltage of the smoothing capacitor 10 rises to a predetermined value in response to rapid charging, the eleventh transistor 30
Since the collector voltage of the transistor 30 also becomes high, the eleventh transistor 30 becomes saturated and turns off again, making it irrelevant to the circuit operation.

(G) Effects of the Invention As described above, according to the present invention, even if the electric field strength changes suddenly from strong to weak, the AGC can follow it, so it is possible to prevent sound interruptions. Furthermore, since a large capacity smoothing capacitor can be used, it is possible to improve various characteristics such as the distortion rate during low frequency modulation. Further, according to the present invention, since the quick charging circuit can be operated only when the electric field strength suddenly changes, it is possible to provide an RF AGC circuit that does not adversely affect the AGC operation during normal reception.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a circuit diagram showing a conventional RF AGC circuit, and FIG. 3 is a circuit diagram showing a specific example of the circuit shown in FIG. 3...AGC transistor, 6...AGC signal generation circuit, 10...smoothing capacitor, 13, 29 ...
…Fast charging circuit.

Claims (1)

[Claims] 1. Detection that generates an output signal indicating the level of the received signal when the electric field strength of the received signal changes from strong to weak in the RF AGC circuit that controls the gain of the RF amplifier circuit according to the control signal obtained from the mixing circuit. a first transistor to which a reference voltage is applied to the base, and a first transistor having a base to which the output signal of the detection circuit is applied, in order to compare the output signal of the detection circuit with a reference voltage and generate a charging current according to the difference voltage; and a smoothing capacitor connected to the collector of the second transistor. , an RF AGC circuit characterized in that it performs rapid charging to a value corresponding to the value of the output signal of the detection circuit.