JPH055705Y2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present application relates to an antenna circuit for a radio receiver that is particularly effective for use in a vehicle.

BACKGROUND TECHNOLOGY In a car-mounted AM electronic tuning radio receiver, the antenna and the antenna input end of the receiver are connected by a long cable, and the antenna capacity is large, so generally, as shown in Fig. 2, A non-tunable antenna circuit is used.

In other words, 1 is an antenna, 2 and 3 are coils for preventing noise in the AM reception band, 4 and 5 are coupling capacitors, and 6 is a level that is connected between the coupling capacitors 4 and 5 via a resistor 7 to control the signal level. A transistor for the control circuit, 8 is an amplifier consisting of an IFT that amplifies the antenna output signal and applies it to the next stage RF tuning circuit, 9 is an AC blocking capacitor connected between the base and emitter of the transistor 6, When a received signal is input from the antenna 1, the received signal is input to the gate of the amplifier 8 via the coupling capacitors 4 and 5, and the received signal is amplified and input to the next stage RF tuning circuit.

The AGC voltage obtained from the subsequent circuit is input to the base of transistor 6, and the AGC voltage obtained from the subsequent circuit is input to the base of transistor 6.
is turned on and off depending on the AGC voltage to control the signal level applied to the gate of the amplifier 8.

Problems to be Solved by the Invention By the way, in the antenna circuit described above, when static electricity is induced in the antenna by a thundercloud, for example, while a car is running, the static electricity flows through the coupling capacitors 4 and 5 to the gate of the amplifier 8. is applied to

That is, the level control circuit consisting of the resistor 7, the transistor 6, and the capacitor 9 has a high impedance, so its level control effect on static electricity is extremely weak.Therefore, the static electricity is hardly attenuated and the amplifier 8 is at a high potential. If negative static electricity is applied to the gate of the antenna 1, there is a risk that the amplifier 8 will be destroyed.This also applies when negative static electricity is induced in the antenna 1.

Therefore, in order to protect the amplifier from static electricity, the present application provides a coupling capacitor connected between the antenna and the amplifier that amplifies the signal output of the antenna,
In the antenna circuit of the radio receiver, a signal level control circuit for controlling the signal level input to the amplifier by the AGC voltage is connected to the intermediate portion of the coupling capacitor. The signal level control circuit is connected to the anode of a first diode whose cathode is grounded and to the anode of a second diode whose anode is grounded.
The AGC voltage is applied to the cathode of a second diode, and the anode of the second diode is connected to one end of a capacitor having one end grounded and to the cathode of a third diode having an anode grounded.

Effect When positive static electricity is induced in the antenna, the static electricity passes through the coupling capacitor and is grounded through the first diode in the forward direction relative to the positive static electricity, that is, the circuit equipped with the first diode. Since the impedance of is extremely small relative to positive static electricity, positive static electricity is greatly attenuated at the intermediate position of the coupling capacitor, and the static electricity applied to the amplifier is significantly reduced.

Also, when negative static electricity is induced in the antenna, the third diode and second diode in the forward direction act on the static electricity, so the impedance of the circuit consisting of the third diode and second diode becomes negative. Since it is extremely small compared to static electricity, negative static electricity is greatly attenuated at the intermediate position of the coupling capacitor, and in this case as well, the static electricity applied to the amplifier is significantly reduced.

Embodiment The embodiment of the present application will be described in detail below based on FIG. 1. As in FIG. 2, 1 is an antenna, 2 and 3 are coils for noise prevention, and 4 and 5 are coupling containers connected to a signal transmission path. , 8 is an amplifier consisting of an IFT connected to the next-stage RF tuning circuit.
A signal level control circuit consisting of the transistor 6 shown in the figure is placed at the midpoint a between the coupling capacitors 4 and 5.
First pin diode 11 whose cathode is grounded
and the cathode of the second pin diode 12 to which the AGC voltage is applied.
The other end of an AC blocking capacitor 13 whose one end is grounded and the cathode of a third diode 14 whose anode is grounded are connected to the anode of the diode.

However, if positive static electricity is now induced in the antenna 1, the static electricity will pass through the intermediate point a between the coupling capacitors 4 and 5, and the first point in the forward direction with respect to the positive static electricity.
The positive static electricity is therefore greatly attenuated by the circuit consisting of the first pin diode 11, and the positive static electricity applied to the gate of the amplifier 8 is significantly reduced. Further, when negative static electricity is induced in the antenna 1, a circuit consisting of the forward third diode 14 and the second pin diode 12 acts on the negative static electricity, and the impedance of this circuit becomes Since it is very small, the negative static electricity is greatly attenuated at the midpoint a between the coupling capacitors 4 and 5, and the static electricity applied to the gate of the amplifier 8 is significantly reduced, and in this case too, the amplifier 8 is protected from being destroyed by static electricity. I can do it.

Note that the second pin diode 12 is activated by the AGC voltage, and its operation controls the operation of the first pin diode 11.
Of course, the signal level input to the gate of the amplifier 8 is controlled by controlling the operation of the pin diode 11.

Effects of the invention Since the present application is configured as described above, the received signal level input to the amplifier can be controlled by the AGC voltage, and the signal level control function can be controlled without any hindrance. With an extremely simple configuration, it is possible to effectively prevent damage to the amplifier from static electricity induced in the antenna.
In particular, in order to reduce the impedance against negative static electricity, a third diode is connected in parallel to the AC blocking capacitor, so the capacitor does not need to have a large capacity and has the advantage of being inexpensive. have

[Brief explanation of drawings]

FIG. 1 is an antenna circuit diagram showing an embodiment of the present application;
FIG. 2 is a conventional antenna circuit diagram. In the figure, 1 is an antenna, 2, 3 are coils, 4, 5
is a coupling capacitor, 13 is a capacitor, and 14 is a diode.

Claims (1)

[Claims for Utility Model Registration] (1) A coupling capacitor is connected between an antenna and an amplifier that amplifies the signal output of the antenna,
In the antenna circuit of a radio receiver, the signal level control circuit is connected between the coupling capacitor and a signal level control circuit that controls the signal level input to the amplifier based on the AGC voltage, and the signal level control circuit is connected between the coupling capacitor and The anode of a first diode whose cathode is grounded is connected to the cathode of a second diode whose anode is applied with an AGC voltage, and the anode of the second diode is connected to the other end of a capacitor whose one end is grounded. An antenna circuit for a radio receiver, characterized in that the anode is connected to the cathode of a third diode whose anode is grounded. (2) The antenna circuit for a radio receiver according to claim 1, wherein the first and second diodes are each pin diodes.