GB2157907A

GB2157907A - High-frequency switching circuit

Info

Publication number: GB2157907A
Application number: GB08430059A
Authority: GB
Inventors: Yoshinobu Yamaguchi
Original assignee: Alps Electric Co Ltd
Current assignee: Alps Alpine Co Ltd
Priority date: 1984-04-18
Filing date: 1984-11-28
Publication date: 1985-10-30
Also published as: JPS60220623A; GB2157907B; GB8430059D0

Abstract

A high-frequency switching circuit comprises a high-frequency amplifying transistor 11 for amplifying a high-frequency signal applied thereto, a switching diode 40 connected between a power supply 6 and a collector of the high-frequency amplifying transistor 11 in the forward direction with respect to a current flowing into the collector through the diode 40, first means 20,8 for deriving an output signal of the high-frequency switching circuit through the switching diode 40, and second means 37 for causing the high- frequency amplifying transistor 11 to stop its high-frequency amplifying operation to perform its attenuation operation. In use, a television signal may be applied at 1; output 7 is applied to the input terminal of, for example, a video tape recorder and output from the diode 40 is applied via a mixer 22, having a local oscillator 6, to an output 8 and thence to an antenna terminal of a television receiver. <IMAGE>

Description

SPECIFICATION High-frequency switching circuit BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a highfrequency switching circuit and particularly a high-frequency switching circuit which is improved in switching characteristic for causing a high-frequency amplifying transistor to stop its amplifying operation to thereby cause the same transistor to perform its attenuating operation.

2. Description of Prior Art Fig. 1 is a block diagram of a main part of the conventional booster mixer for receiving a television signal, in which a television signal applied to an input terminal 1 connected with an television antenna is amplified by a highfrequency amplifier 2 and applied to a distributor. One of the signals distributed by the distributor 3 is applied from its output terminal 7 to an input terminal of, for example, a video tape recorder (VTR), while the other signal is applied to and amplified in a highfrequency amplifier 4 and the amplified output signal is applied to a mixer 5. An output signal of a local oscillator 6 is also applied to the mixer 5 and converted into a television signal of a desired channel and applied from an output terminal to an antenna terminal of a television receiver.

In such an arrangement, there is an advantage that a television signal is produced from the output terminal regardless of the state whether it is in the VTR reproducing operation or not, so that the arrangement may be regarded as a channel of television broadcasting when viewed from the television receiver side and it is unnecessary to change over the booster mixer when the VTR operation mode is changed over between reproducing and recording.

In some locations, however, for example, in the neighborhood of the Heathrow air port in England, a disturbing wave close to the output frequency of the booster mixer comes into the antenna to thereby deteriorate the picture quality in the VTR reproducing operartion. To solve this problem a proposal has been made in which, as shown in Fig. 2, a high-frequency amplifier 4 is conncted with a power supply terminal 9 through a switching circuit 10.The switching circuit 10 is turned off to cause the high-frequency amplifier 4 to stop its amplifying operation and cause the highfrequency amplifier 4 to operate as an attenuator for the signal received from an antenna and the input terminal 1, a high-frequency amplifier 2, and a distributor 3, to thereby attenuate a disturbing signal applied to a television receiver through an output terminal 8 to prevent the reproduced picture from being deteriorated in quality.

Fig. 3 is a circuit diagram of an example of a part of the high-frequency amplifier 2 and the mixer 5 of Fig. 2, in which the same reference numeral designates the same part as in Fig. 2, and in which the reference numeral 11 designates a high-frequency amplifying transistor, 1 2 to 1 6 resistors, 1 7 to 20 capacitors, 21 a choke coil, and 22 a mixing transformer. The collector of the transistor 11 is connected with a power supply terminal 9 through a switching circuit 10, the resistor 16, and the choke coil 21. A distributor 3 is connected at its one distribution output with an output terminal 7 to be connected with a VTR and at its other distribution output with the base of the transistor 11.If the switching circuit 10 connected between the power supply terminal 9 and the transistor 11 is turned off, the amplifying operation of the transistor is stopped.

In many cases a switching transistor is utilized for the switching circuit 10 in view of the cost, the space, etc., and for example, such an arrangement as shown each of the Figs. 4, 5, and 6 is used. In each of Figs. 4, 5, and 6, reference numeral 23 designates a switch control section, 24 a switching portion, 25, 27, 29, and 30 switching transistors, and 16, 28, 31, and 32 resistors. In the arrangement of Fig. 4, it is necessary that the the output of the switching control section 23 takes a high level in the normal operation and therefore there is a disadvantage that the magnitude of voltage drop in the transistor 25 varies depending on the output signal level of the switching control section 23. Accordingly, it is necessary to set the output signal level of the switching control section 23 so as to make small the voltage drop at the switching transistor 25.

In the arrangement as shown in Fig. 5, the switching transistor 27 is turned on when the output signal of the switching control section 23 is in its low level so that it is possible to apply the power supply to the high-frequency amplifying transistor 11, while there is a disadvantage that when the switching transistor 27 be turned off, it is impossible to make the switching transistor 27 be fully turned off if the output signal level of the switching control section 23 is lower than the power source voltage so that the range of the output signal level is restricted. In the arrangement of Fig.

6, the switching section 24 is constituted by the transistors 29 and 30 and the switching control section 23 is sufficient if its output signal is larger than about 1 volt so that the output signal level is hardly restricted, while there is a disadvantage that the number of parts is large.

Accordingly, the high-frequency switching circuit as shown in Fig. 7 has been proposed, in which the same reference numeral as that used in Fig. 3 designates the same part, and in which reference numeral 33 designates a switching control section, 34 a-switching section, 35 a choke coil, 36 a bypass capacitor, 37 a switching transistor, and 38 and 39 resistors. The switching transistor 37 of the switching section 34 is connected with the base of a high-frequency amplifying transistor 11 through the choke coil 35.

When the output signal level of the switching control section 33 is made high, the switching transistor 37 is turned on and the base of the high-frequency amplifying transistor 11 is grounded in DC sense through the switching transistor 37, so that the base current of the high-frequency amplifying transistor 11 is prevented from flowing into the base and the transistor 11 stop its function to amplify the signal distributed by a distributor 3.

If the output signal level of the switching control section 33 is made low, on the contrary, the switching transistor 37 is turned off and the high-frequency amplifying transistor 11 performs its normal amplifying operation.

In the case the high-frequency amplifying transistor 11 was caused to stop its amplifying operation to perform its attenuating operation, however, only the amount of attenuation of about 20 dB could be obtained in the range from 40 MHz to 900 MHz, which amount was substantially the same as that obtained in the arrangement shown in Fig. 3.

To prevent the disturbance as described above, it is necessary that the amount of the attenuation is about 35 dB.

SUMMARY OF THE INVENTION An object of the present invention is therefore to eliminate the above-mentioned drawbacks in the prior art.

Another object of the present invention is to provide a high-frequency switching circuit in which the amount of attenuation effected by a high-frequency amplifying transistor when the transistor is caused to stop its amplifying operation to perform its attenuating operation.

According to an aspect of the present invention, the high-frequency switching circuit comprises a high-frequency amplifying transistor for amplifying a high-frequency signal applied thereto, a switching diode connected between a power supply and a collector of the highfrequency amplifying transistor in the forward direction with respect to a current flowing into the collector through the diode, first means for deriving an output signal of the highfrequency switching circuit through the switching diode, and second means for causing the high-frequency amplifying transistor to stop its high-frequency amplifying operation to perform its attenuation operation.

The above and other objects, features, and advantages of the present invention will be apparent when read the following description in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram showing a main part of the conventional booster mixer; Figure 2 is a block diagram showing a main part of the conventional booster mixer provided with a switching circuit; Figure 3 is a circuit diagram showing the high-frequency amplifier portion of Fig. 2; Figures 4, 5 and 6 are circuit diagrams showing conventional switching circuits; Figure 7 is a circuit diagram showing a high-frequency switching circuit with an improved switching circuit; and Figure 8 is a circuit diagram of an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Fig. 8 is a circuit diagram of an embodiment of the present invention, in which the same reference numeral as used in Fig. 7 designates the same part, and in which reference numeral 40 designates a switching diode such as a pin diode. The switching diode is connected in the forward direction with the collector of a high-frequency amplifying transistor 11, and a capacitor 20, a mixing transformer 22 and an output terminal 8 are connected such that an output signal is derived through the switching diode 40. A series circuit of a capacitor 1 9 and a resistor 1 5 is connected between the collector and the base of the high-frequency amplifying transistor 11 so as to provide a base bias through a series circuit of resistors 14, 15, and 1 3.

When the output signal of the switching control section 33 is in its low level, the switching transistor 37 in the switching section 34 is in its off-state so as allow a rated collector current to flow in the high-frequency amplifying transistor 11, so that the impedance of the switching diode 40 becomes very small to an extent the fact that the switching diode is connected in series with the transistor 11 can be disregarded.

If the output signal of the switching control section 33 is made to be a high level, the switching transistor 37 is turned on so that the collector current becomes zero because the base of the high-frequency amplifying transistor 11 is grounded. Accordingly, no current flows also in the switching diode 40 so that the impedance thereof becomes very high.

When the impedance of the switching diode 40 was made high in such a manner as described above, the amount of attenuation was about 45 dB in the range from 40 MHz to 900 MHz when the high-frequency amplifying transistor 11 was caused to stop its amplifying operation. That is, the obtained amount of attenuation was two times as large as in the arrangements of Figs. 3 and 7.

As the means for causing the high-frequency amplifying transistor 11 to stop its amplifying operation is not restricted to such a switching circuit constituted by the switching control section 33 and the switching section 34 in the embodiment as described above, but any other arrangement may be used.

As described above, in the switching circuit according to the present invention, a switching diode 40 is connected in the forward direction with a collector of a high-frequency amplifying transistor 11, and means is provided for deriving an output signal through the switching diode 40 and for causing the high-frequency amplifying transistor 11 to stop its high-frequency amplifying operation.

When the high-frequency amplifying transistor 11 is cause to stop its amplifying operation, the collector current thereof becomes zero and the current flowing in the switching diode 40 also becomes zero so that the impedance thereof becomes very high. Accordingly, the amount of attenuation becomes large so as to sufficiently attenuate the disturbing signals derived at the output terminal 8 to be connected to a television receiver to make it possible to prevent deterioration in picture quality from occurring in the VTR reproducing operation. Since only one switching diode 40 is sufficient to be added to increase the amount of attenuation, there is a further advantage in the economical view point.

Claims

1. A high-frequency switching circuit comprising: a high-frequency amplifying transistor for amplifying a high-frequency signal applied to a base thereof; a switching diode connected between a power supply and a collector of said highfrequency amplifying transistor in the forward direction with respect to a current flowing into said collector through said diode; first means for deriving an output signal of said high-frequency switching circuit through the switching diode; and second means for causing said high-frequency amplifying transistor to stop its highfrequency amplifying operation to perform its attenuation operation.

2. A high-frequency switching circuit according to Claim 1, in which said first means comprises a first capacitor connected at its one end to said switching diode, a local oscillator, a mixing transformer connected between the other end of said first capacitor and said local oscillator, and an output terminal connected to said mixing transformer and for outputting said output signal therefrom.

3. A high-frequency switching circuit according to Claim 1, in which said second means comprises switching means connected with said high-frequency amplifying transistor for switching the state of he base of said highfrequency amplifying transistor between its grounded and non-grounded states, and control means connected with said switching means for controlling the switching operation of said switching means.

4. A high-frequency switching circuit according to Claim 3, in which said switching means comprises a switching transistor connected with the base of said high-frequency amplifying transistor such that when turned on said switching transistor make the base of said high-frequency amplifying transistor grounded and when turned off, on the contrary, makes the base of said high-frequency amplifying transistor non-grounded, said control means being connected with said switching transistor for controlling the on-off state of said switching means.

5. A high-frequency switching circuit substantially as hereinbefore described, with reference to Fig. 8 of the accompanying drawings.