GB2157906A

GB2157906A - High-frequency switching circuit

Info

Publication number: GB2157906A
Application number: GB08430058A
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: JPS60220622A; GB8430058D0

Abstract

A high-frequency switching circuit has a switching semiconductor element 37 connected to the base of a high- frequency amplifying transistor 11 for grounding the base to cause the latter transistor 11 to stop a high- frequency amplifying operation and to perform an attenuating operation instead. In use, a television signal may be applied at 1; output 7 is applied to an input terminal of, for example, a video tape recorder and output 8 is applied 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 generally relates to a high-frequency amplifying circuit and more particularly a high-frequency switching circuit having a simplified switching arrangement for causing a high-frequency and amplifying transistor to stop its amplifying operation to thereby cause the 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 operation. To solve this problem a proposal has been made in which, as shown in Fig. 2, a high-frequency amplifier 4 is connected with a power supply terminal 9 through a switching circuit 1 0. The switching circuit 10 is turned off to cause the high-freqency amplifier 4 to stop its amplifying operation and cause the high-frequency 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 switcing 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.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the disadvantages in the prior art.

Another object of the present invention is to provide a high-frequency switching circuit having a simplified arrangement for causing a high-frequency amplifying transistor to stop its amplifying 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, and a switching means connected with a base of the high-frequency amplifying transistor, for switching the state of the base between its grounded and non-grounded states such that the high-frequency amplifying transistor is caused to stop its amplifying operation to perform its attenuation operation when the base thereof is grounded.

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 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; and Figure 7 is a circuit diagram showing an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring to the drawings a preferred embodiment of the present invention will be described hereunder.

Fig. 7 is a main part circuit diagram of an embodiment of the present invention, 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.

Since the collector potential of the switching transistor 37 is equal to the base potential of the high-frequency amplifying transistor 11, it is sufficient that the output signal level of the switching control section 33 for on-off controlling the switching transistor 37 is about 1 volt, so that the design of the switching circuit for causing the high-frequency amplifying transistor 11 to stop its amplifying operation becomes easy and the switching circuit can be realized with a simple arrangement.

As the above-mentioned switching section 24, any switching semiconductor element, such as a field effect transistor, other than the bipolar transistor may be used.

As described above, in the switching circuit according to the present invention, a switching semiconductor element, such as a switching transistor 37, is connected with a base of a high-frequency amplifying transistor 11, for grounding the base to cause the high-frequency amplifying transistor to stop its amplifying operation and cause the same transistor to perform its attenuating operation. Thus, the present invention has advantages in that the arrangement of switching circuit is simple and the restriction for the control signal level for performing on-off control of the switching semiconductor element constituting the switching circuit is relieved, so that the design therefor is easy. Accordingly, a high-frequency switching circuit for causing a high-frequency amplifying transistor 11 to stop its amplifying operation to attenuate disturbing signals in the VTR reproducing operation can be economically realized.

Claims

1. A high-frequency switching circuit comprising: a high-frequency amplifying transistor for amplifying a high-frequency signal applied thereto; and switching means connected with a base of said high-frequency amplifying transistors, for switching the state of said base between its grounded and non-grounded states such that said high-frequency amplifying transistor is caused to stop its amplifying operation to perform its attenuation operation when said base thereof is grounded.

2. A high-frequency switching circuit according to Claim 1, in which said switching means includes a semiconductor switching element connected with said base of said high-frequency amplifying transistor, for performing the switching operation for said state of said base.

3. A high-frequency switching circuit according to Claim 2, in which said semiconduc tor switching element is a switching transistor connected to said base of said high-frequency amplifying transistor, for performing the switching operation for said state of said base.

4. A high-frequency switching circuit according to Claim 1, further comprising control means connected with said switching means, for controlling the switching operation of said switching means.

5. A high-frequency switching circuit according to Claim 2, further comprising control means connected with said semiconductor switching element, for controlling the switching operation of said switching means.

6. A high-frequency switching circuit according to Claim 3, further comprising control means connected with said semiconductor switching transistor, for controllng the switching operation of said switching means.

7. A high-frequency switching circuit substantially as hereinbefore described, with reference to Fig. 7 of the accompanying drawings.