JPH0351337B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a switching circuit for switching the level of an audio signal in, for example, a satellite broadcast receiving system into two levels: high output and low output.

"Prior Art" First, an outline of a satellite broadcast television receiver will be explained. Currently, in the United States, the interval is 2 to 5 degrees.
More than 20 private broadcasting geostationary satellites have been launched, each receiving signals using a television receiver equipped with an antenna position control device as shown in Figure 3. In this figure, 1 is a reflector of a parabolic antenna, an antenna 2 is installed at the focal point of this reflector 1, and a low noise amplifier and down converter 3 coupled to this antenna 2 are connected to a receiver via a waveguide. 4 is combined. A monitor TV 5, an audio device 6, and an antenna positioner 7 are connected to the receiver 4, and the audio device 6 is connected to left and right speakers 9, 1 via an audio amplifier 8.
0 is connected. Further, the antenna positioner 7 is coupled to the antenna 2 and an actuator 12 for pointing to the position of the geostationary satellite 11.

In the above configuration, when a certain satellite 11 is selected by the antenna positioner 7, the actuator 12 directs the parabolic antenna reflector 1 to the target satellite. At the same time, the angle of the metal rod inside the antenna 2 is set depending on whether the wave is vertically polarized or horizontally polarized. Further, since each satellite has 24 channels, the antenna positioner 7 selects one of the channels.

The 3.7~4.2GHz signal from the satellite 11 is reflected by the reflector 1, received by the antenna 2, and converted to 430~930MHz by the low noise amplifier and down converter 3.
and sent to receiver 4. The antenna positioner 7 sends to the receiver 4 a control code signal indicating which video channel is selected. The input signal from the antenna 2 is further reduced to 90MHz by the receiver 4, and then separated into a video signal and an audio signal.
sent to. The audio signal separated by the receiver 4 is carried by a subcarrier signal of 5 to 8 MHz and sent to the audio device 6. At this time, a control code signal is sent to the audio device 6 from the antenna positioner 7 to indicate which satellite and which channel the audio is from. That is, information such as the distinction between discrete stereo, matrix stereo, multiplex stereo, and monaural, and the frequency of the subcarrier is sent to the audio device 6. Furthermore, this audio device 6 performs audio intermediate frequency amplification, FM
Detection is performed, and the signal is further amplified by an audio amplifier 8, and audio is output from left and right speakers 9 and 10.

In the above satellite broadcasting receiving system,
When switching the audio signal level in the audio device 6 into two stages, high output and low output, a circuit configuration as shown in FIG. 2 was used. That is, the input terminal 20
When an audio signal is input to the amplification transistor 2
1, is supplied to the base and amplified. Here, when the level changeover switch 22 is switched to the high output contact 23, a high output signal is taken out from the collector of the amplification transistor 21 via the DC blocking capacitor 24, and the level changeover switch 22 is also switched to the low output contact 25. and an amplification transistor 21
The low output signal was taken out via an attenuator 28 consisting of two resistors 26 and 27 coupled to the collector of the . In addition, 29 is a DC blocking capacitor, 30 and 31 are DC bias resistors, and 3
2 is a collector DC load resistance, 33 is an emitter resistance, (Vcc) is a power supply terminal, and 34 is an output terminal.

``Problems to be Solved by the Invention'' Obtaining a low output signal using an attenuator consisting of two resistors as shown in Figure 2 not only requires two useless resistors, but also is wasteful. There were problems such as high power consumption.

"Means for Solving the Problems" The present invention has been made to solve the above-mentioned problems, and it amplifies the input audio signal with an amplification transistor, and then outputs it to a high output via a level switch. In a circuit configured to switch to a low output level, the connection point between the collector of the amplification transistor and the collector resistor is coupled to the high output side of the level changeover switch, and the connection point between the emitter of the amplification transistor and the emitter resistor is connected to the high output side of the level changeover switch. connection point,
This is connected to the low output side of the level changeover switch.

"Effect" When the level selector switch is switched to the high output side,
The output voltage at that time, that is, the collector side voltage Vc
is Vc≒Ei×Rc÷Re, and when switching to the low output side, the output voltage at that time, that is, the voltage Ve on the emitter side, becomes Ve≒Ei. Here, Ei is the input signal voltage input to the input terminal, Rc is the collector resistance, and Re is the emitter resistance.

In addition, the high output signal and low output signal in this case are
The phase will be different by 180 degrees.

“Example” An example of the present invention will be described below with reference to FIG. Note that the same parts as in FIG. 2 are given the same reference numerals.

The connection point between the collector of the NPN type amplification transistor 21 and the collector load resistance 32 is connected to the high output side contact 23 of the level changeover switch 22, and the emitter of the transistor 21 and the emitter resistance 3
3 is connected to the low output side contact 25 of the level changeover switch 22, and the common contact 35 is connected to the output terminal 34 via the DC blocking capacitor 24.

In the above configuration, when the level changeover switch 22 is connected to the contact 23 on the high output side, the output voltage Vc appearing at the collector of the amplification transistor 21 becomes Vc≒Ei×Rc÷Re, and this is directly applied to the output terminal 34 as a high voltage. Obtained as an audio signal at the output level. Further, when the level changeover switch 22 is switched to the low output side contact 25, the output voltage Ve appearing at the emitter of the amplification transistor 21 becomes Ve≈Ei, and this is obtained at the output terminal 34 as a low output level audio signal. In addition, Ei is the input voltage to the amplification transistor 21, Rc is the collector resistance,
Re is the emitter resistance.

"Effects of the Invention" Since the present invention is configured as described above, two resistors that constitute an attenuator can be omitted from the conventional circuit, and since the low output is taken out from the emitter, the output impedance is lowered. be able to.

[Brief explanation of drawings]

FIG. 1 is an electric circuit diagram showing an embodiment of an audio signal output switching circuit according to the present invention, FIG. 2 is a conventional electric circuit diagram, and FIG. 3 is a block diagram of a satellite broadcast receiving system. 1...Reflector, 2...Antenna, 3...Low noise amplifier and down converter, 4...Receiver,
5...Monitor TV, 6...Audio device, 7...Antenna positioner, 8...Audio amplifier, 9,10
... Speaker, 11 ... Satellite, 12 ... Actuator, 20 ... Input terminal, 21 ... Amplification transistor, 22 ... Level changeover switch, 23 ...
...High output side contact, 24...DC blocking capacitor, 25...Low output side contact, 26, 27...Resistor, 28...Attenuator, 29...DC blocking capacitor, 30, 31...DC bias Resistance, 32...
... Collector DC load resistance, 33 ... Emitter resistance, 34 ... Output terminal, 35 ... Common terminal.

Claims (1)

[Claims] 1. In a circuit in which an input audio signal is amplified by an amplification transistor and further switched between high output and low output levels via a level switch, the connection point between the collector of the amplification transistor and the collector resistor is , coupled to the high output side of the level selector switch, and a connection point between the emitter of the amplifying transistor and the emitter resistor is coupled to the low output side of the level selector switch. circuit.