JPS6117536Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a signal switching circuit that switches an arbitrary signal from multiple high-frequency signals input in parallel (for example, video signals from different sources) and supplies it to the necessary signal processing circuit. In particular, the object is to obtain a signal switching circuit that can eliminate problems such as crosstalk of other signals, and that enables remote switching operation or automatic switching operation without complicated shield wiring due to direct current operation. It is something to do.

Television broadcast reception signal, VTR playback signal,
In monitor receivers or video projectors that input video signals from different sources, such as video signals from ITV cameras, input terminals for parallel input video signals (usually located on the back of the device housing) Normally, there is a considerable distance between the switching operation panel (usually located on the front of the device) and the switching operation panel (usually located on the front of the device), so if you simply switch the signal using a rotary switch, etc. In order to prevent talk, etc., complicated shield wire wiring is essential, and it is difficult to obtain satisfactory results in terms of performance and cost.

Taking these points into consideration, it is also possible to adopt a configuration in which an emitter follower transistor with a common emitter resistance is inserted as a buffer amplifier in each signal path, and the supply/disconnection of the DC base bias of any transistor is selectively controlled. However, even in that case, although the shield wiring is simplified, the problem of crosstalk caused by other parallel input signals cannot be resolved, and in order to improve image quality, it is difficult to design and manufacture complicated crosstalk prevention. This required special consideration, and became a bottleneck in improving image quality and reliability.

The present invention aims to improve these drawbacks of the conventional example.

The details of the present invention will be explained below with reference to the drawings showing an example of an embodiment of the circuit.

In this embodiment, a video signal V ₁ supplied from a television receiver, a reproduced signal V ₂ of a VTR, and an ITV
This shows an example of application to a signal switching circuit for the input of a video projector that receives video signals _V3 supplied from
are the corresponding emitter resistances (RO ₁ )
Emitsuta follower transistor with common
It has Q ₁₀ , Q ₂₀ and Q ₃₀ .

Each emitter follower transistor Q ₁₀ , Q ₂₀ ,
The first one supplies the DC bias voltage to the base of Q ₃₀ .
Each cold side of bias circuit B ₁₁ , B ₁₂ , B ₁₂ is
The movable contact () is connected to the fixed contact of the signal selection switch (SW), and is connected to ground.
Through the switching connection, DC voltage is selectively supplied to one of the first bias circuits, and only the corresponding emitter follower transistor is activated, and the video signal input to that signal path is transmitted to the subsequent stage. tell to.

Each of the emitter follower transistors Q ₁₀ ,
In the signal path before Q ₂₀ and Q ₃₀ , switching transistors Q ₁₁ , Q ₂₁ , and Q ₃₁ have their collector-emitter paths shunted to the signal path, that is, when they are conductive, the signal is grounded. Connect like this. Each of the switching transistors Q ₁₁ ,
A resistor is connected to the base of Q ₂₁ and Q ₃₁ from the DC power supply Vcc.
R ₁₃ and diode D ₁₀ (or resistor R ₂₃ and diode
The operating current is supplied through second bias circuits B ₂₁ , B ₂₂ , and B ₂₃ configured with a series circuit of D ₂₀ , resistor R ₃₃ , and diode D ₃₀ .

Each resistor R ₁₃ of the second bias circuit B ₂₁ to _{B 23} ,
Between the connection point of R ₂₃ , R ₃₃ and the diodes D ₁₀ , D ₂₀ , D ₃₀ and the fixed terminal of the signal selection switch (SW), the fixed terminal side is connected in the forward direction, that is, the fixed terminal side is connected to the cathode. as switching diode
Connect D ₁₁ , D ₂₁ , and D ₃₁ . Further, large capacity capacitors C ₁₂ , C ₂₂ and C ₃₂ are connected in parallel to each of the switching diodes D ₁₁ , D _{21 and} D 31,
To protect each switching transistor Q ₁₁ , Q ₂₁ , Q ₃₁ from reverse breakdown voltage and to prevent malfunction of the switching transistor when the signal selection changeover switch (SW) is quickly switched.

Capacitor C ₁₀ connected in series with each signal path,
C ₁₁ , C ₁₂ , C ₂₁ , C ₃₀ , and C ₃₁ are for DC blocking.

Next, the operation of the signal switching circuit of the present invention will be explained.

Now, if you connect the signal selection switch SW to the position shown in the figure, that is, to the fixed terminal, the DC power
The first one consists of a voltage divider circuit of Vcc and resistors R ₁₀ and R ₁₁ .
A DC loop is formed by the bias circuit (B ₁₁ to _{B 13} ), and the emitter follower transistor Q ₁₀
only is in operation. At the same time, the cathode of the diode D ₁₁ is also grounded, so the series circuit of the second bias circuit R ₁₃ and D ₁₀ that turned on the switching transistor Q ₁₁ is connected to its connection point P ₁ , P ₂ , P ₃
is bypassed from the on-state switching diode _D11 .

Therefore, the potential at the connection point _P1 becomes vbe, but since the diode _D10 is interposed between it and the base of the switching transistor _Q11 , the switching transistor _Q11 cannot maintain the on state and turns off. ,
Free the input signal path.

while the other emitter follower transistor Q ₂₀ ,
Since the cold terminals (fixed terminals of the switch SW) of the first bias circuits B ₁₁ and B ₁₂ of Q ₃₀ are released, both transistors Q ₂₀ and Q ₃₀ are inactive, and each input signal is not activated. Cut off. In addition, since the switching diodes D ₂₁ and D ₃₁ are both turned off, both of the above switching transistors
Q ₂₁ and Q ₃₁ have second bias circuits B ₂₂ and B _32, respectively.
An operating current is supplied through the terminals and both remain on to ground the input signal to prevent crosstalk.

Therefore, the selected signal, in this case the video signal _V1 from the television receiver, is transmitted from the emitter of the emitter follower transistor _Q10 to the subsequent stage without containing any crosstalk components.
When the changeover switch SW is switched to another terminal or other terminal, the selected signal is taken out from the emitter of the corresponding emitter follower transistor _Q20 or _Q30 with the same operation.

If the changeover switch SW mentioned above is changed rapidly, for example, if the movable terminal is switched to empty and then switched again without a momentary delay, if capacitor _C12 is not present, switching diode _D11 will become conductive. However, a phenomenon occurs in which the switching transistor _Q11 is locked with a small amount of base current flowing through it, but the switching diode
Since the configuration is such that the negative terminal voltage of the capacitor _C12 charged when _D11 is off is applied to the anode of the diode _D10 as the cathode side of the switching diode _D11 is grounded, the switching transistor Q ₁₁ is immediately turned off, and the aforementioned locking phenomenon is completely prevented. Incidentally, to give examples of the values of each element, R ₁₀ , R ₂₀ , R ₃₀ (8.2KΩ), R ₁₁ ,
R ₂₁ , R ₃₁ (33KΩ), R ₁₃ , R ₂₃ , R ₃₃ (4.7KΩ),
C ₁₂ , C ₂₂ , C ₃₂ B (10 μF).

Since the present invention has the above-described configuration, even if the parallel input terminals of multiple video signals and their switching operation switches are far apart, a selection signal without crosstalk can be extracted without the need for complicated shield wiring. This makes it possible to realize a reliable signal switching circuit that does not cause image quality deterioration.

[Brief explanation of the drawing]

The drawing is an implementation circuit diagram of the signal switching circuit of the present invention. Q ₁₀ , Q ₂₀ , Q ₃₀ ... Emitter follower transistor, RO ₁ ... Emitter resistor, B ₁₁ , B ₁₂ , B ₁₃ ...
...First bias circuit, _Q11 , _Q21 , _Q31 ...Switching transistor, _B21 , _B22 , _B23 ...Second bias circuit, SW...Switching diode.

Claims (1)

[Scope of utility model registration request] A PNP type emitter follower transistor, a first bias circuit that supplies a DC operating voltage to the base of this transistor, and a collector-emitter path connected to the signal path in the previous stage of the transistor so as to form a signal path connection when conductive. a first NPN type switching transistor whose resistor and cathode are connected to the base of the switching transistor;
a second bias circuit that supplies an operating current to the switching transistor through a series connection of diodes, an anode connected to a connection point between the resistor and the first diode, and a cathode connected to one end of the first bias circuit; a second diode connected to both ends of the second diode, and a capacitor connected to both ends of the second diode to ensure that the switching transistor is turned off; and a changeover switch for selectively grounding the one end of the first bias circuit of each signal path, and by switching the changeover switch, a signal selected from a plurality of input signals is connected to the emitter resistor. A signal switching circuit configured to take out the emitter output of the lower transistor.