JPH0315377B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a switching circuit that passes or blocks an input signal, and a switching circuit that prevents the control signal from being output to the output terminal of the switching circuit even if the input signal and the control signal are input from a common input terminal. The present invention provides a switching circuit particularly suitable for integrated circuit implementation.

In integrated circuits, the number of terminals is limited, so when attempting to integrate many functions to a high degree of integration, the integration is often limited not by the number of elements but by the number of terminals. In such a case, it is desirable to have at least one input terminal, and it is sometimes necessary to input a plurality of signals from one terminal. Now, an input signal and a control signal are input from one input terminal of the integrated circuit, and this control signal controls the gate circuit connected after the input terminal, and the input signal is output to the output terminal. Consider the case of switching. Generally, the input terminal of an integrated circuit is biased to an appropriate potential from an internal voltage source via a resistor, and the input signal is often coupled to this input terminal with direct current blocked by a capacitor. In this case, the input terminal has a time constant determined by the product of this capacitance value and the sum of the output impedance of the input signal source and the input impedance of the integrated circuit. Now, when the DC potential of such an input terminal deviates from the bias voltage by more than a certain potential, it is possible to detect this and control a gate circuit connected to the stage subsequent to this input terminal. However, until the potential of the input terminal deviates from the bias potential by the constant potential due to the control signal, it is impossible to avoid outputting this change to the output terminal of the gate circuit. The maximum level of this output control signal is the dynamic range of the input signal itself, and its time width depends on the speed of change of the control signal. Therefore, in order to reduce the number of control signals output, it is necessary to increase the speed of the control signals. By the way, it is possible to some extent to quickly change the potential of the input terminal of an integrated circuit from its bias potential to the potential at which the gate circuit operates. However, on the contrary, it is very difficult to quickly return the potential of the input terminal to its bias potential from outside the integrated circuit because the input terminal has a time constant. Therefore, more control signals are output when the switching circuit is operated to allow the input signal to pass than when the switching circuit is operated to block the input signal. Figures a to d explain this, where a is the input signal, b is the control signal, c is the waveform of the integrated circuit input terminal, and d is the output waveform of the conventional switching circuit. _E1 shown is the bias potential of the input terminal, _E2 is the potential that operates the gate circuit, and when the potential of the input terminal is less than _E2 ,
As shown in Figure d, the output signal is at the average potential _E3 when the input signal is output, but when the potential of the input terminal is between _E1 and _E2 , the input signal alone is Changes caused by the control signal are also output to the output waveform. The same figure c also shows the moment when the control signal is applied.
The potential of the input terminal changes rapidly from E ₁ to E ₂ or less,
When the control signal disappears, it shows how the input terminal returns to _E1 with a time constant, and Figure d shows how the latter is output more widely. The present invention applies in the latter case even if the input terminal of the integrated circuit returns gradually according to its time constant from the potential E ₂ operating _the switching circuit to the bias potential E 1 . As shown, this provides a means by which the change is not output to the output terminal. Below, we will explain one example in the second section.
The explanation will be based on the drawings shown in the figures.

In FIG. 2, V ₁ is an input signal source, R ₁ is the output impedance of the input signal source, and the input signal is input to an input terminal Vin of an integrated circuit or the like via a capacitor C ₁ . Also, V ₂ is a control signal source,
coupled to the base of the transistor T ₁ through a resistor R ₂ , the emitter of which transistor T ₁ is grounded,
The collector is coupled to the input terminal Vin, and the DC potential of the input terminal Vin is controlled by a control signal. _E1 is a stable voltage source provided inside an integrated circuit, etc., and supplies a bias voltage to the input terminal Vin via a resistor _R3 to drive an emitter follower circuit composed of a transistor _{T2 and} a resistor _R4 . Transistors T ₃ , T ₄ , T ₅ , T ₆ and resistors R ₅ , R ₆ , R ₇ constitute a voltage comparison circuit, and one end of resistor 5 is connected to the voltage source E ₁ and the other end is connected to the transistor T ₃ . bound to the collector. Now, when the transistor T ₃ is conducting, a voltage drop occurs across _the resistor R ₅ due to its collector current _. , R ₆ are selected, and since the voltages between the base and emitter of each transistor are approximately equal when they are conductive, they are all set to V _BE , and the voltage of the voltage source E ₁ is set to E ₁ Then, the collector potential of the transistor T ₃ becomes E ₁ when it is on, E ₁ −V _BE because there is no voltage drop across the resistor R ₅ when it is turned off. This voltage comparator circuit sets the collector voltage E ₁ when the transistor T ₃ is cut off to the first
The second reference voltage is _the collector voltage E _{1 -V BE} when conducting _. via the emitter of transistor _T3 . Transistors T ₃ and T ₆ constitute an emitter-coupled amplifier,
The one with higher base potential becomes conductive. Now, when the potential of the input terminal Vin is the bias voltage E ₁ , the base potential of the transistor T ₃ is E ₁ −V _BE , and if the transistor T ₃ is conductive at this time, its collector The potential is E ₁ −V _BE , so the base potential of transistor T ₆ is E ₁ −2V _BE ,
There is no contradiction in the states in which transistor T ₃ is conductive and transistor T ₆ is shut off. In this state, a positive voltage is generated in the control voltage source V ₂ and the transistor
When T ₁ becomes conductive, the potential at the input terminal Vin drops rapidly. When the potential of the input terminal Vin drops by V _BE from the potential E ₁ , the base potential of the transistor T ₃ becomes E ₁ −2V _BE , which becomes equal to the base potential of the transistor T ₆ , and both transistors T ₃ and T ₆ become conductive. .
At this time, since the collector current of the transistor _T3 decreases, the voltage drop across the resistor _R5 decreases, and the collector potential rises.Therefore, the base potential of the transistor _T6 rises from _{E1-2V BE} , and the transistor T ₃
is quickly cut off, transistor T ₆ quickly becomes fully conductive, and the base potential of transistor T ₆ becomes E ₁ −V _BE
becomes. In this state, in order to make transistor T ₃ conductive again, its base potential needs to be set to E ₁ −V _BE . That is, the transistor _T3 is cut off until the input terminal Vin returns to _E1 .

Transistors T ₇ , T ₈ and resistor R ₈ constitute a gate circuit, and a transistor is connected to the base of transistor T _7.
The emitter of _T2 is coupled, and the signal input to the input terminal Vin is transmitted. On the other hand, the collector potential of the transistor _T3 , which is the output of the voltage comparator circuit, is transmitted to the base of the transistor _T8 via an emitter follower circuit composed of a transistor _T9 and a resistor _R9 , and one of these is connected to the gate circuit. The output from the emitters of transistors T ₇ and T ₈ is output to the output terminal Vout. That is, the output terminal
At Vout, when transistor _T3 is conductive, the input signal input to input terminal Vin is outputted by transistor _T7 at an average potential _{E1-2V BE} , and when transistor _T3 is turned off, resistor _R5 and transistor _T9 are output. Through the transistor _T8 , a DC potential _{of E1-2VBE} is output. Therefore, even if the switching circuit switches, its average potential will not change, and even if the control signal is removed, the switching circuit will switch only when the input terminal reaches its bias potential, so the control signal will not change. does not appear at the output terminal.

An example of the application of the present invention is, for example, in a circuit that mixes and outputs a reproduced video signal and a reproduced chroma signal, such as a magnetic recording/reproducing device, by inputting a control signal from the chroma signal input terminal and outputting the potential of the output signal. By fixing the synchronization signal level of the video signal that has been played, a stable synchronization signal is inserted into the playback video signal during special playback of the magnetic recording and playback device, that is, during still, slow, fast forward, rewind playback, etc. It can be used in case. In this case, the video signal is clamped to the voltage source voltage E1 by a clamp circuit, and its synchronous signal portion is clamped to the voltage source voltage _E1 .
At the same time as mixing the chroma signal biased with the voltage source voltage _E1 and inputting it to the gate circuit,
This can be easily realized by inputting the input terminal of the chroma signal biased by the voltage source _E1 to the voltage comparison circuit.

As described above, according to the present invention, even if a control signal is simultaneously input to an input terminal having a time constant coupled to an input signal source via a capacitor, the control signal can be prevented from appearing at the output terminal. can. Of course, when a control signal is applied, the switching circuit operates and outputs a thin pulse at the moment the input signal is cut off. A constant of about 2 μSec is sufficient, and in such a case, the pulse width can be set to about 0.1 μSec. This is a signal with frequency components above 10 MHz that can be attenuated with a simple low-pass film, rendering it virtually uninterferable. Also,
As mentioned above, if a stable synchronization signal is inserted during special playback of the reproduced video signal, this level of interference pulse will not be a problem at all, but if the present invention is not applied, the interference pulse after the inserted synchronization signal Finally, the influence of the control signal appears, and when this is played back on a TV receiver, synchronization may become unstable.

The present invention also provides a switching circuit whose DC potential does not change even when the switching circuit switches, and in which the potential of the input terminal, the reference potential of the voltage comparison circuit, and the output potential thereof are generated from the same voltage source, and The gate circuit outputs the higher (or lower) potential of the input signal, so the circuit configuration is very simple, and it is also characterized by being able to output the same potential even when the switching circuit switches. has.

[Brief explanation of the drawing]

FIG. 1 is a waveform diagram illustrating the operating waveforms of each part of a conventional switching circuit and the present invention, and FIG. 2 is an electrical circuit diagram of a switching circuit according to an embodiment of the present invention. 1...Input signal source, 2...Control signal source, Vin...
…Input terminal, Vout…Output terminal, E ₁ …Stable voltage source, C ₁ …Capacitance, R ₁ to R ₉ …Resistance, T ₁ to T ₉
...transistor.

Claims (1)

[Claims] 1. An input signal source, a control signal source, a first input terminal to which input signals and control signals from the input signal source and the control signal source are input, and the first input terminal connected to a constant potential through a resistor. a voltage source that biases
The voltage source voltage is set as a first reference voltage, and the first
A voltage that differs by a certain voltage from the reference voltage of
a voltage comparison circuit for comparing the voltage of the first input terminal as a reference voltage of the voltage comparison circuit, and second and third input terminals to which the output terminal of the voltage comparison circuit and the first input terminal are respectively coupled in a DC manner. the input signal source is coupled to the first input terminal via a capacitor, the control signal source is coupled to control the DC potential of the first input terminal, and the control signal source is coupled to control the DC potential of the first input terminal; The DC potential of the first input terminal changes from the first reference potential to the second reference potential by the control signal.
, the voltage comparison circuit outputs a first reference voltage, and the first input terminal reaches approximately the first reference potential from the potential exceeding the second reference voltage. When the voltage comparison circuit outputs a second reference voltage, the gate circuit compares the potentials of its second input terminal and third input terminal, and the voltage comparison circuit outputs a second reference voltage. When outputting a voltage, the voltage comparison circuit output voltage is outputted at the second input terminal of the gate circuit, and when the voltage comparison circuit outputs the second reference voltage, the output voltage is outputted at the third input terminal of the gate circuit. A switching circuit configured to output an input signal input to the first input terminal.