JPH0430763B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a signal selection circuit that reliably switches between two signals input from different paths, such as a reproduced FM audio signal of an 8 mm video. .

(b) Prior Art FIG. 1 shows a conventional signal selection circuit that alternately selects two inputs. As is clear from the figure, the first input is input to the base of the first input transistor _Q1 ,
The emitter output is inputted to the base of one first differential pair transistor _Q2 via _the first resistor R'1,
The second input is input to the base of the second input transistor _Q3 , and its emitter output is input to the base of the other second differential pair transistor _Q4 via the second resistor _R2 . In such a signal path, signal selection is performed by the first and second differential pair transistors Q ₂ ,
This is realized by selectively shunting the base input of _Q4 using first and second switching transistors _Q5 and _Q6 . That is, the switching transistor on the conducting side connects the signal path to ground.
Note that these first and second switching transistors Q ₅ ,
Q ₆ has its base connected to the collector and base of control transistor Q ₇ , which is connected between power supply and ground to achieve selective conduction.
Therefore, when the control input input to the base of the control transistor _Q7 is high, the second switching transistor _Q6 becomes conductive and the second input is selected, and when it is low, the first switching transistor Q6 is turned on and the second input is selected.
_Q5 conducts and the first input is selected.

However, as shown in the equivalent circuit of Figure 2,
For example, when the first input is selected, the second switching transistor Q6 conducts, so the second switching transistor _Q6 on the cut-off side
A large current flows through the input transistor _Q3 , causing power loss. Furthermore, although conductive, a resistance component _R6 exists between the emitter and collector of the second switching transistor _Q6 , and a capacitance component _C4 exists between the base and emitter of the second differential pair transistor _Q4 .
Therefore, a portion of the second input, which should be blocked, leaks to the output side.

Therefore, in order to alleviate such inconveniences, it is conceivable to increase the resistance values of the first resistor _R1 and the second resistor _R2 . However, in that case, the collector-emitter capacitance _C5 of the non-conducting first switching transistor _Q5 forms a low-pass filter together with the first resistor _R1 , so the frequency characteristics of the selection signal may be impaired. , which is not desirable.

(c) Purpose of the Invention In view of the above-mentioned points, the present invention proposes a novel signal selection circuit that does not cause power loss or leakage, and does not impair frequency characteristics.

(d) Structure of the Invention The present invention is characterized in that the current to the cut-off side input transistor is cut off by also controlling the constant current load of the input transistor.

(e) Embodiment The present invention will be explained below according to an embodiment illustrated in FIG. In this example, in 8mm video,
Playback FM alternately derived from rotating video head
The signals are used as the first and second inputs, and the RF switching pulse (head output switching signal) is used as the control signal. The selection side circuit of this embodiment is similar to that of Fig. 1, except that the input transistors Q ₁ and Q ₃ are PNP type, and the arrangement with the constant current load is reversed. It is attached in the same way as the conventional circuit, and some of the detailed explanations are omitted because the figures are common.

The feature of this embodiment is that in addition to the switching transistor, the first and second load transistors Q ₈ and Q ₉ , which are constant current loads, are controlled. The control operation of this embodiment will be explained below.

First, when the RF switching pulse, which is a control signal, is at a low level, the first transistor Q ₁₁ on the input side is turned off, and the second and third transistors Q ₁₂ and Q ₁₃ , which constitute a current mirror circuit in the next stage, are turned off.
It becomes ON. When the third transistor Q ₁₃ is turned on, the fourth PNP transistor Q ₁₄ connected to the collector side and forming a current mirror with the second load transistor Q ₉ is turned on, so that the second load transistor Q ₉ is turned on. turns on. Therefore, a constant current is supplied to the second input transistor _Q3 , resulting in a normal operating state. Further, the high level base input of the third transistor _Q13 makes the first switching transistor _Q5 conductive to cut off the first input, and at the same time turns on the fifth transistor _Q15 .
By turning on the fifth transistor _Q15 , the sixth and seventh transistors forming the current mirror circuit are turned on.
Q ₁₆ and Q ₁₇ are turned OFF, and the seventh transistor Q ₁₇
The eighth transistor _Q18 forming a current mirror with the first load transistor _Q8 on the collector side of the
Turn it OFF. 8th transistor Q ₁₈ in OFF state
By turning off the first load transistor _Q8 , the current supply to the first input transistor _Q1 is cut off. The base input of the seventh transistor _Q17 is also supplied to the base of the second switching transistor _Q6 ,
Make Q ₆ non-conductive. As a result, the first input is completely blocked and only the second input is selectively derived.

When the control input is reversed, the above relationship is completely reversed. FIG. 4 equivalently shows this state. As is clear from this figure, the selection side is the same as the conventional one, but on the cutoff side, the second load transistor Q ₉ is turned off, so the emitter of the second input transistor Q ₃ is No current flows,
Moreover, since the second switching transistor _Q6 is also turned on, there is almost no leakage of the second input to the output side.

(F) Effects of the Invention Therefore, according to the present invention, since the base input of the differential pair transistor is shunted and the constant current load of the input transistor is also disabled, signal leakage is completely prevented. Not only that, there is no power loss, and the effect is great.

[Brief explanation of drawings]

FIG. 1 shows a conventional signal selection circuit, FIG. 2 shows its equivalent circuit, FIG. 3 shows a signal selection circuit according to an embodiment of the present invention, and FIG. 4 shows its equivalent circuit diagram. Explanation of main figure numbers, Q ₁ , Q ₃ ... input transistors, Q ₂ , Q ₄ ... differential pair transistors, Q ₅ , Q ₆ ...
...Switching transistor, _Q8 , _Q9 ...Load transistor.

Claims (1)

[Claims] 1. First and second differential pair transistors whose collectors and emitters are commonly connected and a constant current source is arranged between the emitters and ground; and each of the first and second differential pair transistors. The first and second switching transistors connect the collector and emitter between the base and ground, the first load transistor and first input transistor are connected directly between the power supply and ground, and the first load transistor and first input transistor are directly connected between the power supply and ground. a second load transistor and a second input transistor; a first resistor connected between the connection point of the first load transistor and the first input transistor and the first differential pair transistor; and the second load transistor. and a second resistor connected between the connection point of the second input transistor and the second differential pair transistor; and the first load transistor and the second switching transistor, or the second load transistor and the second resistor. a switching control circuit that selectively turns on the first switching transistor;
Signal selection circuits arranged for each.