JPH0528824Y2 - - Google Patents

Description

[Detailed description of the invention] (1) Technical field of the invention This invention relates to a signal switching circuit using an analog switch that turns an input signal on and off using a bipolar transistor, and bootstraps the base of the bipolar transistor with an input signal. By applying this, it is possible to eliminate fluctuations in the base current due to the input signal, thereby lowering the distortion rate and increasing the allowable input.

(2) Prior art There are conventional analog switches that use, for example, C-MOS gates, but this has a low withstand voltage for input signals (10V or less), so its uses are limited, and its distortion characteristics are not very good. There was a runner-up.

Further, as another analog switch, one using a bipolar transistor Tr shown in FIG. 1 has been considered. This is done by connecting the emitter of the transistor Tr to the signal input, the collector to the signal output, the base to the power supply +VCC via the resistor R, and the base to the power supply -VEE via the switch SW. Transistor when on
When the Tr is turned off and the switch SW is turned off, the transistor is turned off.
This is to turn on the Tr. In this analog switch, the withstand voltage is the transistor
This depends on the characteristics of the Tr, so it can be made sufficiently large, but when the transistor Tr is on, the base current of the transistor Tr changes depending on the signal voltage, so this effect causes distortion in the transistor's collector output voltage. Also, transistor
Since the voltage VBE (approximately 0.6V) is required for the Tr to turn on, it was not possible to input a signal with a level higher than +VCC, resulting in poor voltage utilization.

Therefore, it has been considered to improve the analog switch shown in Fig. 1 by using a constant current source 1 instead of the resistor R, as shown in Fig. 2. Although distortion due to base current fluctuation can be prevented, +
It is similar to the one in Figure 1 in that it cannot input signals higher than VCC, and still has the drawback of poor voltage utilization. In addition, the production of the constant current source 1 increases the cost.

(3) Purpose of the invention This invention was made in view of the above points.
The present invention is intended to provide a signal switching circuit which has a low distortion factor, a large allowable input, and a high power utilization rate by improving the circuits shown in FIGS. 1 and 2.

(4) Structure of the device In this device, the emitter of a bipolar transistor is connected to an input terminal into which a signal is input, the collector of the bipolar transistor is connected to an output terminal to which a load can be directly connected, and the base of the bipolar transistor is connected to a terminal. 1 resistor and a second resistor in order to a first voltage source having a polarity that allows a forward current to flow through the base, and connects an intermediate connection point between the first resistor and the second resistor to the emitter terminal. and the base is connected to a second voltage source having a polarity opposite to that of the first voltage source through an on-off switch means for signal switching, and the on-off switch means is turned on. At this time, the base voltage of the bipolar transistor is forcibly set to the second voltage source voltage, and the base and emitter are reverse biased to interrupt the conduction between the emitter and the collector, and the connection is made from the input terminal to the output terminal. While signal transmission is disabled, when the on-off switch means is off, a base current is supplied sequentially through the emitter, the base, the first resistor, and the second resistor of the bipolar transistor, and the current is supplied between the emitter and the collector. is in a conductive state to enable signal transmission from the input terminal to the output terminal, and the capacitor causes the current value flowing between the emitter and base of the bipolar transistor and through the first resistor to become a signal input to the input terminal. This feature is characterized in that it remains almost constant regardless of the situation.

(5) Examples of the invention Examples of the invention will be described below with reference to the attached drawings.

In FIG. 3, the transistor Tr has an emitter connected to an input terminal 10 and a collector connected to an output terminal 11. An input signal is input to the input terminal 10, and a load ZL is connected to the output terminal 11. The input signal can be input with or without a DC potential superimposed. The base of the transistor Tr is a resistor
Power supply + through _R1 first resistor, _R2 second resistor
VCC is connected to the first voltage power supply. Also,
Power supply -VEE is connected to the second voltage source via switch SW on-off switch means. Resistance R ₁ ,
A capacitor C is connected between the connection point of _R2 and the emitter of the transistor Tr.

In this circuit, when the switch SW is on, the transistor Tr is immediately turned off, similar to the conventional ones shown in Figures 1 and 2. Even if the base potential is forced to −VEE
The transistor Tr turns off instantly. ) There is no signal transmission from input terminal 10 to output terminal 11. When the switch SW is off,
The transistor Tr is turned on because a base bias is applied from the power supply +VCC via the resistors R ₁ and R ₂ , and the input signal is transmitted to the output terminal 11 .

On the other hand, regarding the input signal Vi, since its average value (integral value) is at zero potential in a general music signal (alternating current), the electric charge charged to the capacitor C by the resistor _R1 is between the two poles of the capacitor C. Voltage VC
becomes steady at a value where V _C ≒ (VCC)・R ₂ /R ₁ + R ₂ . This voltage between poles VC is
If you set the time constant _R1・C to a value sufficiently larger than the lowest frequency (20Hz) of the input music signal,
Regardless of the input signal voltage Vi, the above-mentioned value is approximately constant. Therefore, as the input signal voltage Vi changes, the potential at the connection point between the resistors R ₁ and R ₂ changes in the same way. At this time the transistor
The base current I _B of the Tr is constant as I _B = V _C − V _BE /R ₂ , so it is independent of the input signal voltage Vi.
The operating state (on resistance) of the transistor Tr becomes constant, and the output of the transistor Tr is not distorted. Furthermore, even if the input signal voltage Vi momentarily exceeds +VCC, the base potential of the transistor Tr is momentarily raised above +VCC by the bootstrap operation by the capacitor C, so the transistor Tr remains conductive without any change. The voltage utilization rate is also extremely good. This bootstrap effect is effective for both positive and negative polarities of the input signal. Also, transistor
Since the base current I _B of the transistor Tr is uniquely determined as shown in the above equation, the conduction state of the transistor Tr can be set completely, and the on-resistance can be made sufficiently small while improving the distortion factor. Since it can reduce the noise generated by the transistor Tr,
As a result, the S/N can be increased.

Next, a specific example of switching two channel signal paths by applying the circuit shown in FIG. 3 will be described with reference to FIG. 4.

In Fig. 4, the input terminals 20 and 21 are the first channel CH-1 and the second channel CH-1, respectively.
-2 signal input terminal. 1st channel
The input terminal 20 of CH-1 is connected via a buffer amplifier 22 to the emitter of the transistor Tr1. The collector of the transistor Tr1 is connected to the output terminal 23. Transistor Tr1
The base of is connected to the power supply +VCC (first voltage source) via resistors R _1a (first resistor) and R _2a (second resistor). Further, the base of the transistor Tr1 is connected to the power supply -VEE (second voltage source) via the transistor Tr3 on/off switch means. The transistor Tr3 is turned on and off by the output Q of the flip-flop circuit 24. resistance
A capacitor C1 is connected between the connection point of R _1a and R _2a and the emitter of the transistor Tr1.

The input terminal 21 of the second channel CH-2 is connected to the emitter of the transistor Tr2 via a buffer amplifier 25. The collector of the transistor Tr2 is connected to the output terminal 23. The base of the transistor Tr2 is connected to the power supply +Vcc via resistors R _1b (first resistor) and R _2b (second resistor). Further, the base of the transistor Tr2 is connected to the power supply +VEE via a transistor Tr4 (on/off switch means). The transistor Tr4 is turned on and off by the output of the flip-flop circuit 24. Resistance R _1b , R _2b
A capacitor C2 is connected between the connection point of the transistor Tr2 and the emitter of the transistor Tr2.

Switches SW1 and SW2 are selection switches for the first channel and second channel, respectively, and drive the flip-flop circuit 24 to drive the transistor Tr.
3.Turn Tr4 on and off. 1st channel CH
-1 selection switch SW1 is turned on,
The output of the flip-flop circuit 24 is Q0 “0”,
Q=“1” and transistor Tr3 is turned off.
Transistor Tr4 is turned on. Therefore,
The transistor Tr1 is turned on and the transistor Tr2 is turned off, and the signal of the first channel CH-1 is transmitted to the output terminal 23. 2nd channel CH-2
When the selection switch SW2 is turned on, the output of the flip-flop circuit 24 is Q=“1”, Q=
The signal becomes "0", turning on the transistor Tr3 and turning off the transistor Tr4. Therefore, the transistor Tr1 is turned off, the transistor Tr2 is turned on, and the signal of the second channel CH-2 is transmitted to the output terminal 23. In the manner described above, switching between the signals of the first channel CH-1 and the second channel CH-2 is realized.

(6) Effects of the invention As explained above, according to this invention, in an analog switch using a bipolar transistor, the base of the transistor is bootstrapped with an input signal, so that the base current remains constant regardless of the input signal. It is possible to prevent distortion from occurring. Further, even if the input signal exceeds the power supply voltage supplied to the base of the transistor, the transistor can be kept turned on with a constant base current, so that the power utilization rate can be increased. In addition, since the bipolar transistor is turned off by forcibly connecting the base to the second voltage source using the on/off switch means, the bipolar transistor can be turned off instantly without being affected by the charge of the bootstrap capacitor. Even when switching a plurality of signals, it is possible to prevent interference at the time of switching without providing a separate muting circuit. Also, the input signal is
The input signal can be input with or without superimposed DC potential, and the bootstrap effect can be applied to either positive or negative polarity of the input signal.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an example of a conventional signal switching circuit, Fig. 2 is a circuit diagram showing a conventional signal switching circuit improved from the circuit in Fig. 1, and Fig. 3 shows an embodiment of this invention. FIG. 4 is a circuit diagram showing a specific example of a signal switching circuit that switches between two channels of signals using this invention. 10,20,21...input terminal, 11,23...
...output terminal, T _r , T _r1 , T _r2 ... bipolar transistor, ZL ... load, R ₁ , R _1a R _1b ... first resistor, R ₂ , R _2a , R _2b ... second resistor , +VCC...
First voltage source, -VEE...Second voltage source, C, C
1, C2...Capacitor, SW, T _r3 , T _r4 ...On/off switch means.

Claims (1)

[Claims for Utility Model Registration] The emitter of a bipolar transistor is connected to an input terminal into which a signal is input, the collector of the bipolar transistor is connected to an output terminal to which a load can be directly connected, and the base of the bipolar transistor is connected to a first terminal. The base is connected to a first voltage source having a polarity that allows a forward current to flow through a resistor and a second resistor in order, and the intermediate connection point between the first resistor and the second resistor is connected to the emitter. A capacitor is connected between them, and the base is connected to a second voltage source having a polarity opposite to that of the first voltage source through an on/off switch means for signal switching.
when the on/off switch means is on, the base voltage of the bipolar transistor is forcibly set to the second voltage source voltage to reverse bias between the base and the emitter, and the emitter and the collector are connected to the voltage source of the bipolar transistor. disabling signal transmission from the input terminal to the output terminal by cutting off conduction between the emitter, the base, the first resistor, and the second resistor of the bipolar transistor when the on-off switch means is off; A base current is sequentially supplied between the emitter and the collector to enable signal transmission from the input terminal to the output terminal, and between the emitter and the base of the bipolar transistor and between the first A signal switching circuit characterized in that the value of the current flowing through the resistor is approximately constant regardless of the signal input to the input terminal.