JP4174909B2 - Switch circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a switch circuit for switching a signal such as a video signal.
[0002]
[Prior art]
As a video signal switching circuit, a differential type switching circuit is used. FIG. 3 shows a conventional current mode switch circuit.
The conventional current mode switch circuit includes SWa1 and SWa2, transistors Qa1 to Qa5, and current sources Ia1 to Ia3. Transistor Qa1 and transistor Qa2 constitute a first differential amplifier, and transistor Qa3 and transistor Qa4 constitute a second differential amplifier. The transistor Qa5 is an output transistor.
[0003]
INa1 and INa2 are video input terminals, and OUTa is an output terminal. VCC is a power supply voltage, and GND is a ground.
The switches SWa1 and SWa2 are made of semiconductors and may be controlled by the output of the logic circuit, or may be configured to directly apply a high level or a low level.
[0004]
When the switch SWa1 is closed, SWa2 is open, and when the switch SWa2 is closed, SWa1 is open. When SWa1 is opened , the video signal applied to INa1 is output from the output terminal OUTa via the base of transistor Qa1, the emitter of transistor Qa1, the emitter of transistor Qa2, the collector and base of transistor Qa2, and the base of transistor Qa5. The
[0005]
When SWa2 is opened , the video signal applied to INa2 is output from the output terminal OUTa via the base of transistor Qa3 → the emitter of transistor Qa3 → the emitter of transistor Qa4 → the collector / base of transistor Qa4 → the base of transistor Qa5. Is output. Thus, when the switch SWa1 is opened , the video signal applied to the INa1 is output to the output terminal OUTa, and when the switch SWa2 is opened , the video signal applied to the INa2 is output to the output terminal OUTa. .
[0006]
[Problems to be solved by the invention]
However, the signal input from INa1 or INa2 passes through the high impedance line (La) of the collector / base of the transistor Qa2, the collector / base of the transistor Qa4, and the base of the transistor Qa5, and passes through the emitter of the transistor Qa5 to OUTa. Is output.
[0007]
By the way, when SWa1 is turned on and the input signal V1 is output to OUTa, the signal V2 is caused by the parasitic impedance (Z2) formed between INa2 and the line La and the impedance (Z1) between GND and the line La. Leak into OUTa. That is, as shown in FIG. 4, the signal V <b> 2 is generated even though the switch SWa <b> 1 is OFF.
V2 × Z1 / (Z1 + Z2) (1)
Of V2 leaks to the line La.
[0008]
According to equation (1), when Z1 is large, leakage (crosstalk) increases, and when Z1 is small, leakage decreases. Further, when Z2 is large, the leakage is small, and when Z2 is small, the leakage is large.
The conventional device has a problem that the impedance (Z1) between the GND and the line La is large, and crosstalk occurs.
[0009]
In particular, in a high frequency region such as a video signal, there is a problem that Z2 is small and large crosstalk occurs.
The transistors Qa2 and Qa4 have a full feedback amplifier configuration, and it is difficult to obtain good frequency characteristics. To ensure good frequency characteristics, the currents of the current source Ia1 and the current source Ia2 must be increased. There was a problem of not becoming.
[0010]
The present invention has been made in view of the above problems, and an object of the present invention is to reduce crosstalk and improve frequency characteristics in a high frequency region in a switch circuit.
[0011]
[Means for Solving the Problems]
Multiple The invention described in claim 1, a plurality of emitter follower transistors and the emitter common connection (e.g., a transistor Qb2 in Figure 1, transistor Qb4) and, respectively connected to the base over the scan of the emitter follower transistor of the plurality of the common base transistor (e.g., transistor Qb1 in FIG. 1, transistor Qb3) and are respectively connected to the plurality of signal input terminals respectively connected to the collectors of the common base transistor of the plurality of, in the base of the plurality of common base transistor A plurality of switch circuits (for example, switch SWb1 and switch SWb2 in FIG. 1), and an output transistor (for example, transistor Qb5 in FIG. 1) having a base connected to the emitter of the emitter follower transistor, A current source is connected to each of the bases of the plurality of base-grounded transistors, a signal is applied to each of the plurality of signal input terminals, and an open switch is connected by opening one of the plurality of switches. The signal applied to the signal input terminal connected to the collector of the grounded base transistor is output from the emitter of the output transistor, and the emitter follower transistor (eg, transistor Qc2, transistor Qc4 in FIG. 2). A differential circuit configuration transistor (for example, transistor Qc6 in FIG. 2) that forms a differential circuit with the emitter follower transistor is provided between the emitter of the output transistor and the base of the output transistor (for example, transistor Qc5 in FIG. 2). Emission of dynamic circuit configuration transistor Is connected to the emitter of the emitter follower transistor, the collector of the differential circuit configuration transistor is connected to the base, and the current source and the base of the output transistor are connected to the collector-base connection point of the differential circuit configuration transistor. Are connected, and an output signal is obtained from the emitter of the output transistor .
[0012]
According to the first aspect of the present invention, there are provided a plurality of emitter follower transistors having emitters connected in common, and a grounded base transistor connected to each base of the plurality of emitter follower transistors. The signal input terminal is connected, a switch circuit is connected to each base of the base-grounded transistor, and a signal connected to the collector of the base-grounded transistor is opened by opening a switch connected to the base. By outputting from the emitter of the emitter follower transistor, crosstalk at high frequencies can be reduced and frequency characteristics in the high frequency region can be improved.
Further, by providing a diode in the forward direction between the emitter of the emitter-follower transistor and the base of the output transistor, the temperature characteristic based on the base-emitter voltage of the transistor is halved and can be used for the power supply voltage VCC. The voltage can also be used up to the power supply voltage VCC-VF (VF: voltage between the base and emitter of the transistor ) .
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described with reference to the drawings.
A current mode switch circuit of the present invention is shown in FIG.
The current mode switch circuit of the present invention includes SWb1 and SWb2, transistors Qb1 to Qb5, and current sources Ib1 to Ib4. Transistors Qb2 and Qb4 are emitter follower circuits. The emitters of the transistors Qb2 and Qb4 are directly connected. The transistor Qb5 is an output transistor.
[0016]
INb1 and INb2 are video input terminals, and OUTb is an output terminal. VCC is a power supply voltage, and GND is a ground.
Further, the switches SWb1 and SWb2 are made of a semiconductor and may be controlled by an output of a logic circuit, or may be configured to directly apply a high level or a low level.
[0017]
Further, SWb2 is open when the switch SWb1 is closed, and SWb1 is open when the switch SWb2 is closed. When SWb1 is opened, the video signal applied to INb1 is output from the output terminal OUTb via the collector of transistor Qb1, the emitter of transistor Qb1, the base of transistor Qb2, the emitter of transistor Qb2, and the base of transistor Qb5.
[0018]
When SWb2 is opened , the video signal applied to INb2 is output from the output terminal OUTb via the collector of transistor Qb3 → the emitter of transistor Qb3 → the base of transistor Qb4 → the emitter of transistor Qb4 → the base of transistor Qb5. The Thus, when the switch SWb1 is opened , the video signal applied to the INb1 is output to the output terminal OUTb, and when the switch SWb2 is opened , the video signal applied to the INb2 is output to the output terminal OUTb. .
[0019]
In this current mode switch circuit, a signal input from INb1 or INb2 passes through the low impedance line (Lb) of the emitter of transistor Qb2, the emitter of transistor Qb4, and the base of transistor Qb5, and is output to OUTb.
Further, since the signal flows through the low impedance line, it is difficult for other signals to jump in. In particular, the parasitic capacitance is advantageous for crosstalk at a high frequency.
[0020]
In addition, since the transistor Qb2 and the transistor Qb4 constitute an emitter follower circuit, it is possible to ensure frequency characteristics in a high frequency region.
Further, since the circuit of FIG. 1 is not configured as a differential amplifier, it is not necessary to make the characteristics of the transistors Qa1 and Qa2 and the transistors Qa3 and Qa4 uniform as shown in FIG.
[0021]
By the way, in the current mode switch circuit of FIG. 1, the emitter of the transistor Qb2 or the transistor Qb4 has a potential that is lower than the power supply voltage VCC by the base-emitter voltage (VF) of the transistor. Further, the emitter of the transistor Qb5 is a potential that is further lower than the base of the transistor Qb5 by the base-emitter voltage (VF) of the transistor. Therefore, if the saturation voltage (about 0.2 V) between the collectors and emitters of the transistors Qb1 and Qb3 is ignored with respect to the inputs INb1 and INb2, the emitter of the transistor Qb5 is twice the base-emitter voltage of the transistor (approximately The potential is lowered by 2 × VF).
[0022]
Further, the base-emitter voltage of the transistor has a temperature characteristic of −2 mV / ° C. and is output via two base-emitter voltages. Even compared with the conventional example that is output, the temperature characteristic is doubled. Further, 2 × VF is fixed, and the width that can be used for amplification is the power supply voltage VCC-2 × VF, which is smaller than the conventional example. That is, since the base-emitter voltage of the transistor is about 0.7V, when the power supply voltage VCC is 3V, if the saturation voltage between the collectors and emitters of the transistors Qb1 and Qb3 is ignored, 1.4V Only (3-1.4 × 2 = 1.4) can be used.
[0023]
An improved current mode switch circuit is shown in FIG.
The improved current mode switch circuit includes SWc1 and SWc2, transistors Qc1 to Qc6, and current sources Ic1 to Ic5. Transistors Qc2 and Qc4 are emitter follower circuits. The emitters of the transistors Qc2 and Qc4 are directly connected. The transistor Qc5 is an output transistor. The transistor Qc6 is a transistor for raising the emitter potential of the transistor Qc5 by the base-emitter voltage (VF) of the transistor. The current mode switch circuit of FIG. 2 differs from FIG. 1 in that a current source Ic5 and a transistor Qc6 are added.
[0024]
INc1 and INc2 are video input terminals, and OUTc is an output terminal. VCC is a power supply voltage, and GND is a ground.
When SWc1 is closed, the video signal applied to INc1 passes through the collector of transistor Qc1, the emitter of transistor Qc1, the base of transistor Qc2, the emitter of transistor Qc2, the emitter of transistor Qc6, the base of transistor Qc6, and the base of transistor Qc5. And output from the output terminal OUTc.
[0025]
When SWc2 is opened , the video signal applied to INc2 is changed from the collector of transistor Qc3 to the emitter of transistor Qc3 to the base of transistor Qc4 to the emitter of transistor Qc4 to the emitter of transistor Qc6 to the base of transistor Qc6 to the base of transistor Qc5. Through the output terminal OUTc.
[0026]
Thus, when the switch SWc1 is opened , the video signal applied to the INc1 is output to the output terminal OUTc, and when the switch SWc2 is opened , the video signal applied to the INc2 is output to the output terminal OUTc. . The transistor Qc6 has a function of increasing the emitter potential of the transistor Qc5 by the base-emitter voltage (VF) of the transistor.
[0027]
As a result, the temperature characteristic is halved as compared with the current mode switch circuit of FIG. 1, and the use width is widened to the power supply voltage VCC-VF. In the above-described embodiment, an example using a transistor has been described. However, the present invention can also be implemented using an FET. In the above embodiment, an example of two signal inputs has been described. However, the number of input signals is not limited to two, and a plurality of input signals can be handled. In this case, a desired signal is output by opening the switch of the grounded base transistor connected to the input terminal to which the output signal is applied.
[0028]
In the above embodiment, an example of a video signal has been described. However, the present invention is not limited to a video signal and may be another signal.
Further, although the amount of crosstalk is substantially determined by the impedance and parasitic capacitance of the lines Lb and Lc, the lines Lb and Lc are kept at a low impedance, and the crosstalk can be reduced.
[0029]
In addition, since the emitter follower circuit is used, high frequency characteristics can be obtained. For example, the conventional one of about 20 MHz can be extended to 100 to 200 MHz.
[0030]
【The invention's effect】
As described above, according to the present invention, the following various effects can be achieved.
As described above, according to the present invention, the following various effects can be achieved.
According to the first aspect of the invention, a plurality of emitter-follower transistors connected in common emitter jitter, and a plurality of common base transistor connected to the bases of the emitter follower transistor of the plurality, the plurality of common base transistor A plurality of signal input terminals respectively connected to the collectors of the plurality of transistors, and a plurality of switch circuits respectively connected to the bases of the plurality of grounded base transistors, and signals are applied to the plurality of signal input terminals, respectively. The signal applied to the signal input terminal connected to the collector of the grounded base transistor to which the opened switch is connected by opening one of the plurality of switches, the emitter of the emitter follower transistor To reduce crosstalk at high frequencies. Comb, it is possible to improve the frequency characteristic in the high frequency range.
Further, by providing a diode in the forward direction between the emitter of the emitter-follower transistor and the base of the output transistor, the temperature characteristic based on the base-emitter voltage of the transistor is halved and can be used for the power supply voltage VCC. The voltage can also be used up to the power supply voltage VCC-VF (VF: voltage between the base and emitter of the transistor ) .
[0031]
According to the invention of claim 2, by providing the diode in the forward direction between the emitter of the emitter follower transistor and the base of the output transistor, the temperature characteristic based on the base-emitter voltage of the transistor is halved. A voltage that can be used for the power supply voltage VCC can also be used up to the power supply voltage VCC-VF (VF: voltage between the base and emitter of the transistor).
[Brief description of the drawings]
FIG. 1 is a diagram for explaining a first embodiment of the present invention;
FIG. 2 is a diagram for explaining a second embodiment of the present invention.
FIG. 3 is a diagram for explaining a conventional example.
FIG. 4 is a diagram for explaining crosstalk.
[Explanation of symbols]
Qa1 to Qa5, Qb1 to Qb5, Qc1 to Qc6 Transistors Ia1 to Ia3, Ib1 to Ib4, Ic1 to Ic5 Current sources INa1, INa2, INb1, INb2, INc1, INc2 Input terminals OUTa, OUTb, OUTc Output terminals VCC Power supply voltage SWa1, SWa2, SWb1, SWb2, SWc1, SWc2 Switch V Crosstalk source Z1, Z2 Impedance

Claims (1)

A plurality of emitter-follower transistors commonly connected to an emitter, a plurality of grounded base transistors connected to the bases of the plurality of emitter-follower transistors, and a plurality of signal input terminals respectively connected to collectors of the plurality of grounded base transistors A plurality of switch circuits respectively connected to the bases of the plurality of grounded base transistors, and an output transistor having a base connected to the emitter of the emitter follower transistor,
A current source is connected to each of the bases of the plurality of grounded base transistors,
A signal is applied to each of the plurality of signal input terminals,
By opening one of the plurality of switches, a signal applied to the signal input terminal connected to the collector of the grounded base transistor to which the opened switch is connected is output from the emitter of the output transistor. And
A differential circuit configuration transistor is provided between the emitter of the emitter follower transistor and the base of the output transistor to form a differential circuit with the emitter follower transistor,
An emitter of the differential circuit constituting transistor is connected to an emitter of the emitter follower transistor;
In the differential circuit configuration transistor, a collector and a base are connected,
A current source and a base of the output transistor are connected to a connection point between a collector and a base of the differential circuit configuration transistor,
An output signal is obtained from an emitter of the output transistor .

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