Buffer circuit, driver circuit, and semiconductor testing apparatus

There is provided a buffer circuit that can deal with input and output signals having a large voltage swing. Such a buffer circuit is designed for outputting an output signal corresponding to an input signal. The buffer circuit includes an input/output circuit for maintaining an output impedance at a constant level, and outputting the output signal having an output voltage which is substantially the same as the input voltage of the input signal, the transistors 434 and 424 that are connected to the respective ends of the input/output circuit in series, where the transistors 434 and 424 protect the input/output circuit by reducing power consumption of the input/output circuit in such a manner as to supply voltages that correspond to the input voltage or the output voltage in terms of level, to the respective ends of the input/output circuit, and the control circuit 420 for (i) when the input voltage is smaller than a reference level, supplying a predetermined constant voltage to the transistor 434 as the base voltage, and (ii) when the input voltage is equal to or larger than the reference level, supplying a voltage that is obtained by decreasing the input voltage by a predetermined voltage to the transistor 434 as the base voltage. Here, the decreased voltage is larger than the predetermined constant voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a buffer circuit, a driver circuit, and a semiconductor testing apparatus. The present invention specifically relates to a buffer circuit and a driver circuit that can deal with input and output signals with a large voltage swing, and a semiconductor testing apparatus including such a driver.

2. Related Art

FIG. 1shows the configuration of a conventional buffer circuit100. The buffer circuit100includes an input terminal101, a transistor102, a schottky diode104, a constant current source106, a constant current source108, a schottky diode110, a transistor112, a transistor114, a schottky diode116, a schottky diode118, a transistor120, and an output terminal122.

The transistor102is an NPN transistor. The base electrode of the transistor102is connected to the input terminal101, the collector electrode is connected to a positive power supply voltage (Vcc), and the emitter electrode is connected to the anode of the schottky diode104. The schottky diode104is connected at the anode to the emitter electrode of the transistor102, and connected at the cathode to the base electrode of the transistor120and the constant current source106. The constant current source106is connected to the cathode of the schottky diode104and a negative power supply voltage (Vee) so as to be positioned therebetween. The constant current source106causes a constant current to flow, between the positive power supply voltage and the negative power supply voltage, into the transistor102and the schottky diode104.

The transistor112is a PNP transistor. The base electrode of the transistor112is connected to the input terminal101, the collector electrode is connected to the negative power supply voltage, and the emitter electrode is connected to the cathode of the schottky diode110. The schottky diode110is connected at the anode to the constant current source108, and connected at the cathode to the emitter electrode of the transistor112. The constant current source108is connected to the positive power supply voltage and the anode of the schottky diode110so as to be positioned therebetween. The constant current source108causes a constant current to flow, between the positive power supply voltage and the negative power supply voltage, into the schottky diode110and the transistor112.

The transistor114is an NPN transistor. The base electrode of the transistor114is connected to the constant current source108and the anode of the schottky diode110, the collector electrode is connected to the positive power supply voltage, and the emitter electrode is connected to the schottky diode116. The schottky diode116is connected at the anode to the emitter electrode of the transistor114, and connected at the cathode to the output terminal122. The schottky diode118is connected at the anode to the output terminal122, and connected at the cathode to the emitter electrode of the transistor120. The transistor120is a PNP transistor. The base electrode of the transistor120is connected to the constant current source106and the cathode of the schottky diode104, the emitter electrode is connected to the cathode of the schottky diode118, and the collector electrode is connected to the negative power supply voltage. Here, the currents supplied from the constant current sources106and108are substantially the same. According to this construction, the transistor114, the schottky diode116, the schottky diode118, and the transistor120are supplied with a constant current all the time.

Here, it is assumed that in the above-described buffer circuit100, the positive power supply voltage is +19 V, the negative power supply voltage is −6.5 V, and the input signal (input) and the output signal (output) have a voltage in a range from −3 V to +15 V. When a voltage decrease due to the transistors is 0.8 V, and a voltage decrease due to the schottky diodes is 0.5 V, the base voltage of the transistor114ranges from −1.7 V to +16.3 V, and the base voltage of the transistor120ranges from −4.3 V to +13.7 V.

FIG. 2shows the configuration of a conventional buffer circuit200. The buffer circuit200includes an input terminal201, a transistor202, a schottky diode204, a resistance206, a schottky diode208, a constant current source210, a constant current source212, a schottky diode214, a resistance216, a schottky diode218, a transistor220, a transistor222, a transistor224, a schottky diode226, a schottky diode228, a transistor230, a transistor232, and an output terminal234.

The transistor202is an NPN transistor. The base electrode of the transistor202is connected to the input terminal201, the collector electrode is connected to a positive power supply voltage (Vcc), and the emitter electrode is connected to the anode of the schottky diode204. The schottky diode204is connected at the anode to the emitter electrode of the transistor202, and connected at the cathode to the base electrode of the transistor230and the resistance206. The resistance206is connected to the cathode of the schottky diode204and the anode of the schottky diode208so as to be positioned therebetween. The schottky diode208is connected at the anode to the resistance206, and connected at the cathode to the base electrode of the transistor232and the constant current source210. The constant current source210is connected to the cathode of the schottky diode208and a negative power supply voltage (Vee) so as to be positioned therebetween. The constant current source210causes a constant current to flow, between the positive power supply voltage and the negative power supply voltage, into the transistor202, the schottky diode204, the resistance206and the schottky diode208.

The transistor220is a PNP transistor. The base electrode of the transistor220is connected to the input terminal201, the collector electrode is connected to the negative power supply voltage, and the emitter electrode is connected to the cathode of the schottky diode218. The schottky diode218is connected at the cathode to the emitter electrode of the transistor220, and connected at the anode to the base electrode of the transistor224and the resistance216. The resistance216is connected to the cathode of the schottky diode214and the anode of the schottky diode218so as to be positioned therebetween. The schottky diode214is connected at the cathode to the resistance216, and connected at the anode to the base electrode of the transistor222and the constant current source212. The constant current source212is connected to the anode of the schottky diode214and the positive power supply voltage so as to be positioned therebetween. The constant current source212causes a constant current to flow, between the positive power supply voltage and the negative power supply voltage, into the schottky diode214, the resistance216, the schottky diode218, and the transistor220.

The transistor222is an NPN transistor. The base electrode of the transistor222is connected to the constant current source212and the anode of the schottky diode214, the collector electrode is connected to the positive power supply voltage, and the emitter electrode is connected to the collector electrode of the transistor224. The transistor224is an NPN transistor. The base electrode of the transistor224is connected to the resistance216and the anode of the schottky diode218, the collector electrode is connected to the emitter electrode of the transistor222, and the emitter electrode is connected to the anode of the schottky diode226. The schottky diode226is connected at the anode to the emitter electrode of the transistor224, and connected at the cathode to the output terminal234and the anode of the schottky diode228. The schottky diode228is connected at the anode to the cathode of the schottky diode226and the output terminal234, and connected at the cathode to the emitter electrode of the transistor230. The transistor230is a PNP transistor. The base electrode of the transistor230is connected to the cathode of the schottky diode204and the resistance206, the emitter electrode is connected to the cathode of the schottky diode228, and the collector electrode is connected to the emitter electrode of the transistor232. The transistor232is a PNP transistor. The base electrode of the transistor232is connected to the cathode of the schottky diode208and the constant current source210, the emitter electrode is connected to the collector electrode of the transistor230, and the collector electrode is connected to the negative power supply voltage. Here, the currents supplied from the constant current sources210and212are substantially the same. According to this construction, the transistor222, the transistor224, the schottky diode226, the schottky diode228, the transistor230and the transistor232are supplied with a constant current all the time.

Here, it is assumed that in the above-described buffer circuit200, the positive power supply voltage is +19 V, the negative power supply voltage is −6.5 V, and the input signal (input) and the output signal (output) have a voltage in a range from −3 V to +15 V. When a voltage decrease due to the transistors is 0.8 V, and a voltage decrease due to the schottky diodes is 0.5 V, the base voltage of the transistor224ranges from −1.7 V to +16.3 V, the base voltage of the transistor230ranges from −4.3 V to +13.7 V, the base voltage of the transistor222ranges from −0.9 V to +17.1 V, and the base voltage of the transistor232ranges from −5.1 V to +12.9 V.

Since no prior art documents related to the present invention have been found, the explanation regarding such documents will be omitted.

Recent development of a higher-speed semiconductor device has created a demand for a higher-speed buffer circuit to be used in a driver circuit that is included in a semiconductor testing apparatus and supplies a test signal for testing the semiconductor device. Since such a buffer circuit has been increasingly formed by a chip, it is difficult to increase the permissible voltage level of the base voltages of the transistors used in the buffer circuit in addition to realization of higher-speed configuration.

In the buffer circuit100shown inFIG. 1, the voltage swing of the input signal is directly reflected in the base voltages of the transistors114and120. This makes it impossible to increase the voltage swing of the input signal. Accordingly, the buffer circuit100can not output an output signal having a large voltage swing. The buffer circuit200shown inFIG. 2additionally has a tracking circuit, and thus can maintain the base voltages of the transistors224and230at a constant level. However, the voltage swing of the input signal is directly reflected in the base voltages of the transistors222and232. This makes it impossible to increase the voltage swing of the input signal. As a result, the buffer circuit200can not output an output signal having a large voltage swing.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a buffer circuit, a driver circuit, and a semiconductor testing apparatus that can solve the above-described problems. This object is achieved by combining the features recited in the independent claims. The dependent claims define further effective specific example of the present invention.

A first embodiment of the present invention provides a buffer circuit for outputting an output signal corresponding to an input signal. The buffer circuit includes an input/output circuit for maintaining an output impedance at a constant level, and outputting the output signal having an output voltage which is substantially same as an input voltage of the input signal, a first transistor and a second transistor that are connected to respective ends of the input/output circuit in series, wherein the first and second transistors protect the input/output circuit by reducing power consumption of the input/output circuit in such a manner as to supply voltages that correspond to the input voltage or the output voltage in terms of level, to the respective ends of the input/output circuit, and a first control circuit for (i) when the input voltage is smaller than a first reference level, supplying a predetermined constant voltage to the first transistor as a base voltage thereof, and (ii) when the input voltage is equal to or larger than the first reference level, supplying a voltage that is obtained by decreasing the input voltage by a first predetermined voltage to the first transistor as the base voltage. Here, the decreased voltage is larger than the predetermined constant voltage.

The input/output circuit may include an input terminal for receiving the input signal, an output terminal for outputting the output signal, a third transistor whose base electrode is connected to the input terminal, and whose collector electrode is connected to a positive power supply voltage, wherein the third transistor is an NPN transistor, a first diode whose anode is connected to an emitter electrode of the third transistor, a first constant current source that is connected to a cathode of the first diode, wherein the first constant current source supplies a constant current to the third transistor and the first diode, a fourth transistor whose base electrode is connected to the input terminal, and whose collector electrode is connected to a negative power supply voltage, wherein the fourth transistor is a PNP transistor, a second diode whose cathode is connected to an emitter electrode of the fourth transistor, and a second constant current source that is connected to an anode of the second diode, wherein the second constant current source supplies a constant current to the fourth transistor and the second diode.

The input/output circuit may further include a fifth transistor whose base electrode is connected to the cathode of the first diode, and whose collector electrode is connected to an emitter electrode of the first transistor, wherein the fifth transistor is a PNP transistor, a third diode whose anode is connected to the output terminal, and whose cathode is connected to an emitter electrode of the fifth transistor, a sixth transistor whose base electrode is connected to the anode of the second diode, and whose collector electrode is connected to an emitter electrode of the second transistor, wherein the sixth transistor is an NPN transistor, and a fourth diode whose anode is connected to an emitter electrode of the sixth transistor, and whose cathode is connected to the output terminal.

The first transistor may be a PNP transistor, and a collector electrode and an emitter electrode of the first transistor may be respectively connected to a negative power supply voltage and the input/output circuit. The second transistor may be an NPN transistor, and a collector electrode and an emitter electrode of the second transistor may be respectively connected to a positive power supply voltage and the input/output circuit.

The first control circuit may include a first constant voltage source for, when the input voltage is smaller than the first reference level, supplying the predetermined constant voltage to the first transistor as the base voltage thereof.

The first control circuit may include a seventh transistor whose base electrode is supplied with the input signal, wherein the seventh transistor is a PNP transistor, a fifth diode whose cathode is connected to an emitter electrode of the seventh transistor, a first voltage decreasing circuit for decreasing a positive power supply voltage and supplying the decreased voltage to an anode of the fifth diode, a third constant current source that is connected to the positive power supply voltage and the first voltage decreasing circuit so as to be positioned therebetween, wherein the third constant current source supplies a constant current to the first voltage decreasing circuit, the fifth diode, and the seventh transistor, an eighth transistor whose base electrode is supplied with the input signal, wherein the eighth transistor is an NPN transistor, a sixth diode whose anode is connected to an emitter electrode of the eighth transistor, a second voltage decreasing circuit that is connected to a cathode of the sixth diode, wherein the second voltage decreasing circuit supplies the voltage obtained by decreasing the input voltage by the first predetermined voltage, to the first transistor as the base voltage and the decreased voltage is larger than the predetermined constant voltage, and a fourth constant current source that is connected to the second voltage decreasing circuit and a negative power supply voltage so as to be positioned therebetween, wherein the fourth constant current source supplies a constant current to the eighth transistor, the sixth diode, and the second voltage decreasing circuit.

The buffer circuit may further include a second control circuit for (i) when the input voltage is larger than a second reference level, supplying a predetermined constant voltage to the second transistor as a base voltage thereof, and (ii) when the input voltage is equal to or smaller than the second reference level, supplying a voltage obtained by increasing the input voltage by a second predetermined voltage, to the second transistor as the base voltage, wherein the increased voltage is smaller than the predetermined constant voltage.

The second control circuit may include a second constant voltage source for, when the input voltage is larger than the second reference level, supplying the predetermined constant voltage to the second transistor as the base voltage.

The second control circuit may include a ninth transistor whose base electrode is supplied with the input signal, wherein the ninth transistor is an NPN transistor, a seventh diode whose anode is connected to an emitter electrode of the ninth transistor, a third voltage decreasing circuit for increasing a negative power supply voltage and supplying the increased voltage to a cathode of the seventh diode, a fifth constant current source that is connected to the negative power supply voltage and the third voltage decreasing circuit so as to be positioned therebetween, wherein the fifth constant current source supplies a constant current to the third voltage decreasing circuit, the seventh diode, and the ninth transistor, a tenth transistor whose base electrode is supplied with the input signal, wherein the tenth transistor is a PNP transistor, an eighth diode whose cathode is connected to an emitter electrode of the tenth transistor, a fourth voltage decreasing circuit that is connected to an anode of the eighth diode, wherein the fourth voltage decreasing circuit supplies the voltage obtained by increasing the input voltage by the second predetermined voltage, to the second transistor as the base voltage and the increased voltage is smaller than the predetermined constant voltage, and a sixth constant current source that is connected to the fourth voltage decreasing circuit and a positive power supply voltage so as to be positioned therebetween, wherein the sixth constant current source supplies a constant current to the tenth transistor, the eighth diode, and the fourth voltage decreasing circuit.

A second embodiment of the present invention provides a buffer circuit for outputting an output signal corresponding to an input signal. The buffer circuit includes an input/output circuit for maintaining an output impedance at a constant level, and outputting the output signal having an output voltage which is substantially same as an input voltage of the input signal, a first transistor and a second transistor that are connected to respective ends of the input/output circuit in series, wherein the first and second transistors protect the input/output circuit by reducing power consumption of the input/output circuit in such a manner as to supply voltages that correspond to the input voltage or the output voltage in terms of level, to the respective ends, and a second control circuit for (i) when the input voltage is larger than a second reference level, supplying a predetermined constant voltage to the second transistor as a base voltage thereof, and (ii) when the input voltage is equal to or smaller than the second reference level, supplying a voltage obtained by increasing the input voltage by a second predetermined voltage to the second transistor as the base voltage, wherein the increased voltage is smaller than the predetermined constant voltage.

A third embodiment of the present invention provides a driver circuit for supplying a test signal to a semiconductor device. The driver circuit includes an input/output circuit for maintaining an output impedance at a constant level, and supplying the test signal which has an output voltage substantially same as an input voltage of an input test signal, to the semiconductor device, a first transistor and a second transistor that are connected to respective ends of the input/output circuit in series, wherein the first and second transistors protect the input/output circuit by reducing power consumption of the input/output circuit in such a manner as to supply voltages that correspond to the input voltage or the output voltage in terms of level, to the respective ends, and a first control circuit for (i) when the input voltage is smaller than a first reference level, supplying a predetermined constant voltage to the first transistor as a base voltage thereof, and (ii) when the input voltage is equal to or larger than the first reference level, supplying a voltage obtained by decreasing the input voltage by a first predetermined voltage, to the first transistor as the base voltage, wherein the decreased voltage is larger than the predetermined constant voltage.

A fourth embodiment of the present invention provides a semiconductor testing apparatus for testing a semiconductor device. The semiconductor testing apparatus includes a pattern generating section for generating a test signal to be input into the semiconductor device, a driver circuit for supplying the test signal to the semiconductor device, a comparator circuit for comparing a test signal output from the semiconductor device with a threshold voltage, and a judging section for judging whether the semiconductor device is good or bad based on a result of the comparison done by the comparator circuit. Here, the driver circuit includes an input/output circuit for maintaining an output impedance at a constant level, and supplying a test signal having an output voltage which is substantially same as an input voltage of the test signal, to the semiconductor device, a first transistor and a second transistor that are connected to respective ends of the input/output circuit in series, wherein the first and second transistors protect the input/output circuit by reducing power consumption of the input/output circuit in such a manner as to supply voltages that correspond to the input voltage or the output voltage in terms of level, to the respective ends of the input/output circuit, and a first control circuit for (i) when the input voltage is smaller than a first reference level, supplying a predetermined constant voltage to the first transistor as a base voltage thereof, and (ii) when the input voltage is equal to or larger than the first reference level, supplying a voltage that is obtained by decreasing the input voltage by a first predetermined voltage to the first transistor as the base voltage, wherein the decreased voltage is larger than the predetermined constant voltage.

Here, all the necessary features of the present invention are not listed in the summary of the invention. The sub-combinations of the features may become the invention.

The present invention can provide a buffer circuit and a driver circuit that can deal with input and output signals having a large voltage swing, and a semiconductor testing apparatus including such a driver.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will now be described through preferred embodiments. The embodiments do not limit the invention according to claims and all the combinations of the features described in the embodiments are not necessarily essential to means for solving the problems of the invention.

FIG. 3shows an example of the configuration of a semiconductor testing apparatus300in accordance with one embodiment of the present invention. The semiconductor testing apparatus300includes a pattern generating section302, a waveform shaping section304, a driver circuit306, a judging section308, and a comparator circuit310. The pattern generating section302generates a test signal to be input into a semiconductor device (device under test: DUT)312, and supplies the generated test signal to the waveform shaping section304. The pattern generating section302also generates an expected value signal corresponding to the test signal to be input into the semiconductor device312, and supplies the expected value signal to the judging section308. The waveform shaping section304shapes the test signal supplied from the pattern generating section302. The driver circuit306supplies the test signal shaped by the waveform shaping section304, to the semiconductor device312. The comparator circuit310compares the test signal output from the semiconductor device312with a threshold voltage, and supplies the result of the comparison to the judging section308. The judging section308compares the comparison result supplied from the comparator circuit310with the expected value signal supplied from the pattern generating section302, to judge whether the semiconductor device312is good or bad.

In the semiconductor testing apparatus300in accordance with the present embodiment, the driver circuit306has a tracking circuit. This enables the semiconductor testing apparatus300to deal with a test signal with a large voltage swing. For this reason, the semiconductor device312tested by the semiconductor testing apparatus300can include semiconductor devices with various characteristics.

FIG. 4shows an example of the configuration of the driver circuit306in accordance with the present embodiment. The driver circuit306includes an input terminal400, a transistor401, a schottky diode402, a constant current source404, a constant current source406, a schottky diode408, a transistor410, a transistor412, a transistor414, a transistor416, a transistor418, a control circuit420, a control circuit422, a transistor424, a transistor426, a schottky diode428, a schottky diode430, a transistor432, a transistor434, and an output terminal436. Here, the driver circuit306is illustrative of the buffer circuit in accordance with the present invention. A schottky diode serves as an example of the diode in accordance with the present invention. The present invention may use other types of diodes than schottky diodes for the diode.

The input terminal400, the transistor401, the schottky diode402, the constant current source404, the transistor410, the schottky diode408, the constant current source406, the transistor426, the schottky diode428, the schottky diode430, the transistor432, and the output terminal436are example constituents of the input/output circuit in accordance with the present invention. These constituents maintain the output impedance at a constant level, and output, through the output terminal436, an output signal (output) having a voltage substantially the same as the voltage of an input signal (input) that is input through the input terminal400. The input/output circuit in accordance with the present invention may have a different circuit construction from the construction shown inFIG. 4.

The transistors434and424are respectively connected to the transistors432and426in series. The transistors434and424respectively supply voltages corresponding to the input voltage or output voltage to the transistors432and426, so as to reduce their power consumption. In this manner, the transistors432and426are protected. Here, the transistors432and426are illustrative of the ends of the input/output circuit in accordance with the present invention.

When the input voltage is smaller than a first reference level, the control circuit420supplies a predetermined constant voltage to the transistor434as the base voltage. When the input voltage is equal to or larger than the first reference level, the control circuit420supplies a voltage obtained by decreasing the input voltage by a first predetermined voltage, to the transistor434as the base voltage. Here, the decreased voltage is larger than the predetermined constant voltage. When the input voltage is larger than a second reference level, the control circuit422supplies a predetermined constant voltage to the transistor424as the base voltage. When the input voltage is equal to or smaller than the second reference level, the control circuit422supplies a voltage obtained by increasing the input voltage by a second predetermined voltage, to the transistor424as the base voltage. Here, the increased voltage is smaller than the predetermined constant voltage.

The following specifically describes the circuit configuration. The transistor401is an NPN transistor. The base electrode of the transistor401is connected to the input terminal400, the collector electrode is connected to the emitter electrode of the transistor412and a positive power supply voltage (Vcc) via the transistor412, and the emitter electrode is connected to the anode of the schottky diode402. The schottky diode402is connected at the anode to the emitter electrode of the transistor401, and connected at the cathode to the collector electrode of the transistor416and the constant current source404via the transistor416. The constant current source404is connected to the cathode of the schottky diode402via the transistor416. The constant current source404is connected to the schottky diode402and a negative power supply voltage (Vee) so as to be positioned therebetween. The constant current source404causes a constant current to flow, between the positive power supply voltage and the negative power supply voltage, into the transistor412, the transistor401, the schottky diode402, and the transistor416.

The transistor410is a PNP transistor. The base electrode of the transistor410is connected to the input terminal400, the collector electrode is connected to the emitter electrode of the transistor418and the negative power supply voltage via the transistor418, and the emitter electrode is connected to the cathode of the schottky diode408. The schottky diode408is connected at the cathode to the emitter electrode of the transistor410, and connected at the anode to the collector electrode of the transistor414and the constant current source406via the transistor414. The constant current source406is connected to the anode of the schottky diode408via the transistor414. The constant current source406is connected to the schottky diode408and the positive power supply voltage so as to be positioned therebetween. The constant current source406causes a constant current to flow, between the positive power supply voltage and the negative power supply voltage, into the transistor414, the schottky diode408, the transistor410, and the transistor418.

The transistor412is an NPN transistor. The base electrode of the transistor412is connected to the output terminal (out2) of the control circuit422, the collector electrode is connected to the positive power supply voltage, and the emitter electrode is connected to the collector electrode of the transistor401. The transistor414is a PNP transistor. The base electrode of the transistor414is supplied with the output terminal (out2) of the control circuit422, the emitter electrode is connected to the constant current source406, and the collector electrode is connected to the anode of the schottky diode408, the base electrode of the transistor426, and the input terminal (in) of the control circuit422. The transistor416is an NPN transistor. The base electrode of the transistor416is supplied with the output terminal (out1) of the control circuit420, the collector electrode is connected to the cathode of the schottky diode402and the base electrode of the transistor432, and the emitter electrode is connected to the constant current source404. The transistor418is a PNP transistor. The base electrode of the transistor418is supplied with the output terminal (out1) of the control circuit420, the emitter electrode is connected to the collector electrode of the transistor410, and the collector electrode is connected to the negative power supply voltage.

The transistor432is a PNP transistor. The base electrode of the transistor432is connected to the cathode of the schottky diode402and the collector electrode of the transistor416, the collector electrode is connected to the emitter electrode of the transistor434, and the emitter electrode is connected to the cathode of the schottky diode430. The schottky diode430is connected at the anode to the output terminal436and the cathode of the schottky diode428, and connected at the cathode to the emitter electrode of the transistor432. The transistor426is an NPN transistor. The base electrode of the transistor426is connected to the anode of the schottky diode408and the collector electrode of the transistor414, the collector electrode is connected to the emitter electrode of the transistor424, and the emitter electrode is connected to the anode of the schottky diode428. The schottky diode428is connected at the anode to the emitter electrode of the transistor426, and connected at the cathode to the output terminal436and the anode of the schottky diode430.

The transistor434is a PNP transistor. The base electrode of the transistor434is connected to the output terminal (out2) of the control circuit420, the collector electrode is connected to the negative power supply voltage, and the emitter electrode is connected to the collector electrode of the transistor432. The transistor424is an NPN transistor. The base electrode of the transistor424is connected to the output terminal (out1) of the control circuit422, the collector electrode is connected to the positive power supply voltage, and the emitter electrode is connected to the collector electrode of the transistor426.

Here, the currents supplied from the constant current sources404and406are substantially the same. According to this construction, the transistor424, the transistor426, the schottky diode428, the schottky diode430, the transistor432, and the transistor434are supplied with a constant current all the time, and the output impedance of the driver circuit306is maintained at a constant level. In addition, the base voltage is dispersed between the transistors424,426,432and434. In this manner, the driver circuit306can deal with input and output signals having a large voltage swing, with the permissible level of the voltages supplied to the transistors424,426,432and434being satisfied.

FIG. 5shows an example of the configuration of the control circuit420in accordance with the present embodiment. The control circuit420includes an input terminal (in)500, a constant current source502, one or more transistors504, a schottky diode506, a transistor508, a transistor510, a transistor512, a transistor514, a schottky diode516, one or more transistors518, a transistor520, a constant current source522, a schottky diode524, a constant voltage source526, an output terminal (out1)528, and an output terminal (out2)530.

Here, the control circuit420is illustrative of the first control circuit in accordance with the present invention. The present invention may use other circuit configurations than the control circuit420to realize the first control circuit. The transistors504and518are illustrative of the voltage decreasing circuit in accordance with the present invention. The present invention may use other types of elements than transistors to realize the voltage decreasing circuit.

The transistor508is a PNP transistor. The base electrode of the transistor508is connected to the input terminal500, the emitter electrode is connected to the cathode of the schottky diode506, and the collector electrode is connected to the emitter electrode of the transistor510. The base electrode of the transistor508is supplied with the input signal which is input through the input terminal400of the driver circuit306and of which the input voltage is decreased by the transistor401and the schottky diode402. The schottky diode506is connected at the cathode to the emitter electrode of the transistor508, and connected at the anode to the last transistor504. The transistors504decrease the positive power supply voltage (Vcc), and supply the decreased voltage to the anode of the schottky diode506. The transistors504are NPN transistors. The transistors504are connected to one another in series between the constant current source502and the anode of the schottky diode506. The first transistor504is connected at the base and collector electrodes to the constant current source502, and connected at the emitter electrode to the base and collector electrodes of the next transistor504. The last transistor504is connected at the base and collector electrodes to the emitter electrode of the previous transistor504, and connected at the emitter electrode to the anode of the schottky diode506. The transistor510is connected at the base electrode to the output terminal528, the emitter electrode of the last transistor518, the cathode of the schottky diode524, and the base and collector electrodes of the transistor520. The transistor510is connected at the emitter electrode to the collector electrode of the transistor508, and connected at the collector electrode to the negative power supply voltage (Vee). In the present embodiment, the number of the transistors504connected in series is eleven. However, it is obviously possible to adjust the voltage decrease freely by changing the number of the transistors504.

The constant current source502is connected to and provided between the positive power supply voltage and the transistors504. The constant current source502causes a constant current to flow, between the positive power supply voltage and the negative power supply voltage, to the transistors504, the schottky diode506, the transistor508, and the transistor510.

The transistor514is an NPN transistor. The base electrode of the transistor514is connected to the input terminal500, the emitter electrode is connected to the anode of the schottky diode516, and the collector electrode is connected to the emitter electrode of the transistor512. The base electrode of the transistor514is supplied with the input signal that is input through the input terminal400of the driver circuit306and of which the input voltage is decreased by the transistor401and the schottky diode402. The schottky diode516is connected at the anode to the emitter electrode of the transistor514, and connected at the cathode to the base and collector electrodes of the first transistor518. The transistors518outputs a voltage obtained by decreasing the input voltage of the input signal that is input through the input terminal400of the driver circuit306by a first predetermined voltage, through the output terminal530to the transistor434as the base voltage. The decreased voltage is larger than a predetermined constant voltage. The transistors518are NPN transistors. The transistors518are connected to one another in series between the cathode of the schottky diode516and the constant current source522. The first transistor518is connected at the base and collector electrodes to the cathode of the schottky diode516, and connected at the emitter electrode to the base and collector electrodes of the next transistor518. The last transistor518is connected at the base and collector electrodes to the emitter electrode of the previous transistor518, and connected at the emitter electrode to the base electrode of the transistor510, the base and collector electrodes of the transistor520, the cathode of the schottky diode524, and the output terminal528. In the present embodiment, the number of the transistors518connected in series is thirteen. However, it is obviously possible to adjust the first predetermined voltage freely by changing the number of the transistors518.

The transistor520is connected at the base and collector electrodes to the base electrode of the transistor510, the emitter electrode of the last transistor518, the cathode of the schottky diode524, and the output terminal528, and connected at the emitter electrode to the output terminal530and the constant current source522. The constant current source522is connected to and provided between the transistors518and the negative power supply voltage. The constant current source522causes a constant current to flow into the transistor512, the transistor514, the schottky diode516, the transistors518, and the transistor520.

When the input voltage of the input signal input through the input terminal400is smaller than a first reference level, the constant voltage source526outputs a predetermined constant voltage to the transistor434as the base voltage, through the output terminal530. In the present embodiment, the constant voltage source526is connected to the anode of the schottky diode524. The constant voltage output through the output terminal530is obtained by decreasing the voltage generated by the constant voltage source526, by a voltage decrease due to the schottky diode524and a voltage decrease due to the transistor520. Also, the constant voltage source526outputs a voltage, through the output terminal528, which is obtained by decreasing the voltage generated by the constant voltage source526by the voltage decrease due to the schottky diode524. When the input voltage is smaller than the first reference level, the constant voltage source526outputs the constant voltage as the base voltage of the transistor434as mentioned above. This constant voltage may preferably has a level that does not damage the high-frequency characteristics of the basic components of the buffer circuit including the transistor401, the schottky diode402, the schottky diode408, the transistor410, the transistor426, the schottky diode428, the schottky diode430and the transistor432.

FIG. 6shows an example of the configuration of the control circuit422in accordance with the present embodiment. The control circuit422includes an input terminal (in)600, a constant current source602, one or more transistors604, a schottky diode606, a transistor608, a transistor610, a transistor612, a transistor614, a schottky diode616, one or more transistors618, a transistor620, a constant current source622, a schottky diode624, a constant voltage source626, an output terminal (out1)630, and an output terminal (out2)628.

The control circuit422is illustrative of the second control circuit in accordance with the present invention. The present invention may use different circuit configurations than the control circuit422to realize the second control circuit. The transistors604and618are illustrative of the voltage decreasing circuit in accordance with the present invention. The present invention may use different types of elements than transistors to realize the voltage decreasing circuit.

The transistor608is an NPN transistor. The base electrode of the transistor608is connected to the input terminal600, the emitter electrode is connected to the anode of the schottky diode606, and the collector electrode is connected to the emitter electrode of the transistor610. The base electrode of the transistor608is supplied with the input signal which is input through the input terminal400of the driver circuit306and of which the input voltage is increased by the transistor410and the schottky diode408. The schottky diode606is connected at the anode to the emitter electrode of the transistor608, and connected at the cathode to the last transistor604. The transistors604are NPN transistors, and connected to one another in series between the constant current source602and the schottky diode606. The last transistor604is connected at the base and collector electrodes to the cathode of the schottky diode606, and connected at the emitter electrode to the base and collector electrodes of the previous transistor604. The first transistor604is connected at the base and collector electrodes to the emitter electrode of the next transistor604, and connected at the emitter electrode to the constant current source602. The transistor610is connected at the base electrode to the output terminal628, the base and collector electrodes of the last transistor618, the anode of the schottky diode624, and the emitter electrode of the transistor620, connected at the emitter electrode to the collector electrode of the transistor608, and connected at the collector electrode to the positive power supply voltage (Vcc). In the present embodiment, the number of the transistors604connected in series is eleven. However, it is obviously possible to adjust the voltage increase freely by changing the number of the transistors604.

The constant current source602is connected to and provided between the negative power supply voltage (Vee) and the transistors604. The constant current source602causes a constant current to flow, between the positive power supply voltage and the negative power supply voltage, into the transistors604, the schottky diode606, the transistor608, and the transistor610.

The transistor614is a PNP transistor. The base electrode of the transistor614is connected to the input terminal600, the emitter electrode is connected to the cathode of the schottky diode616, and the collector electrode is connected to the emitter electrode of the transistor612. The base electrode of the transistor614is supplied with the input signal which is input through the input terminal400of the driver circuit306, and of which the input voltage is increased by the transistor410and the schottky diode408. The schottky diode616is connected at the cathode to the emitter electrode of the transistor614, and connected at the anode to the emitter electrode of the first transistor618. The transistors618output a voltage obtained by increasing the input voltage of the input signal input through the input terminal400of the driver circuit306by a second predetermined voltage, to the transistor424as the base voltage through the output terminal630. The increased voltage is smaller than a predetermined constant voltage. The transistors618are NPN transistors, and connected to one another in series between the anode of the schottky diode616and the constant current source622. The first transistor618is connected at the emitter electrode to the anode of the schottky diode616, and connected at the base and collector electrodes to the emitter electrode of the next transistor618. The last transistor618is connected at the emitter electrode to the base and collector electrodes of the previous transistor618, and connected at the base and collector electrodes to the base electrode of the transistor610, the emitter electrode of the transistor620, the anode of the schottky diode624, and the output terminal628. In the present embodiment, the number of the transistors618connected in series is thirteen. However, it is obviously possible to adjust the second predetermined voltage freely by changing the number of the transistors618.

The transistor620is connected at the emitter electrode to the base electrode of the transistors610, the base and collector electrodes of the last transistor618, the anode of the schottky diode624, and the output terminal628, and connected at the base and collector electrodes to the output terminal630and the constant current source622. The constant current source622is connected to and provided between the transistors618and the positive power supply voltage. The constant current source622causes a constant current to flow into the transistor612, the transistor614, the schottky diode616, the transistors618, and the transistor620.

When the input voltage of the input signal input through the input terminal400is larger than the second reference level, the constant voltage source626outputs a predetermined constant voltage through the output terminal630to the transistor424as the base voltage. In the present embodiment, the constant voltage source626is connected to the cathode of the schottky diode624. The constant voltage output through the output terminal630is obtained by increasing the voltage generated by the constant voltage source626by a voltage increase due to the schottky diode624and a voltage increase due to the transistor620. The constant voltage source626also outputs a voltage, through the output terminal628, which is obtained by increasing the voltage generated by the constant voltage source626by the voltage increase due to the schottky diode624.

FIG. 7shows an example of the transition of the base voltage of the transistor434in accordance with the present embodiment. InFIG. 7, the horizontal axis represents the input voltage of the input signal input through the input terminal400, and the vertical axis represents the base voltage of the transistor434which is output through the output terminal (out2)530of the control circuit420to be supplied to the transistor434. It is assumed that in the driver circuit306illustrated byFIGS. 4 to 6the positive power supply voltage is +19 V, the negative power supply voltage is −6.5 V, the input signal (input) and output signal (output) have a voltage ranging from −3 V to +15 V, the voltage decrease due to the transistors is 0.8 V, the voltage decrease due to the schottky diodes is 0.5 V, and the constant voltage source526is −4.1 V.

When the input voltage of the input signal is smaller than +8.4 V which is the first reference level, the base voltage to be supplied from the control circuit420to the transistor434is −5.4 V which is the predetermined constant voltage. When the input voltage of the input signal is equal to or larger than +8.4 V of the first reference level, the base voltage to be supplied from the control circuit420to the transistor434is larger than −5.4 V of the predetermined constant voltage. As explained above, the constant voltage source526is provided so as to maintain the base voltage of the transistor434at a constant level when the input voltage of the input signal is smaller than the first reference level. Accordingly, the control circuit420does not need to operate to cope with the change of the input voltage, thereby enabling the driver circuit306to operate at high speed. As stated above, when the input voltage is smaller than the first reference level, a voltage of constant level is supplied from the constant voltage source526to the transistor434as the base voltage, and when the input voltage is equal to or larger than the first reference level, the input voltage is decreased by the transistors518and then supplied to the transistor434as the base voltage. According to this construction, the voltage swing of the input signal is not directly reflected in the base-collector voltage of the transistor434. As a result, the voltage swing of the input and output signals can be increased with the permissible level of the voltage supplied to the transistor434being satisfied.

FIG. 8shows an example of the transition of the base voltage of the transistor424in accordance with the present embodiment. InFIG. 8, the horizontal axis represents the input voltage of the input signal that is input through the input terminal400, and the vertical axis represents the base voltage of the transistor424which is output through the output terminal (out1)630of the control circuit422to be supplied to the transistor424. It is assumed that in the driver circuit306illustrated byFIGS. 4 to 6the positive power supply voltage is +19 V, the negative power supply voltage is −6.5 V, the input signal (input) and output signal (output) have a voltage ranging from −3 V to +15 V, the voltage decrease due to the transistors is 0.8 V, the voltage decrease due to the schottky diodes is 0.5 V and the constant voltage source626is +16.6 V.

When the input voltage of the input signal is larger than +4.1 V which is the second reference level, the base voltage to be supplied from the control circuit422to the transistor424is +17.9 V which is the predetermined constant voltage. When the input voltage of the input signal is equal to or smaller than +4.1 V of the second reference level, the base voltage to be supplied from the control circuit422to the transistor424is smaller than +17.9 V of the predetermined constant voltage. As explained above, the constant voltage source626is provided so as to maintain the base voltage of the transistor424at a constant level when the input voltage of the input signal is larger than the second reference level. Accordingly, the control circuit422does not need to operate to cope with the change in the input voltage. As stated above, when the input voltage is larger than the second reference level, a voltage of constant level is supplied from the constant voltage source626to the transistor424as the base voltage, and when the input voltage is equal to or smaller than the second reference level, the input voltage is increased by the transistors618and then supplied to the transistor424as the base voltage. According to this construction, the voltage swing of the input signal is not directly reflected in the base-collector voltage of the transistor424. As a result, the voltage swing of the input and output signals can be increased with the permissible level of the voltage supplied to the transistor424being satisfied.

As for the driver circuit306illustrated byFIGS. 4 to 8, as shown inFIG. 4, the base voltage of the transistors412and414falls in a range of +10 V to +17.1 V, the base voltage of the transistors416and418falls in a range of −4.6 V to +2 V, the base voltage of the transistor424falls in a range of +10.8 V to +17.9 V, the base voltage of the transistor426falls in a range of −1.7 V to +16.3 V, the base voltage of the transistor432falls in a range of −4.3 V to +13.7 V, and the base voltage of the transistor434falls in a range of −5.4 V to +1.2 V. By providing the tracking circuit including the control circuits420and422, the base voltage can be dispersed between the transistors424,426,432and434. This allows the input and output voltages to have a large voltage swing. As a consequence, the driver circuit306in accordance with the present embodiment can appropriately deal with input and output signals with a large voltage swing.

While the present invention has been described with the embodiment, the technical scope of the invention is not limited to the above described embodiment. It is apparent to persons skilled in the art that various alternations and improvements can be added to the above-described embodiment. It is apparent from the scope of the claims that the embodiment added with such alternations or improvements can be included in the technical scope of the invention.