A level shift circuit

Abstract: A level shift circuit which drops the output voltage of the prior stage circuit to an input voltage level required at the next stage circuit, includes a source follower enhancement type FET (Q₁) gate of which is connected to an input terminal (10), a drain of which is connected to a positive power supply (VDD), and a source of which is connected to an anode of a level shift diode (D₁,D₂) a current adjusting enhancement type FET (Q₂) a drain of which is connected to a cathode of the level shift diode (D₁,D₂), a drain and a gate of which are connected with each other via a constant current source (3) provided therebetween, and a source of which is connected to a negative power supply (VEE), an output terminal (20) which is taken out from the connection node of the level shift diode (D₂) and the constant current source (3), a resistor (5) connected between the constant current source (3) and the negative power supply (VEE), and the current adjusting enhancement type FET (Q₂) having its gate-to-source voltage set by a current flowing through the resistor (5) to adjust the current flowing through the source follower enhancement type FET (Q₁).

Description: FIELD OF THE INVENTION The present invention relates to a level shift circuit which drops the output voltage of the prior stage circuit to an input voltage level of the next stage circuit, more particularly, to a level shift circuit which suppresses the variation in the level shift amount caused by the variations of the device characteristics of a source follower enhancement type field effect transistor (hereinafter referred to as FET). BACKGROUND OF THE INVENTION Figure 4 is a diagram showing a circuit construction of a prior art level shift circuit. In figure 4, reference numeral 500 designates a level shift circuit. A drain of a source follower enhancement type FET 1(Q1) is connected to a positive power supply 30, and a gate thereof is connected to an input terminal 10 to which an output signal of the prior stage circuit is input, and a source thereof is connected to the anode of the level shift diode 2. A constant current source 3 is connected between the cathode of the level shift diode 2 and the negative power supply 40 and an output terminal 20 is taken out from the connection node of the cathode of the level shift diode 2 and one end of the constant current source 3, thus constituting a level shift circuit. Here, while in the above example the level shift diode 2 comprises two diodes D1 and D2 connected in series to each other, the number of diodes is not restricted thereto and can be determined in accordance with the voltage to be dropped. In addition, the constant current source 3 is constituted by a depletion FET having its gate and its source connected with each other. A description is given of an operation hereinafter. When it is supposed that a current which is made to flow through the level shift circuit 500 by the constant current source 3 is I const and that a source follower enhancement type FET 1 is operated at the saturation region, the level shift amount V gs between its gate and its source, is represented by the following formula (1). V gs = I const K + V th here, K : gain coefficient of the enhancement type FET 1(Q1) V th : threshold voltage of the enhancement type FET 1(Q1) From the equation (1), by setting the current I const flowing through the constant current source 3 to an appropriate value, the level shift amount V gs at the source follower enhancement type FET 1(Q1) is determined. The reason why he enhancement type FET 1(Q1) is employed for the source follower FET is because the enhancement type FET an produce a desired level shift amount by a less current I const than that of the depletion type FET, thereby resulting in a low power dissipation. Here, when the voltage difference between the gate and the source in the source follower enhancement type FET 1(Q1) is larger than its junction voltage  B (about 0.7 V), a forward direction current flows between the gate and source of the FET, and the level shift amount V gs falls within 0 to 0.7 V and does not exceed that range. In this level shift circuit, in order to increase the level shift amount of the entire circuit V shift to a value larger than the junction voltage  B (about 0.7 V), the level shift diode 2 is employed. The level shift amount at one level shift diode D1, D2 is approximately equal to its junction voltage difference  B (about 0.7 V) and the level shift amount of the entire circuit V shift is represented by the following formula (2) and it means that providing an appropriate number of level shift diodes enables setting of a level shift amount of arbitrary value larger than 0.7 V. V shift = I const K + V th +  B x (the number of diodes) In the prior art level shift circuit constituted as described above, while producing a level shift circuit of such construction, if the device parameters of the source follower enhancement type FET 1(Q1) such as threshold voltages and gain coefficients are deviated from predetermined set values, the variations in the device parameters vary the level shift amount V gs against the current I const of the source follower enhancement type FET 1(Q1), thereby resulting in variations in the output voltage of the entire circuit of the level shift circuit, that is, the level shift amount. Here, the variations in the threshold value of the enhancement type FET are about several tens mV on the same wafer, and they become large to about 100 mV on different wafers. From Patent Abstracts of Japan, Vol. 14, No. 396, for JP-A-02 149 014 a level shift circuit is known comprising, between a positive power supply and a negative power supply, a series connected arrangement of a source follower FET, a level shift diode and a current adjusting FET, the circuit input being connected to the gate of the source follower FET and the circuit output providing the level shifted voltage being connected to the connection node of the cathode of the level shift diode and the drain electrode of the current adjusting FET within said series arrangement. For adjusting the current flowing through the source follower FET the gate-to-source voltage of the current adjusting FET is set by the voltage drop at a circuit component connected parallelly to the gate-to-source-path and being fed by a constant current source connected to the positive power supply; said parallely connected circuit component has the shape of a diode the cathode of which is offset from the source terminal of the current adjusting FET by a resistor connected between such source terminal and the negative power supply. It is an object of the present invention to provide a level shift circuit that provides a desired constant output voltage even when the device parameters such as a threshold voltage and a gain coefficient of a source follower enhancement type FET are deviated from the design values in its production, and that has a circuit configuration different from the known level shift circuit just mentioned before. According to a first aspect of the present invention, a level shift circuit which drops the output voltage of the prior stage circuit to an input voltage level required at the next stage circuit, includes a source follower enhancement type field effect transistor a gate of which is connected to an input terminal, a drain of which is connected to a positive power supply, and a source of which is connected to an anode of a level shift diode, a current adjusting enhancement type field effect transistor a drain of which is connected to a cathode of the level shift diode, a drain and a gate of which are connected with each other via a constant current source provided therebetween, and a source of which is connected to a negative power supply, an output terminal which is taken out from the connection node of the level shift diode and the constant current source, and a resistor connected between the constant current source and the negative power supply. The current adjusting enhancement type field effect transistor has its gate-to-source voltage set by a current flowing through the resistor, thereby adjusting the current flowing through the level shift diode. In this construction, since the current adjusting enhancement type FET is connected in parallel to the constant current source, even when the threshold value or the gain coefficient of a source follower enhancement type FET vary between wafers in wafer processes, the similar variations also arise in the current adjusting enhancement type FET produced on the wafer in the vicinity of the source follower enhancement type FET, thereby adjusting a current flowing through the source follower enhancement type FET. According to a second aspect of the present invention, a level shift circuit of the above construction further includes a capacitance connected in parallel with the level shift diode. In this construction, since a capacitance is connected in parallel with a level shift diode, the high frequency signal input to the input terminal is output to the output terminal through this capacitance, so that the deterioration in the waveform due to the input capacitance of the source follower FET and the internal resistance of the level shift diode are prevented. According to a third aspect of the present invention, a level shift circuit of the above-described construction further includes a direct connection of the source of the source follower FET and the drain of the current adjusting FET removing the level shift diode. Because the level shift diode is removed, the level shift amount comprises only the gate-to-source voltage of the source follower FET and a buffer circuit having a level shift function is constituted to have the level shift amount below 0.7 V. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagram illustrating a circuit construction of a level shift circuit according to a first embodiment of the present invention. Figure 2 is a diagram illustrating a circuit construction of a level shift circuit according to a second embodiment of the present invention; Figure 3 is a diagram illustrating a circuit construction of a level shift circuit according to a third embodiment of the present invention; and Figure 4 is a diagram illustrating a circuit construction of a level shift circuit according to the prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT EMBODIMENT 1 Figure 1 is a diagram illustrating a level shift circuit according to a first embodiment of the present invention. In figure 1, reference numerals shown in figure 5 designates the same or corresponding elements. Reference numeral 100 designates a level shift circuit of this first embodiment. In this level shift circuit 100, a drain of a current adjusting enhancement type FET 4(Q2) is connected to an output terminal 20 of the level shift circuit 500 shown in figure 5 and a source of the current adjusting enhancement type FET 4(Q2) is connected to a negative power supply 40. A gate of the current adjusting FET 4(Q2) is connected to the other end of a constant current source 3 (opposite side end of the output terminal 20) and a bias resistor 5 is inserted between the gate and the source of the current adjusting enhancement type FET 4(Q2). In this circuit, the gate-to-source voltage of the current adjusting FET 4(Q2) is determined by that a current from the constant current source 3 flows through the resistor 5. A description is given of the operation. In this level shift circuit, the current flowing through the circuit is a sum of a current I const determined by the constant current source 3 and a current I var flowing through the current adjusting enhancement type FET 4(Q2). Accordingly, supposed that the design value of the threshold voltage of the enhancement type FET is V th , the level shift amount V gs1 between the gate and source of the source follower enhancement type FET 1(Q1) is represented by the following formula (3): V gs1 = I const + I var K1 + V th here, K1: gain coefficient of the source follower enhancement type FET 1(Q1). The current I var flowing through the current adjusting enhancement type FET 4(Q2) is represented by the following formula (4): I var = K2 (V gs2 - V th ) 2 here, K2 : gain coefficient of the current adjusting enhancement type FET 4(Q2) V gs2 : a voltage between the gate and source of the current adjusting enhancement type FET 4(Q2) Since the source follower enhancement type FET 1(Q1) and the current adjusting enhancement type FET 4(Q2) are arranged on a wafer quite close to each other, and differences in the threshold voltage between both transistors in the manufacture are smaller than several tens mV. Accordingly, in a case where the junction depth of the implantation region of the FET is produced to be shallower than a predetermined depth and the threshold voltage of FET on a wafer is deviated by + ΔV th from the design value V th in the safer process, the level shift amount V gs1 at the source follower FET 1(Q1) also rises by ΔV th while the current I var flowing through the current adjusting enhancement type FET 4(Q2) which is determined by the following formula (5): Δ(I var ) 1/2 = -(K2) 1/2 · ΔV th is reduced. Therefore, the rising of the level shift amount V gs1 at the source follower FET 1(Q1) is suppressed by the reduction in the current I var flowing through the current adjusting FET 4(Q2), whereby the variation in the level shift amount of the entire circuit is suppressed. On the other hand, in a case where the junction depth of the implantation region of the FET is produced to be deeper than a predetermined depth and the threshold voltage of the FET on a wafer is deviated from the design value V th by - ΔV th , the level shift amount V gs1 at the source follower FET 1(Q1) is reduced by -ΔV th . In this case, since the current I var flowing through the current adjusting FET 4(Q2) increases, the reduction in the level shift amount V gs1 at the source follower FET 1(Q1) is suppressed by the increase in the current I var flowing through the current adjusting FET 4(Q2), and the variations in the level shift amount of the entire circuit are suppressed. In addition, the level shift circuit of the present invention can suppress the variations in the gain coefficient. That is, supposed that the gain coefficient K1 of the source follower FET 1(Q1) varies in the direction of increase, the gate-to-source voltage of the source follower FET 1(Q1) is reduced. At the same time, the current I var increases because the gain coefficient K2 of the current adjusting FET 4(Q2) has increased, and it functions to increase the gate-to-source voltage of the source follower FET 1(Q1), thereby suppressing the variations in the level shift amount of the entire circuit. Also in a case where the gain coefficient K1 of the source follower FET 1(Q1) varies in the direction of reduction, the variations in the level shift amount of the entire circuit are suppressed. In the level shift circuit of this first embodiment, a current adjusting enhancement type FET 4(Q2), between the gate and the source of which a resistor 5 for biasing is inserted, is connected in parallel with the constant current source 3 so that a current from the constant current source 3 flows through the resistor 5. Therefore, even when the threshold voltage and the gain coefficient of the source follower enhancement type FET 1(Q1) increase or reduce to be more or less than the design values, the threshold voltage and the gain coefficient of the current adjusting enhancement type FET 4(Q2) also increase or reduce by the same amount and the variations in the level shift amount V gs1 between the gate and source of the source follower FET 1(Q1) is suppressed by the variations in the current I var flowing through the current adjusting FET 4(Q2), thereby resulting in suppressing the variations in the level shift amount of the entire circuit. EMBODIMENT 2 Figure 2 is a diagram showing a circuit construction of a level shift circuit according to a second embodiment of the present invention. In figure 2, the same reference numerals designate the same or corresponding elements as those shown in figure 1. In a level shift circuit 200 of this second embodiment, a capacitance 6 for passing a high frequency signal is connected in parallel to the level shift diode 2 of the level shift circuit 100 of the first embodiment shown in figure 1. In the level shift circuit of this second embodiment, in addition to that the same effects as those of the level shift circuit of the first embodiment are obtained, the dc level of the input signal is level-shifted via the source follower enhancement type FET 1(Q1) and the level shift diode 2, and a high frequency signal is output to the output terminal 2 through the capacitance 6, thereby preventing the waveform deterioration of the high frequency signal due to the input capacitance of the source follower enhancement type FET 1(Q1) and the internal resistance of the level shift diode 2, resulting in enabling a transmission of a high speed signal. EMBODIMENT 3 Figure 3 is a diagram showing a circuit construction of a level shift circuit of a third embodiment of the present invention. In the figure, the same reference numerals designate the same or corresponding elements as those in figure 1. In the level shift circuit 300 of this third embodiment, the level shift diode 2 is removed from the level shift circuit 100 of the first embodiment shown in figure 1, and the source of the source follower FET 1(Q1) is directly connected to the output terminal 20, and the drain of the current adjusting FET 4(Q2) is connected to the source of the source follower FET 1(Q1). In the level shift circuit of this third embodiment, the level shift amount comprises only the gate-to-source voltage of the source-follower FET 1(Q1). Accordingly, the level shift circuit of this embodiment can be used as a buffer circuit having a level shift function of the level shift amount of below 0.7 V. As discussed above, in a level shift circuit according to the present invention, a current adjusting enhancement type FET is connected in parallel with a constant current source, and therefore, even when the threshold value and gain coefficient of the source follower enhancement type FET vary largely in the manufacture, similar variations arise in the current adjusting FET produced in the vicinity of the source follower FET, and the variations in the current flowing through the source follower FET are suppressed. As a result, a desired constant output voltage is always obtained. In addition, in a level shift circuit according to the present invention, a capacitance is connected in parallel with the level shift diode connected to the source of the source follower FET, and therefore, the input high frequency signal is output without receiving waveform deterioration due to the input capacitance of the source follower FET and the internal resistance of the level shift diode, thereby enabling the transmission of a high speed signal.

Claims: A level shift circuit (fig. 1) which drops the output voltage of the prior stage circuit to an input voltage level required at the next stage circuit, comprising: a source follower enhancement type field effect transistor (1) a gate of which is connected to an input terminal (10), a drain of which is connected to a positive power supply (30), and a source of which is connected to an anode of a level shift diode (2); a current adjusting enhancement type field effect transistor (4) a drain of which is connected to a cathode of said level shift diode (2), a drain and a gate of which are connected with each other via a constant current source (3) provided therebetween, and a source of which is connected to a negative power supply (40); an output terminal (20) which is taken out from the connection node of said level shift diode (2) and said constant current source (3); a resistor (5) connected between said constant current source (30) and said negative power supply (40); and said current adjusting enhancement type field effect transistor (4) having its gate-to-source voltage set by a current flowing through said resistor (5) to adjust the current flowing through said source follower enhancement type field effect transistor (1). The level shift circuit (fig. 2) of claim 1, wherein a capacitor (6) is connected in parallel with said level shift diode (2). A level shift circuit (fig. 3) which drops the output voltage of the prior stage circuit to an input voltage level required at the next stage circuit, comprising; a source follower enhancement type field effect transistor (1) a gate of which is connected to an input terminal (10) and a drain of which is connected to a positive power supply (30); a current adjusting enhancement type field effect transistor (4) a drain of which is connected to the source of said source follower enhancement type field effect transistor (1), a drain and a gate of which are connected with each other via a constant current source (3), and a source of which is connected to a negative power supply (40); an output terminal (20) taken out from the connection node of the source of said source follower enhancement type field effect transistor (1) and said constant current source (3); a resistor (5) connected between said constant current source (3) and a negative power supply (40); and said current adjusting enhancement type field effect transistor (4) having its gate-to-source voltage set by the current flowing through said resistor (5) thereby to adjust the current flowing through said source follower enhancement type field effect transistor (1).

Inventors: MIYASHITA MIYO, MIYASHITA, MIYO, MIYASHITA, MIYO, C/O MITSUBISHI DENKI K.K., HIKARI
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1993
Main classification: H03K19
IPC codes: H03F3, H03K19