CIRCUIT FOR PRODUCING A CONSTANT VOLTAGE

Abstract: A circuit for producing a constant voltage comprises first and second MOSFETs, and first and second bias voltage producing devices. The first and second MOSFETs to which first and second input voltages are applied, respectively, are connected in series. The first bias voltage producing device produces a potential difference, which is equal to a threshold voltage of the first MOSFET, to be applied across drain and gate of the first MOSFET, and the second bias voltage producing device produces a potential difference, which is equal to a threshold voltage of the second MOSFET, to be applied across drain and gate of the second MOSFET, so that a wide range of an output voltage is produced at a connecting point of the first and second MOSFETs. Even more, the output voltage is stabilized in level, even if the threshold voltages fluctuate in a semiconductor device fabricating process.

Description: The invention relates to a circuit for producing a constant voltage, and more particularly to a circuit in which a wide range of a voltage is produced with a stabilized characteristic. A circuit for producing a constant voltage is generally used to supply a predetermined voltage, which is different from an externally input voltage, to a semiconductor device. One type of a conventional circuit for producing a constant voltage comprises first and second P type MOS field effect transistors (each defined "P-MOSFET" hereinafter) connected in series. In the circuit, gate and drain of the first P-MOSFET are connected to source and substrate potential of the second P-MOSFET, source and substrate potential of the first P-MOSFET are connected to a first voltage input terminal, and gate and drain of the second P-MOSFET are connected to a second voltage input terminal, wherein a connecting point between the gate and the drain of the first P-MOSFET and the source and the substrate potential of the second P-MOSFET is connected to a constant voltage output terminal. In operation, first and second voltages V₁ and V₂ (V₁>V₂) are applied to the first and second voltage input terminals, respectively. A current of the first P-MOSFET is decreased to increase an output voltage at the constant voltage output terminal, and is "zero" when the output voltage ranges a value of V₁ -|V T1 | to the voltage V₁, where V T ₁ is a threshold voltage of the first P-MOSFET. On the other hand, a current of the second P-MOSFET is "zero" when the the output voltage ranges the voltage V₂ to a value of V₂+|V T2 |, where V T2 is a threshold voltage of the second P-MOSFET, and is increased to increase the output voltage. When the currents of the first and second P-MOSFETs are equal to each other, a predetermined output voltage is obtained at the constant voltage output terminal in a stabilized state. A stabilized output voltage V s is defined in the equation (1). V s =V₂+|V T2 | +{ (V₁-|V T1 | )-(V₂+|V T2 | )}x g m1 g m1 +g m2 where g m1 is a mutual transfer conductance of the first P-MOSFET, and g m2 is a mutual transfer conductance of the second P-MOSFET. According to the conventional circuit for producing a constant voltage, however, there is a disadvantage that a range of an output voltage is narrow, as understood from reasons to be described later. Further, there is a disadvantage that the output voltage V s fluctuates in accordance with the threshold voltages V T1 and V T2 changed dependent on the conditions of the fabricating process of MOSFETs, as understood from the equation (1). GB 2 073 519 relates to an intermediate potential generation circuit having a large current driving capacity. The circuit comprises first and second MOSFETs connected in series and each having one conduction type and first and second voltage sources connected respectively to the MOSFETs. The output voltage of this circuit has a width equivalent to the instability factor of the circuit and which is dependant on threshold voltages within the circuit. Clearly this circuit does not provide a high precision output voltage suitable for applications of the present invention. Accordingly, an object of the invention is to provide a circuit for producing a constant voltage from which a wide range of a constant output voltage is supplied. A further object of the invention is to provide a circuit for producing a constant voltage in which a constant voltage is produced without being affected by a threshold voltage of MOSFETs. Accordingly, the invention provides a circuit for producing a constant voltage comprising, first and second MOSFETs connected in series and being of one conduction type; bias means for each of said first and second MOSFETs; and first and second voltage sources connected to said first and second MOSFETs, respectively; characterised in that the bias means is connected between gate and drain of each of the first and second MOSFETs and includes third and fourth MOSFETs each of said one conduction type, and serves to produce potential differences equal to the threshold voltages of the respective first and second MOSFETs, whereby a wide range of a stabilized output voltage is produced at a connecting point of said first and second MOSFETs. The invention will be explained in more detail by way of example only in conjuction with appended drawings; wherein, Fig. 1 is a circuitry diagram of a conventional circuit for producing a constant voltage including two P-MOSFETs connected in series, Fig. 2 to 4 are graphical diagrams showing currents of the two P-MOSFETs relative to an output voltage of the conventional circuit, respectively, Fig. 5 is a circuitry diagram of a circuit for producing a constant.voltage in a first embodiment according to the invention, Fig. 6 is a graphical diagram showing currents of two P-MOSFETs connected in series in the circuit of the first embodiment relative to an output voltage of the circuit, and Fig. 7 is a circuitry diagram of a circuit for producing a constant voltage in a second embodiment according to the invention. Before explaining a circuit for producing a constant voltage in the first and second embodiments according to the invention, the aforementioned conventional circuit for producing a constant voltage will be explained in conjunction with Figs. 1 to 4. Fig. 1 shows a structure of the conventional circuit in which the first and second P-MOSFETs M₁ and M₂ are connected in series. In the circuit, the drain and the gate of the first P-MOSFET M₁ are respectively connected to the source and the substrate potential of the second P-MOSFET M₂, the source and the substrate potential of the first P-MOSFET M₁ is connected to the first voltage input terminal V 1N1 the gate and the drain of the second P-MOSFET is connected to the second voltage input terminal V 1N2 , and the connecting point between the gate and the drain of the P-MOSFET M₁ and the source and the substrate potential of the P-MOSFET M₂ is connected to the output terminal V OUT . Fig. 2 shows the currents flowing through the P-MOSFETs M₁ and M₂ in the circuit for producing a constant voltage relative to an output voltage at the output terminal V OUT . When the threshold voltages of the first and second P-MOSFETs M₁ and M₂ are V T1 and V T2 , and the input voltages V₁ and V₂ are applied to the input terminals V 1N1 and V 1N2 as explained before, no current flows through the first P-MOSFET M₁ when the output voltage ranges V₁-|V T1 | to V₁, and a current flows through the first P-MOSFET M₁ in reversely proportional to the output voltage when it is below V₁-|V T1 |, while no current flows through the second P-MOSFET M₂ when the output voltage ranges V₂ to V₂+|V T2 |, and a current flows through the second P-MOSFET M₂ in proportional to the output voltage when it is above V₂+|V T2 |. When the currents flowing through the P-MOSFETs M₁ and M₂ are equal to each other, the stabilized output voltage V s is obtained at the output terminal V out . The level of the stabilized output voltage V s is determined in accordance with the aforementioned equation (1). Here, it is assumed that the input voltage V₁ is 10 V, the input voltage V₂ is 5 V, the threshold voltages V T1 and V T2 are -1 V, and the ratio of the mutual transfer conductances g m1 and g m2 is 2/1. Thus, lines M₁ and M₂ indicating currents flowing through the first and second P-MOSFETs M₁ and M₂ relative to the output voltage at the output terminal V OUT are obtained as shown in Fig. 3, so that the stabilized output voltage V s is 8 V. In this situation, the lines M₁ and M₂ changes as shown in Fig. 4, where the threshold voltages V T1 and V T2 of the first and second P-MOSFETs M₁ and M₂ change from -1 V to -0.5 V, so that the stabilized output voltage V s changes from 8 V to 8.17 V. This is one of the aforemention disadvantages. Further, it is clearly understood from Fig. 2 that a range of the output voltage at the output terminal V OUT is narrow. This is the other disadvantage. These disadvantages are overcome in a circuit for producing a constant voltage according to the invention. Next, a circuit for producing a constant voltage in the first embodiment according to invention will be explained in conjunction with Figs. 5 and 6. In Fig. 5, there is shown the circuit for producing a constant voltage which comprises P-MOSFETs M₁₁, and M₁₂, M₁₃ and M₁₄. In the circuit, the P-MOSFETs M₁₁ and M₁₂ are connected in series between first and second voltage input terminals V 1N1 and V 1N2 , source and substrate potential of the P-MOSFET M₁₃ are connected to drain of the P-MOSFET M₁₁, gate and drain of the P-MOSFET M₁₃ are connected to gate of the P-MOSFET M₁₁, source and substrate potential of the P-MOSFET M₁₄ are connected to drain of the P-MOSFET M₁₂, gate and drain of the P-MOSFET M₁₄ are connected to gate of the P-MOSFET M₁₂, and a connecting point of the P-MOSFETs M₁₁ and M₁₂ is connected to an output terminal V OUT . In operation, input voltage V₁ and V₂ are applied to the first and second voltage input terminals V 1N1 and V 1N2 . Here, it is assumed that threshold voltages of the P-MOSFETs M₁₁, M₁₂, M₁₃ and M₁₄ are equal to each other to be "VTH". Thus, a gate voltage V G11 of the P-MOSFET M₁₁ is obtained in the presence of the P-MOSFET M₁₃ as follows. V G11 =V D11 - |V TH | where V D11 is a drain voltage of the P-MOSFET M₁₁. Then, a current flowing through the P-MOSFET M₁₁ is indicated by a line M₁₁ in Fig. 6, and is reversely proportional to the drain voltage V D11 equal to an output voltage at the output terminal V OUT , where the output voltage is below the first input voltage V₁. On the other hand, a gate voltage V G12 of the P-MOSFET M₁₂ is obtained in the presence of the P-MOSFET M₁₄ as follows. V G12 =V D12 -|V TH | where V D12 is a drain voltage of the P-MOSFET M₁₂. Then, a current flowing through the P-MOSFET M₁₂ is indicated by a line M₁₂ in Fig. 6, and is proportional to a source voltage equal to the output voltage, where the output voltage is above the second input voltage V₂. The stabilized output voltage V s is obtained from a crossing point of the lines M₁₁ and M₁₂, and is determined in accordance with the equation (4). V s =V₂+(V₁-V₂)x g m11 g m11 +g m12 where g m11 is a mutual transfer conductance of the P-MOSFET M₁₁ and g m12 is a mutual transfer conductance of the P-MOSFET M₁₂. As understood from the equation (4), the output voltage at the output terminal V OUT can be arbitrarily set, in the range between the voltages V₁ and V₂ applied to the first and second voltage input terminals V 1N1 and V 1N2 , in accordance with the setting of the mutual transfer conductances g m11 and g m12 . Even more, the output voltage does not change under the conditions that the threshold voltages of the P-MOSFETs M₁₁, M₁₂, M₁₃ and M₁₄ are equal to each other, even if the threshold voltages change. In Fig. 7, there is shown a circuit for producing a constant voltage in the second embodiment according to the invention, wherein like parts are indicated like reference symbols in the first embodiment. In the circuit, first and second P-MOSFETs M₁₁ and M₁₂ are connected in series between first and second voltage input terminals V 1N1 and V 1N2 , source and substrate potential of P-MOSFET M₁₃ are connected to drain of the P-MOSFET M₁₁, gate and drain of the P-MOSFET M₁₃ are connected to gate of the P-MOSFET M₁₁, source and substrate potential of P-MOSFET M₁₄ are connected to drain of the P-MOSFET M₁₂, and gate and drain of the P-MOSFET M₁₄ are connected to gate of the P-MOSFET M₁₂. In the circuit, further, drain of N type depletion MOSFET M₁₅ is connected to a connecting point between the gate of the P-MOSFET M₁₁ and the gate and the drain of the P-MOSFET M₁₃, gate and source of the N type depletion MOSFET M₁₅ are connected to a ground potential terminal V G1 connected to the ground potential, drain of N type depletion MOSFET M₁₆ is connected to a connecting point between the gate of the P-MOSFET M₁₂ and the gate and the drain of the P-MOSFET M₁₄, gate and source of the N type depletion MOSFET M₁₆ are connected to a ground potential terminal V G2 connected to the ground potential, and a connecting point between the first and second P-MOSFETs M₁₁ and M₁₂ is connected to an output terminal V OUT . In operation, the same characteristic of an output voltage as that in the first embodiment is obtained at the output terminal V OUT . Even more, minute currents flow from the connecting point between the gate of the P-MOSFET M₁₁ and the gate and the drain of the P-MOSFET M₁₃ and the connecting point between the gate of the P-MOSFET M₁₂ and the gate and the drain of the P-MOSFET M₁₄ through the N type depletion MOSFETs M₁₅ and M₁₆ to the ground potential terminals V G1 and V G2 , respectively, where the first and second voltages V₁ and V₂ applied to the input terminals V 1N1 and V 1N2 fluctuate, so that the gates of the P-MOSFETs M₁₁ and M₁₂ are under a floating state, thereby avoiding an operation instability of the circuit. In a circuit for producing a constant voltage according to the invention, as explained above, first and second MOSFETs each having one conduction type are connected in series between first and second voltage sources, and bias means is connected between gate and drain of each MOSFET, wherein the bias means produces a potential difference equal to a threshold voltage of each MOSFET, so that a wide range of an output voltage can be produced, and an output voltage characteristic is maintained to be constant, even if a threshold voltage changes in a semiconductor device fabricating process.

Claims: A circuit for producing a constant voltage (V out ) comprising, first and second MOSFETs (M₁₁, M₁₂) connected in series and being of one conduction type; bias means for each of said first and second MOSFETs; and first and second voltage sources (V₁, V₂) connected to said first and second MOSFETs, respectively; characterised in that the bias means (M₁₃, M₁₄) is connected between gate and drain of each of the first and second MOSFETs and includes third and fourth MOSFETs (M₁₃, M₁₄) each of said one conduction type, and serves to produce potential differences equal to the threshold voltages of the respective first and second MOSFETs, whereby a wide range of a stabilized output voltage is produced at a connecting point of said first and second MOSFETs. A circuit for producing a constant voltage according to Claim 1, characterised in that each of said first and second MOSFETs (M₁₁, M₁₂) is such that substrate potential is equal to source potential; the source and substrate potential of said third MOSFET (M₁₃) being connected to said drain of said first MOSFET (M₁₁), and drain and the gate of said third MOSFET being connected to said gate of said first MOSFET; and the source and substrate potential of said fourth MOSFET (M₁₄) being connected to said drain of said second MOSFET (M₁₂), and drain and the gate of said third MOSFET being connected to said gate of said second MOSFET. A circuit for producing a constant voltage according to Claim 2, characterised in that the circuit further comprises, means (M₁₅, M₁₆) for floating said gates of said first and second MOSFETs when first and second voltages of said first and second voltage sources fluctuate. A circuit for producing a constant voltage according to Claim 3, characterised in that said means for floating includes first (M₁₅) and second (M₁₆) N type depletion MOSFETs; the drain of said first N type depletion MOSFET being connected to a connecting point of said drain and said gate of said third MOSFET, and the source and the gate of said first N type depletion MOSFET being connected to the ground potential; and the drain of said second N type depletion MOSFET being connected to a connecting point of said drain and said gate of said fourth MOSFET, and the source and the gate of said second N type depletion MOSFET being connected to the ground potential.

Inventors: JINBO, TOSHIKATSU
Applicants: NEC CORPORATION
Application year: 1989
Main classification: G05F3
IPC codes: G05F1, H03F1, G05F3