Stable reference voltage generator circuit

Abstract: A circuit for generating a stable reference voltage (Vref) as temperature and process parameters vary, comprising at least one field-effect transistor (M1) and an associated resistive bias element (R) connected in series between a supply voltage (Vcc) and ground (GND), further comprises a second field-effect transistor (M2) connected to the first transistor such that the reference voltage (Vref) can be picked up as the difference between the respective threshold voltages of the two transistors. This provides a reference voltage which is uniquely stable against variations in temperature and process parameters.

Description: Field of the Invention This invention relates to a circuit for generating a stable reference voltage. In particular, the invention relates to a circuit capable of providing a reference voltage which is compensated for temperature and process parameters, and is highly stable with respect to the value of a supply voltage. Background Art As is known, many types of electronic circuits require a reference voltage Vref which be stable over time. Several solutions have been proposed to derive, for example, such a reference voltage Vref from the supply voltage Vcc to the electronic circuit. The simplest way of achieving this is, for example, to provide a resistive partition of the supply Vcc. In other words, it might suffice that a resistive divider be connected between a supply voltage pole and ground, with the reference voltage being picked up from a resistor linking node. But this solution is not devoid of serious problems: integrated circuit resistors are made to wide manufacturing tolerances, which disallows their values to be known with any accuracy; this may result in a reference voltage being obtained which varies from the target voltage; and the integration of the resistors is disadvantageous from the standpoint of circuit area occupation, which reflects unfavorably on integrations costs. In addition, the reference voltage may be affected by thermal drift from the circuit operating temperature and/or interferences with the supply voltage. An improved resistive divider can be implemented using a transistor-type of divider as shown in Figure 1 herewith. A series of three MOS transistors can provide, for example, a reference voltage which is unaffected by temperature. The last-mentioned solution would, however, have a drawback in that it produces a reference voltage which is closely dependent on the supply voltage Vcc. Furthermore, the latter voltage cannot amount to anything less than three times the threshold voltage of the MOS transistors, which rules out the use of the circuit with low voltages. A voltage regulator is known from the Patent Abstract of Japan No. JP-A-60 252 923 (HITACHI K.K.). Such a regulator includes insulation type field effect transistors (IGFETs), connected in series in order to produce an output reference voltage, which is the difference between the Fermi voltages of the IGFETs. Also known from the Patent Abstract of Japan No. JP-A-60 243 717 (HITACHI K.K.) is a semiconductor integrated circuit device, which includes two MOS transistors and obtains a reference voltage as difference between their drain voltages, i.e. their Fermi levels, such difference being approximately equal to the difference between the threshold voltages of the MOS transistors. Finally, in the article entitled "MOS Voltage Reference Based on Polysilicon Gate Work Function", to OGUEY et al., published on IEEE Journal of Solid-State Circuits in June 1980, it is described a positive reference voltage source, comprising two NMOS transistors supplied with two current sources, the output voltage being the difference in threshold voltages of the transistors, only when the two current generator provide the same current value. All these known circuits requires complex current generators for a correct operation. Further prior approaches can only provide a stable reference voltage at the expense of increased circuit complexity. And even so, the reference voltage cannot be set in an accurate way. The underlying technical problem of this invention is, therefore, to provide a circuit arrangement which is uniquely simple and ensures an accurate and constant reference voltage as temperature and process parameter vary, while being quite stable with respect to the voltage supply. Summary of the Invention The solutive idea on which the invention stands is one of using a first, natural p-channel MOS transistor associated with a second, n-channel Mos transistor which is also a natural one; the reference voltage is obtained as the difference between the threshold voltages VT of these two transistors. Based on this solutive idea, the technical problem is solved by a circuit as defined in the characterizing parts of the appended claims. The features and advantages of a circuit according to the invention will be apparent from the following description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings. Brief Description of the Drawings In the drawings: Figure 1 is a diagram showing schematically a reference voltage generating circuit according to the prior art; and Figure 2 is a diagram showing the circuit of this invention. Detailed Description With reference to the drawing Figures, generally indicated at 1 is an electronic circuit for generating a stable reference voltage, which can function as an input of a comparator 2. The circuit 1 allows a reference voltage, denoted by Vref, to be obtained from a voltage supply Vcc. More particularly, the circuit 1 is connected between the voltage supply Vc and a ground GDN, and comprises a bias resistor R, a first transistor M1, and a second transistor M2. The resistor R may be replaced with a bias MOS transistor of the p-channel type having its gate electrode grounded; this being a preferable circuit embodiment with integrated circuits. The transistors M1 and M2 are field-effect transistors of the MOS type. Each of them has a first or drain terminal D, a second or source terminal S, and a control gate terminal G. The first transistor M1 is a natural p-channel MOS, and the second transistor M2 is a natural n-channel MOS. Transistors of the so-called "natural" type have an advantage in that their threshold voltages are related in an analogous manner to temperature and/or process parameters. Accordingly, the difference between their threshold voltages will be kept constant as such parameters vary. In addition, both transistors M1 and M2 are connected in the circuit 1 in a diode configuration, that is with their respective gate and drain terminals connected together. Specifically, the gate terminal G1 of transistor M1 is shorted to the drain terminal D1, while the gate terminal G2 of the second transistor M2 is shorted to the drain terminal D2. The first transistor M1 has its source terminal S1 connected to the bias resistor R and its drain terminal D1 connected to ground at GDN. The other end of the bias resistor R is connected to the voltage supply Vcc. The source terminal S1 is in common with the drain terminal D2 of the second transistor M2. The other source terminal S2, of transistor M2, is the point whence the desired reference voltage Vref is picked up. With this arrangement, the voltage at the source terminal S2 of transistor M2 is equal to the difference between the threshold voltage VT(p-ch nat) of transistor M1 and the threshold voltage VT(n-ch nat) of transistor M2. Assuming, for example, the threshold voltage of a natural p-channel transistor to be about 1.7 V (VT(p-ch nat)=1.7V), and the threshold voltage of a natural n-channel transistor to be about 0.6 V (VT(n-ch nat)=0.6V), then the value of the reference voltage Vref (given as Vref=VT(p-ch nat)-VT(n-ch nat)) would be approximately 1.1 V. Temperature and process parameter variations would change the threshold voltages of the transistors in the same direction (to increase or decrease them), and cancel out when their difference is taken. The resultant reference voltage will, therefore, be unaffected by temperature and process parameters. A reference voltage obtained by simulation within a broad range of temperatures (-40°C to +150°C) has revealed a Gaussian distribution centered on the desired value of 1.1 V and very little scattered around it, which was the objective of the invention and obviates the problems of conventional circuits. The circuit arrangement of this invention is very simple, but quite effective.

Claims: A circuit for generating a stable reference voltage (Vref) as temperature and process parameters vary, comprising at least one field-effect transistor (M1) and an associated resistive bias element (R) connected in series between a supply voltage (Vcc) and ground (GND), and a second field-effect transistor (M2) having at least one terminal in common with the first transistor (M1), said common terminals being the source (S1) of the first transistor and the drain (D2) of the second transistor, respectively, whereby a highly stable reference voltage (Vref) can be picked up as the difference between the respective threshold voltages of the field-effect transistors, characterized in that said first and second field-effect transistors (M1,M2) are natural MOS transistors of opposite conductivity types. A circuit according to Claim 1, characterized in that the first (M1) of said transistors is a natural p-channel MOS. A circuit according to Claim 1, characterized in that the second (M2) of said transistors is a natural n-channel MOS. A circuit according to Claim 1, characterized in that both said transistors (M1, M2) are connected in the circuit in a diode configuration with their respective gate (G1,G2) and drain (D1,D2) terminals connected together. A circuit according to Claim 1, characterized in that the second transistor (M2) has its drain terminal (D2) connected to the resistive element (R) and its source terminal (S2) available for picking up the reference voltage (Vref).

Inventors: GOLLA CARLA, PADOAN SILVIA, GOLLA, CARLA, PADOAN, SILVIA
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1993
Main classification: G05F3
IPC codes: G05F3, H01L21, H01L27