Cascode CMOS Bandgap Reference

1. Field of the Invention

The present invention relates to a CMOS bandgap voltage reference and, more particularly, to a CMOS bandgap voltage reference which utilizes cascoded MOS devices to provide increased temperature stability of the bandgap voltage reference.

2. Description of the Prior Art

The bandgap voltage reference, since introduced by Widlar, has become widely used as a means for providing a reference voltage in bipolar integrated circuits. In general, the bandgap reference relies on the principle that the base to emitter voltage, V.sub.BE, of a bipolar transistor will exhibit a negative temperature coefficient, while the difference of base to emitter voltages, .DELTA.V.sub.BE, of two bipolar transistors will exhibit a positive temperature coefficient. Therefore, a circuit capable of summing these two voltages will provide a relatively temperature independent voltage reference. One such circuit arrangement is disclosed in U.S. Pat. No. 4,429,122 issued to R. J. Widlar on Feb. 3, 1981. In CMOS technology, the basic Widlar arrangement may be directly applied, since bipolar devices may be created using standard CMOS processes. However, the bipolar devices available in CMOS are not as stable as those directly developed in bipolar technology, and additional control requirements are needed to provide a relatively temperature stable bandgap reference. U.S. Pat. No. 4,287,439 issued to H. Leuschner on Sept. 1, 1981, discloses one exemplary CMOS bandgap arrangement. Here, the circuit utilizes two substrate bipolar transistors with the emitter of one being larger than the other. The transistors are connected in an emitter follower arrangement with resistors in their respective emitter circuits from which a voltage is obtained to generator the bandgap reference. A later arrangement, disclosed in U.S. Pat. No. 4,380,706 issued to R. S. Wrathall on Apr. 19, 1983, relates to an improvement of on the Leuschner circuit wherein an additional transistor is inserted between the output of the amplifying stage and the substrate bipolar transistors to provide an output voltage of twice the bandgap voltage.

There exist many factors which affect the performance of these and other CMOS bandgap references. One factor not addressed by these prior art arrangements is the temperature dependence of the resistors used in association with the substrate bipolar transistors to provide the needed ratio between the emitter currents. Therefore, true temperature stability cannot be achieved without addressing this problem. One solution is disclosed in U.S. Pat. No. 4,375,595 issued R. W. Ulmer et al on Mar. 1, 1983. In the Ulmer et al arrangement, switch capacitors are used at the inputs associated with V.sub.BE and .DELTA..sub.VBE to sample both voltages. Proper selection of the capacitor ratio provides a weighted sum of both voltages to the amplifier inputs which will be substantially independent of temperature. This particular solution to the resistance-related temperature coefficient problem, however, requires an external clock source and relies on the proper selection of the capacitor values used. The need remains, therefore, for a CMOS bandgap reference which provides increased temperature stability in relation to the resistor-based temperature coefficient which is relatively easy to implement and does not require external circuitry.

The problem remaining in the prior art has been solved in accordance with the present invention which relates to a CMOS bandgap voltage reference and, more particularly, to a CMOS bandgap reference which utilizes cascoded MOS devices to provide increased temperature stability of the bandgap reference as related to the temperature coefficient of the resistors used in the reference circuit.

It is an aspect of the present invention to utilize cascoded MOS devices disposed between the substrate bipolar resistors and a power supply to such augment the value of the bandgap current to a level where only relatively small resistors are needed to provide the desired bandgap voltage level. Since p+ diffusion resistors have a better temperature coefficient than the larger P tub resistors, the associated temperature stability is significantly reduced over prior art arrangements.

Another aspect of the present invention is to provide a constant current source at a minimal increase (the addition of one MOS transistor) in circuit complexity.

A further aspect of the present invention relates to providing a bandgap reference which can operate at lower supply voltages by correctly sizing the transistors used to form the cascode arrangement.

Other and further aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.

Referring now to the drawings, where like numerals represent like parts in several views:

FIG. 1 illustrates a basic prior art CMOS bandgap voltage reference;

FIG. 2 illustrates an exemplary CMOS bandgap voltage reference formed in accordance with the present invention; and

FIG. 3 illustrates an alternative CMOS bandgap voltage reference formed in accordance with the present invention which can operate at lower supply voltages than the arrangement illustrated in FIG. 2.

Bandgap voltage references are frequently used in many integrated circuits. As CMOS technology becomes more and more prevalent, the need for a bandgap reference which can be formed using CMOS processes has become essential. A exemplary prior art CMOS bandgap reference 10 is illustrated in FIG. 1. A pair of bipolar transistors 12 and 14 are npn substrate transistors, where both collectors are coupled together and connected to a first power supply, denoted VDD in FIG. 1. In formation, the n-type substrate itself is defined as the collector regions, a p-type well formed in the substrate defines the base regions of transistors 12 and 14, and n-type diffusions in the p-type well form the emitters of transistors 12 and 14. It is to be noted that transistors 12 and 14 could also be pnp transistors, which would thus utilize a p-type substrate and diffusions and an n-type well. A complete description of this formation process can be found in the article "Precision Curvature-Compensated CMOS Bandgap Reference", by B. Song et al appearing in IEEE Journal of Solid-State Circuits, Vol. SC-18, No. 6, December 1983 at pp. 634-43. The base to emitter voltage of transistor 12, denoted V.sub.BE12, is applied as a first, positive input to an operational amplifier 16. The detailed internal structure of operational amplifier 16 has not been shown for the sake of simplicity, since there exist many different CMOS circuits capable of performing the difference function of operational amplifier 16. A resistor 18 is connected between the emitter of transistor 12 and the output of operational amplifier 16. A resistor divider network comprising a pair of resistors 20 and 22 is connected between the emitter of transistor 14 and the output of amplifier 16, where the interconnection of resistors 20 and 22 is applied as a second, negative input to operational amplifier 16, as shown in FIG. 1. The bandgap voltage reference, V.sub.BG, measured across the terminals as shown, can be represented by the equation ##EQU1## where V.sub.T is the thermal voltage kT/q, I.sub.s12 is the saturation current of transistor 12 and I.sub.s14 is the saturation current of transistor 14. In order to provide a temperature coefficient which will be substantially equal to zero, large-valued resistors (on the order of 100k) are needed to keep the bandgap current (I.sub.12 +I.sub.14) at a reasonable level while still providing a substantially zero temperature coefficient. In MOS technology, the actual p-type tub is used to form resistors of such large magnitude, where a problem with this lies in the fact that p-tub resistors are well known in the art to exhibit a very large temperature coefficient. Therefore, the temperature coefficient of p-tub resistors 18, 20, and 22 will significantly degrade the temperature coefficient of bandgap voltage reference 10.

FIG. 2 illustrates a cascode bandgap voltage reference 30 formed in accordance with the present invention which overcome the problem related to the temperature coefficient of the p-tub resistors. As shown, resistors 18 and 20 of FIG. 1 are replaced with resistors 32 and 34, respectively, where resistors 32 and 34 are on the order of 15-20k, instead of 100k as was the case for the prior art arrangement. Therefore, resistors 32 and 34 may be formed from small p+ diffusions, which due to their decreased resistivity, exhibit a temperature coefficient which is significantly less than that associated with p-tub resistors. To compensate for the decreased resistor size, the present invention utilizes a cascode MOS circuit 36 connected as shown in FIG. 2, where the individual transistors forming circuit 36 are sized to provide the required level for the bandgap voltage. In particular, circuit 36 includes a pair of MOS transistors 40 and 42 connected in series between resistor 32 and VSS, where the drain of transistor 40 is connected to resistor 32, the source of transistor 40 is connected to the drain of transistor 42, and the gate of transistor 40 is coupled to the output of operational amplifier 16. The gate of transistor 42 is coupled to its drain, and the source of transistor 42 is connected to VSS. Circuit 36 further includes a pair of MOS transistors 44 and 46 connected in a like manner between resistor 34 and VSS, where the gate of transistor 44 is connected to the gate of transistor 40 and the gate of transistor 46 is connected to the gate of transistor 42. As shown in FIG. 2, transistors 44 and 46 are formed to have a width-to-length (Z/L) ratio n times greater than that of transistors 40 and 42. As shown below, the n factor provides the compensation for the decrease in resistor size as compared with prior art arrangements. In particular, the bandgap voltage, V.sub.BG, of circuit 30 can be defined by the following equation ##EQU2## Comparing equations (1) and (2), it can be seen that utilizing a bandgap reference circuit formed in accordance with the present invention results in substituting the factor n(R.sub.34 /R.sub.32) the prior art factor R.sub.22 /R.sub.20. Therefore, if, n=10, the value of the needed resistors may be decreased from approximately 100K to approximately 10K, thus allowing low temperature coefficient p+ diffusion resistors to be utilized in place of high temperature coefficient p-tub resistors.

An added advantage of utilizing the cascode MOS arrangement of the present invention is that a constant current source may also be realized from merely adding one additional transistor to the above-described circuit. As shown in FIG. 2, an MOS transistor 50 may be included where the gate of transistor 50 is connected to the gates of transistors 42 and 46, and the source of transistor 50 is connected to VSS. Transistor 50, as shown, comprises a Z/L ratio m times larger than transistors 40 and 42. The current flowing through transistor 50, denoted I.sub.BIAS, is defined by the following expression ##EQU3## An additional advantage of the present invention arises from the fact that the output of operational amplifier 16 does not have to sink the bandgap current, as does the prior art arrangement of FIG. 1. Instead, the output of operational amplifier 16, as stated above is coupled to cascode circuit 36 at the gate terminals of transistors 40 and 44.

The minimum range between supply voltages VDD and VSS for the circuit of FIG. 2 can be expressed as

where V.sub.TH(n) is defined as the threshold voltage for transistors 44 and 46 and V.sub.ON is also associated with transistors 44 and 46. In order to operate at lower supply voltages, a ratioed cascode current mirror, included in the circuit illustrated in FIG. 3, may be utilized to eliminate the V.sub.TH(n) term from equation (3). As shown, a current mirror formed from a pair of MOS transistors 62 and 64 supply a like current I' to the drain terminals of a pair of transistors 66 and 68, respectively. Transistor 66 is connected between transistor 62 and VSS, where the gate of transistor 66 is connected to the gates of transistors 42 and 46. The gate to source voltage, V.sub.GS, of transistor is equal to the quantity V.sub.TH(n) +V.sub.ON. In order to eliminate the V.sub.TH(n) component, transistor 68, as shown in FIG. 3, is chosen to comprise a Z/L ratio which is one-fourth that of transistors 40 and 42. Therefore, it follows that V.sub.GS of transistor 68 is equal to the quantity V.sub.TH(n) +2V.sub.ON. Since the drain to source voltage, V.sub.DS, for both transistors 44 and 46 has been altered to equal V.sub.ON, the minimum voltage difference between VDD and VSS can be expressed as ##EQU4##