All MOS ransistors Bandgap Reference Using Chopper Stabilization echniue H. D. Roh J. Roh DUANQUANZHEN Q. Z. Duan Abstract A 0.6-, 8-μW bandgap reference without BJs is realized in the standard CMOS 0.13μm technology. All MOS transistors bandgap reference circuit with very low supply voltage 0.6 is designed. he chopper stabilization techniue is used to improve the accuracy of the bandgap reference voltage. he measurement results have confirmed that the chopper stabilization techniue reduces bandgap voltage error from 100m to 30m comparing to the one without chopper stabilization techniue. Keywords- Bandgap reference; chopper stabilization technology; all MOS transistors I. INRODUCION Nowadays, with the increased accuracy of the circuits, bandgap reference becomes a much more important block in many applications. For the bandgap reference circuit, on one hand, the accuracy of the bandgap reference becomes more and more important. For example, the bandgap reference works as the common-mode voltage in ADC. It plays an important role and greatly improves the performance of ADC. On the other hand, the demand for low voltage operation becomes especially apparent. For example, the bandgap reference works in batteryoperated mobile products. So the high precision bandgap reference circuit with low supply voltage is expected to appear. In CMOS technology, the conventional bandgap reference circuit usually uses the parasitic bipolar junction transistors (BJs) or diodes to get the high precision bandgap voltage [1], [2], [3]. However, it is impossible to realize, when the bandgap reference circuit works with 0.6 of supply voltage, since base-emitter voltage of BJ and two terminals' voltage of diodes are bigger than 0.6. Besides, the bandgap voltage usually is about 1.25 [4], [5], it greatly exceeds 0.6. However, this paper designed all MOS transistors bandgap reference circuit that works with very low supply voltage level. What's more, we know that it's very difficult to get the high-precision bandgap voltage with low supply voltage. So a lot of methods are proposed to improve the performance of the bandgap reference circuit, such as second-order temperature compensation [6] and resistors temperature compensation [1]. o get the high-precision, the more significant matching of current mirrors becomes necessary. So long channel transistors [1] and cascode current mirror [7] are used to get higher precision bandgap reference. However, the offset noise brought by the OA (operational amplifier) greatly reduces the performance of the bandgap reference [7]. hus, this paper proposed to use the chopper stabilization techniue to reduce these errors. In brief, a 0.6, 8μW all MOS transistors bandgap reference circuit with low supply voltage is presented in this paper, where the chopper stabilization techniue is proposed to reduce the offset errors of amplifier. his paper analyzed the conventional bandgap reference circuit in section II. he proposed bandgap reference circuit is introduced in section III. he measurement and conclusion are presented in section I and section, respectively. II. CONENIONAL BANDGAP REFERENCE he bandgap reference circuit is widely used in low voltage applications [4]. In this circuit, P1, P2 and P3 have the same size and the resistances of and are the same. herefore, the bandgap reference voltage is expressed as follows [4]: ( ) ref f 1 f ( ln( N)) f 1 is proportional to the absolute temperature (PA), thus the bandgap reference voltage ref is given by k ( ln( N)) ref f 1 (1) (2) his work was partially supported by Mid-career Researcher Program through NRF grant funded by the MES (No.2010-0000203). CAD tool was supported by IDEC. 978-1-4244-8631-1/10/$26.00 2010 IEEE - 353 -
Where k is Boltzmann s constant, is electronic charge, is absolute temperature and N is the number ratio of the diodes. Where >1 is a nonideality factor and k. When Bandgap reference oltage buffer OA with chopper switches Bandgap core Unit gain buffer DD DD M1 2 M2 M3 M6 b1 M5 M1 M2 M3 M29 M30 M4 M27 M28 I1 - + A1 + - + 1 OS1 _ I2 I3 REF1 +OS2- + - A2 - + M4 REF M15 M7 M8 M9 M10 P M16 OU I1 N P I2 I3 M25 M26 R5 REF IX1 IX2 IX3 IX4 + GS2 - na P N A + GS1 - Offchip capacitor R5 bg1 R6 bg2 M17 M11 M12 M13 M14 N M18 IX1 IX2 IX3 IX4 D1 D2 M23 M24 M31 M21 M22 b2 M20 R6 M32 bg2 bg1 (a) Figure 1. Proposed bandgap reference. (a) Structure. (b) Schematic. (b) he term k ln( N) is apparently proportional to the absolute temperature (PA), which is used to compensate for the negative temperature coefficient of f1.herefore, choosing proper, and N, we can get the term 1 which is small independence on the temperature R k ( f 1 ln( N)) and about 1.25. hen can be freely chosen, thus the bandgap voltage REF can be freely changed from 1.25 [4]. III. PROPOSED BANDGAP REFERENCE A. Realization of the low supply voltage Figure 1 shows the proposed bandgap reference circuit. he proposed bandgap reference circuit structure and schematic are illustrated in Figure 1. (a) and Figure 1. (b), respectively, where M1, M2 and M3 have the same size, and and are the same. As the base-emitter voltage of BJs and the voltage of diodes are bigger than 0.6, designing a bandgap reference circuit with a supply voltage of 0.6 in traditional bandgap reference circuit becomes impossible. However, MOSFEs working in the weak inversion region have the similar characteristics to BJs and diodes, since the effect of diffusion current becomes more significant than that of drift current [8]. In this case, GS is much smaller than 0.6. hus, in this paper MOSFEs are used to substitute BJs and diodes. MOSFEs current characteristics in the weak inversion region can be expressed as follows [8]: GH DS I I e (1 e ) D 0 D (3) MOSFEs work in the weak inversion region, since DS is much larger than DS, the term e approximately is 0. hen (3) can be written as D D0 G H I I e (4) As shown in Figure 1, we assume that the gain of OA in bandgap reference circuit A1 is infinite, so P and N are approximately same. From the Figure 1. (a), we can see that N euals to GS1. hus, P and N can be expressed as follows I GS 2 X 2 GS1 herefore, I X2 can be written as 1 I I ln [ - ( D1 D02 )] X 2 H1 H2 I I D01 D1 Since I D1 and I D2 are approximately same, the current I D0 varies with the scale of width and length W/L, then (6) can be transformed as: 1 k W2 L2 I [ - ln( )] X 2 H1 H2 W 1 L 1 hen I X1 is expressed by I (5) (6) (7) - 354 -
I GS1 (8) X 1 As and are the same, I X1 and I X4 are the same and I X2 and I X3 are the same. I1 is the sum of I X1 and I X4, and I1 is mirrored to I3. 1 3 X1 X 2 I I I I (9) I I GS 2 X 2 GS1 OS1 (12) Similar to (10), the bandgap reference voltage REF1 is expressed as follows: k W2 L2 { [ - ln( )]} (13) REF1 GS1 H1 H 2 OS1 W 1 L 1 herefore, the bandgap reference voltage REF1 becomes: k W2 L2 { [ - ln( )]} REF1 GS1 H1 H 2 W 1 L 1 (10) So the the term k W2 L2 ln( ) is used to compensate for the W1 L1 negative temperature coefficient of GS1. Properly choosing,, W and L, the bandgap reference voltage with small temperature coefficient can be realized. herefore, the MOSFEs working in inversion region are proposed to take the place of BJs and diodes in this paper, the bandgap reference circuit with very low supply voltage of 0.6 is designed, as shown in Figure 1. Besides, in order to prevent reverse current flowing into I3, a unit gain buffer realized by OA is added to the badngap reference circuit. herefore, REF is eual to REF1. he unit gain buffer with resistors works as a regulator, and through using different resistance of R5, R6 and, different bandgap voltages bg1 and bg2 are obtained, which are expressed as follows: bg1 bg 2 R6 R5 R6 R5 R6 REF REF (11) herefore, the different bandgap reference voltages can be obtained at the same time, and they can be used as supply voltages in different circuit blocks or common-mode inputs in ADC and so on. What s more, as shown in Figure 1, the offchip capacitors are added to the bandgap voltage bg1 and bg2, which are used to protect the bandgap reference voltages. B. Reduce the errors using chopper stabilization techniue Since the ideal OA does not exist in reality, in the other words, the input of the bandgap reference amplifier always has offset noise caused by process errors, which is one of main factors reducing the performance of the bandgap reference circuit [7], especially for the bandgap reference circuit working with low supply voltage, as shown in Figure 1. Furthermore, the OA working with low supply voltage has much bigger offset noise comparing to the OA with high supply voltage. herefore, the chopper stabilization techniue is proposed to reduce these errors in this paper, as shown in Figure 1. (b). We assume that the amplifier s offset error is OS1, then (5) can be written as: Figure 2. Analysis chopper stabilization Since the OA in the unit gain buffer also is not ideal. It brings the offset errors to the bandgap reference circuit, too. So if we assume that the offset noise is OS2, the bandgap reference voltage REF can be written as: { [ - REF GS1 H1 H 2 OS1 k W2 L2 ln( )]} W 1 L 1 OS 2 (14) From (14), we can find that the offset noises OS1 and OS2 are directly delivered to bandgap reference voltage, which will greatly affect the precision of the bandgap reference voltage. How to diminish the offset noise becomes necessary. herefore, in this paper, the chopper stabilization techniue is proposed to minimize the offset noise caused by process errors [9], [10]. As illustrated in Figure 1, the chopper stabilization techniue is applied to each of operational amplifier. Figure 2 shows the analysis of the chopper stabilization techniue. he input signal ( P - N ) is switched prior to being fed into the operational amplifier, so the input signal ( P - N ) is modulated to the chopping freuency (high freuency) through chopper switches controlled by chopping clock signal, as shown in Figure 2. (a). As the signal is introduced into the operational amplifier, the offset noise of operational amplifier is added to it. As shown in Figure 2. (b), we get 1 which is the mixture containing the signal in high-freuency and the offset noise in the lowfreuency, since the offset noise isn t chopped by the chopper switches. hen 1 is amplified by the operational amplifier. he output signal of operational amplifier is chopped by the - 355 -
chopper switches at the output stage which demodulates the input signal to baseband (low freuency) and modulates the offset noise to the high freuency. So we get the output mixture 2 contains the signal in low freuency and the noise in high freuency, as shown in Figure 2. (c). he noise in the high freuency can be filtered by a low pass filter implemented with RC, and then the only signal is left, as shown in Figure 2. (d). In this paper, ceramic capacitors are used for the low-pass filter. As chopper switches are added to both the OA of bandgap reference circuit and the OA in the unity buffer, the chopper techniue successfully reduces the offset errors brought by these two operational amplifiers, and much more significant bandgap reference voltage is obtained using this method. I. MESUREMEN Figure 3 shows the microphotograph of the bandgap reference circuit. Figure 4 which is obtained from 21-samples shows the measurement result of the bandgap reference with and without chopper switches. he result shows that the variation of the bandgap reference using chopper switches is within 30m, while the variation of the bandgap reference voltage without chopper switches is about 100m. From the measurement result, we can get that the much more significant bandgap reference voltage is obtained by using the chopper stabilization techniue. he chopper stabilization techniue greatly reduces the offset errors of the amplifier in the bandgap circuit. Besides, the target bandgap voltage is 300m, however, the measurement voltage is about 275m, which is estimated to be an error caused by the inaccurate control of the resistance due to the absence of a separate trimming circuit. Since resistors usually have very big variation, the variation is about 10% to 20%. However, the separate trimming circuit usually increases the silicon area and we also need spend much more time to test the circuit, which will make the cost much higher. Figure 3. Die phtograph. CONCLUSIONS A low-voltage, low power all MOS transistors bandgap reference circuit in the standard 0.13μm CMOS process is presented in this paper. MOSFEs which work in weak inversion region are used to exchange the BJs and diodes. As a result, the low supply voltage 0.6 bandgap reference circuit with all MOS transistors appears in this paper. What s more, in this paper the chopper stabilization techniue is proposed to minimize the errors brought by the OAs of both bandgap reference circuit and unity buffer, thereby a good performance bandgap reference circuit arises. he experimental results have proved that the chopper stabilization techniue greatly reduce the errors comparing to the bangap reference circuit without chopper switches. Figure 4. Output voltage of bandgap reference vesus samples In short, a 0.6, 8μW and all the MOS transistors bandgap reference circuit has been presented in this paper. MOSFEs which work in weak inversion region are used to instead of BJs and diodes to realize low supply voltage bandgap reference circuit, and the chopper stabilization techniue is adopted to reduce the errors brought by operational amplifiers, as shown in Figure 1. Ideally the threshold voltage H1 and H2 are the same, however, H1 and H2 are processdependent constants. So the threshold voltages will bring some errors. Since the effect on H is placed prior to the chopper switches, it s difficult to remove the effect of H using chopper stabilization techniue. Accordingly, the variation of the threshold voltage affects the accuracy of the bandgap reference voltage. In addition, due to the mismatch among M1, M2 and M3, the bandgap reference circuit also suffers from some errors, which can be reduced using a good layout method. 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