In the design of analog and mixed digital/analog integrated circuits, voltage reference is one of the most important circuit modules. The cleverly designed bandgap voltage reference is widely used in a variety of integrated circuits, such as LDO and DC-DC integrated regulators, radio frequency circuits, high-precision A/D and D/A converters, etc., because it is almost independent of power supply voltage, process, and temperature changes. With the increasing complexity and precision of large-scale integrated circuits, higher requirements are also placed on the temperature stability of bandgap reference voltages. Traditional bandgap reference voltage sources can only generate a fixed voltage of approximately 1.2 V, which cannot meet the application in low-voltage occasions. The current-mode bandgap circuit uses a positive temperature coefficient current branch (PTAT) and a negative temperature coefficient current branch (CTAT) in parallel to generate a reference current that is independent of temperature. Then let this current generate a reference voltage on the resistor. The current-mode bandgap structure can obtain a reference voltage of any size. This paper proposes a new current-mode bandgap structure and uses the first-order temperature compensation technology to design a new BiCMOS bandgap reference circuit with good temperature characteristics, high power supply rejection ratio, and fast startup. The circuit structure is simple and meets the requirements of low output voltage.

　　1 Design of Bandgap Voltage Reference

　　1.1 Structure and principle of traditional current mode reference source

　　In the traditional current mode bandgap reference circuit, two shunt resistors of equal resistance are added to the two input terminals of the operational amplifier, and the output reference is obtained by the sum of the two currents flowing through the resistors. The circuit structure is shown in Figure 1. In Figure 1, the emitter area of ​​Q1 is N times that of Q2. Since the amplifier is in deep negative feedback, the voltages at points A and B are equal. The current flowing through R1 is I1, which is the PTAT current, and the current flowing through R2 is I2, which is the CTAT current, then:

　　By properly selecting the values ​​of R1, R2 and N, an output voltage Vref with zero temperature coefficient can be obtained. By changing R3, different reference voltages can be obtained.

　　1.2 Design of a Novel BiCMOS Bandgap Reference Circuit

　　The common current-mode bandgap circuit structure adds shunt resistors of equal resistance at both ends of the input of the operational amplifier. The output reference is obtained by passing the sum of two currents through the resistor to obtain a relatively small reference voltage. This structure of the reference circuit has the problem of the third degenerate state. The existence of the third degenerate state greatly limits the application of the current-mode reference circuit. This design uses a current-mode structure bandgap reference to obtain an output voltage of any size, and eliminates the problem of the third degenerate state through a special structure. By adding a trimming circuit to fine-tune the output voltage, the accuracy of the reference source is improved. The core circuit of the bandgap reference source is shown in Figure 2.

　　In Figure 2, each MOS tube has the same aspect ratio. Transistors Q1 and Q2 have the same emitter area, Q3 and Q4 have the same emitter area, and the emitter area ratio of Q1 to Q3 is 1:n. Rs and Rt are trimming resistors. Amplifiers AMP1 and AMP2 are in deep negative feedback. AMP1 makes the voltages at points a and b equal, while AMP2 makes the voltage VR2 equal to Vbe3. The currents through the M1, Q1, Q2 branch and the M2, Q3, Q4 branch are equal and are set to I1. The current through the M6, R2 branch is set to I2. The following expressions can be obtained:

　　Where: I1 has a positive temperature coefficient and I2 has a negative temperature coefficient. I2 and I2 are mirrored to M3 and M7 respectively and summed to obtain a reference current that does not change with temperature. This current generates a reference voltage Vref through R3, R4 and trimming resistors Rs and Rt. Due to the randomness of the IC process, the thin film resistor will have a (10% change, so this design uses an external trimming circuit to accurately control the output reference voltage. The output voltage can be fine-tuned by controlling the number of trimming resistors through laser trimming or digital circuits. As a general conclusion, consider the number of series resistors Rs as x and the number of parallel resistors Rt as y, and get:

　　From equation (6), we can know that by adjusting the value of R2/R1, the temperature coefficient of Vref is approximately zero. By increasing the number x of series resistors Rs, Vref is increased, while increasing the number y of parallel resistors Rt can reduce Vref.

　　The reverse input of AMP1 is connected in series with two (instead of one) forward diodes to ground, which reduces noise and also suppresses the influence of the amplifier's offset voltage on Vref. In order to further reduce the influence of the op amp offset on the reference voltage, a larger ratio of the emitter junction area of ​​Q1 and Q3 can be considered. In addition, due to the introduction of the trimming circuit, the output voltage Vref can be stabilized at 0.5 V.

　　1.3 Generation of secondary voltage

　　In order to improve the power supply rejection ratio, the main power supply voltage Vcc is not used directly for power supply, but the main power supply voltage Vcc is used to generate a secondary voltage Vcc1 for power supply (as shown in Figure 2), so as to improve the power supply rejection ratio of this new bandgap reference circuit. The circuit is shown in Figure 3.

　　In this circuit, AMP3 is in a deep negative feedback state. According to the operational amplifier virtual short principle, the function of capacitor C is to remove the influence of the AC component of the power supply voltage.

　　1.4 Circuit startup and degeneracy point analysis

　　Because the conventional current mode bandgap structure introduces a new current channel, each branch has two current channels, so there is a third possible degenerate state. The literature provides a solution to the third degenerate state, but its startup circuit is complex. This design implements the current mode structure without introducing an additional current path, so there are only two degenerate states: zero-point state and working state. Therefore, the required startup circuit is simple, and its structure is shown in Figure 4.

　　Point M in Figure 4 is connected to point M at the output end of AMP1 in the core circuit. When AMP1 outputs a high level, the PMOS in the core circuit cannot be turned on. At this time, the startup circuit turns on M10 through the action of the inverter, and the drain of M10 is connected to point a in the core circuit, so that M10 starts to charge point a, so that the circuit is out of the zero current state. After the circuit is turned on, point M outputs a low level to turn off M10, and the startup circuit is separated from the main circuit.
 

        1.5 Design of operational amplifier in circuit

　　The important performance indicators of the amplifier considered in this design are large open-loop DC gain and high power supply rejection ratio. The op amp structure is shown in Figure 5, which adopts a two-stage amplification structure: the first stage is a double-ended input and single-ended output folded cascode structure with a cascode PMOS as the load; the second stage is a common source amplifier (capacitors are used between the two stages for compensation). Such a structure provides a sufficiently high DC gain, and the application of the cascode load not only improves the open-loop DC gain but also increases the power supply rejection ratio.

　　2 Bandgap reference circuit simulation results

　　The circuit uses the process model library of Xfab 0.35μm BiCMOS and is simulated by Cadence Specte simulator. When the power supply voltage is 3.3 V, Figures 6 and 7 are the temperature dependence and power supply rejection ratio (PSRR) curves respectively. The results show that the bandgap reference outputs a stable voltage of 0.5 V, and the temperature drift is 15 ppm in the temperature range of -40 to +125°C. The circuit exhibits good temperature characteristics. At the same time, the power supply rejection ratio of the reference voltage source can reach -103 dB at low frequency, and the power supply rejection ratio is less than -100 dB before 40 kHz. Figure 8 is the output voltage of the circuit at different operating voltages. It can be seen that the normal startup voltage of the circuit is 2 V, and the change of the reference voltage after the circuit is started is less than 0.06 mV.

　　3 Conclusion

　　As an important module in analog circuits, bandgap reference voltage circuit has a significant impact on the A/D acquisition accuracy and the performance of power management chips. This paper designs a high-precision, high power supply rejection ratio, low-voltage bandgap reference circuit, and realizes external adjustment of the reference voltage. The results show that the circuit can provide a stable 0.5 V reference voltage output at a power supply voltage of 3.3 V, -40 ~ +125 ℃, a temperature drift of 15 ppm, and a power supply rejection ratio of -103 dB at low frequency, meeting the design requirements.