Research on a Low-Temperature Drift CMOS Bandgap Voltage Reference

In recent years, due to the rapid development of integrated circuits, reference voltage sources have been widely used in analog integrated circuits, mixed analog and digital circuits, and system integrated chips (SOCs). They also play a vital role in the application and development of high-tech analog electronic technology. Their accuracy and stability will directly affect the performance of the entire system. Therefore, designing a good reference source is of great practical significance.

1 Basic Principles of Bandgap Reference Circuit

The purpose of a bandgap reference voltage source is to generate a quantity that remains constant with temperature changes. Since the temperature coefficient of the base voltage VBE of a bipolar transistor is approximately -2.2 mV/K at room temperature (300 K), and the base-emitter voltage difference VT of two bipolar transistors with different current densities has a temperature coefficient of +0.086 mV/K at room temperature, and since the voltage temperature coefficients of VT and VBE are opposite, they are multiplied by a suitable coefficient and then weighted with the former, thereby offsetting the temperature drift characteristics of VBE within a certain range and obtaining an output voltage VREF with approximately zero temperature drift. This is the basic design idea of ​​a bandgap voltage source.

1.1 Bandgap reference voltage source core circuit

The core structure of the circuit proposed in this paper is shown in Figure 1. In the circuit, the bipolar transistor constitutes the core of the circuit, realizing the linear superposition of VBE and VT, and obtaining an output voltage with an approximate zero temperature coefficient. In Figure 1, the emitter area of ​​the bipolar transistors Q1 and Q2 is the same, and the emitter area of ​​Q3 and Q4 is the same. Considering the design requirements, the emitter area of ​​Q1 and Q2 is 8 times the emitter area of ​​Q3 and Q4.

Assuming that the base current of the bipolar transistor is zero and the gain of the op amp is large enough, the voltages at points a and b are equal, that is:

In the actual circuit, it is calculated that when R3/R1=2.3066, an output reference voltage with approximately zero temperature coefficient at room temperature can be obtained.

1.2 Overall circuit of bandgap reference voltage source

The overall circuit of the bandgap reference voltage source consists of four parts: Part A is the startup circuit, Part B provides the bias voltage, Part C is the operational amplifier, and Part D is the core part of the bandgap voltage source. The core part is composed of bipolar transistors, which realizes the linear superposition of VBE and VT and obtains an output voltage with an approximate zero temperature coefficient. The overall circuit is shown in Figure 2.

1.3 Impact of Op Amp Offset on Reference Source

The design of the op amp in the reference source is very important. The offset of the op amp is a major error source of the reference source. Due to asymmetry, the op amp will be affected by input offset. Assuming the offset voltage is Vos, the output formula including the offset voltage is calculated as:

It can be seen that the size of Vos may cause a considerable error in the output voltage of the reference source. In addition, Vos itself is a function of temperature. Compared with the ideal operational amplifier, it will introduce a certain error, and the error introduced by the operational amplifier power supply rejection ratio PSRR can be converted into the offset input voltage Vos, which will also be related to the power supply. In this way, in order to reduce the impact of the offset on the reference voltage, the offset of the operational amplifier should be as small as possible. However, there are many reasons for the offset, such as the mismatch between transistors, the mismatch of the threshold voltage of the operational amplifier input stage tube, the limited gain of the operational amplifier, etc. Therefore, in fact, Vos is difficult to completely eliminate, but by increasing the gain of the operational amplifier and carefully designing the layout, its impact on the reference voltage can be reduced and the accuracy of the reference voltage source can be improved.

1.4 Power Supply Rejection Ratio

The power supply rejection ratio (PSRR) is the circuit's ability to suppress changes in the frequency of the power supply voltage. It is the ratio of the open-loop gain from the input to the output of the op amp to the gain from the power supply to the output of the op amp, expressed as KPSR. For the bandgap reference, since the output voltage is independent of Vdd, changes in Vdd will basically not affect the output reference voltage. However, as the operating frequency increases, the output voltage will be affected by the fluctuation of Vdd at high frequencies due to capacitive coupling, thereby affecting the stability of the output voltage. This point is taken into consideration in the specific circuit design. A current mirror with a self-biased cascode structure is used in the circuit, and a pair of ground filter capacitors are connected to the output end. The power supply rejection characteristics of the output voltage are greatly improved.

1.5 Starting circuit

The startup circuit is also an important part of the bandgap reference source. As shown in part A of Figure 2, the circuit may have zero output. Because when the inputs at both ends of the amplifier are at zero level, the circuit is in a non-working state, so a startup circuit is needed to break this balance. The startup circuit introduced in the figure is composed of Mp1~Mp6 and Mn1~Mn4. Its working principle is that the inverter composed of Mp1~Mp4 and Mn1 drives Mn2 and Mn3, so that Mn2 and Mn3 are turned on, thereby indirectly providing a bias voltage to the two differential input terminals of the operational amplifier through points a and b, ensuring that the input differential pair will not be turned off when the system is powered on. When the circuit works normally, the startup circuit is turned off.

2 Simulation Results

2.1 Temperature characteristics

The simulation of this circuit is based on Chartered 0.25 μm models. The simulation software is T-SPICE, the power supply voltage is 3.3 V, and the ratio of R3/R1 is 2.306 6. Such a result is relatively easy to achieve in layout design. The unit resistor can be connected in series, which is conducive to reducing the error caused by layout mismatch. The unit resistor W=3μm, L=10 μm, the block resistance R=330 Ω, and the first layer of polycrystalline is used. Figure 3 shows the simulation results of the output voltage temperature characteristics.

The temperature varies between -20 and 70°C, and the output voltage temperature characteristic is shown in Figure 3. Its temperature coefficient is about 10 ppm/°C. Therefore, it can be seen that the temperature characteristic of the output voltage is not always zero, but is zero within a temperature range and positive or negative at other temperatures. This is caused by the change of base-emitter voltage, collector current, offset voltage and resistance with temperature.

2.2 Power Supply Rejection Characteristics

Figure 4 shows different power supply rejection conditions obtained by scanning in the range of 1 Hz to 10 GHz. The rejection at low frequencies is not very good, at around -10 dB, and needs to be improved; the rejection at high frequencies is very good, basically stable at around -120 dB. Compared with traditional circuits, the circuit proposed in this article can be used in various systems, especially high-frequency systems, which is unmatched by traditional circuits.

2.3 Noise characteristics

Noise is one of the main factors affecting the stability of the bandgap reference source. Usually, noise is divided into external noise and internal noise. External noise is generally caused by changes in the power supply voltage and interference from other circuits. Internal noise mainly includes thermal noise and flicker noise. The size of flicker noise is inversely proportional to the frequency, so it is mainly flicker noise at low frequencies, and thermal noise at high frequencies. For high-frequency thermal noise, an RC low-pass filter can be added at the output end Vref to solve it, while the low-frequency noise from coupling to the power supply needs to be considered and can be reduced by improving the power supply rejection ratio. Figure 5 shows the noise characteristics of the circuit at the output and power supply voltage. The noise at the output is 10.4 nv/Rt at low frequencies and almost 0 nv/Rt at high frequencies, with very good performance. The noise at the power supply voltage is about 9.6 nv/Rt.

2.4 Other circuit parameters

The simulation results of other aspects of the circuit performance are shown in Table 1. The simulation results in Table 1 are measured under the condition of a power supply voltage of 3.3 V. The effective current refers to the current from the power supply to the ground when the circuit is working normally, and the shutdown current refers to the leakage current from the power supply to the ground when the circuit is not working.
 

3 Conclusion

This paper studies a CMOS bandgap reference voltage source using first-order temperature compensation technology in 0.25 μm N-well CMOS process. After the circuit parameters are optimized, the T-SPICE simulation results are as follows: the output reference voltage is 1.403 1 V under 3.3 V power supply voltage, when the temperature changes between -20 and 70℃, the temperature coefficient of the circuit reaches 10x10-6/℃, and the power consumption of the circuit at room temperature is 5.283 1 mW. The power supply rejection ratio characteristics of the circuit at low frequency are not very good and need to be further improved. The power supply rejection ratio at high frequency is very good, so this circuit can be widely used in low-power, low-temperature drift, and high-frequency integrated circuits.