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 has very practical significance.

1 Basic Principles of Bandgap Reference Circuit

The purpose of the bandgap reference voltage source is to generate a constant quantity with temperature changes. Since the base voltage VBE of the bipolar transistor has a temperature coefficient of 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, VT and VBE have opposite voltage temperature coefficients, multiplying them by a suitable coefficient and then weighting them 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, which is the basic design idea of ​​the bandgap voltage source. 1.1 Core Circuit of Bandgap Reference Voltage Source 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 approximately zero temperature coefficient. In FIG1 , the emitter areas of the bipolar transistors Q1 and Q2 are the same, and the emitter areas of Q3 and Q4 are the same. Considering the design requirements, the emitter areas of Q1 and Q2 are 8 times the emitter areas of Q3 and Q4.

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

1.2 Bandgap reference voltage source overall circuit The overall circuit of the bandgap reference voltage source consists of 4 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 to obtain 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 reference source output voltage. 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 offset on the reference voltage, the offset of the op amp should be as small as possible. However, there are many reasons for offset, such as mismatch between transistors, mismatch of threshold voltage of op amp input stage tubes, limited gain of op amps, etc. Therefore, in fact, Vos is difficult to completely eliminate, but by increasing the gain of the op amp 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, the change of 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 the coupling of capacitors , thus affecting the stability of the output voltage. This is taken into consideration in the specific circuit design. A self-biased cascode current mirror is used in the circuit. At the same time, a pair of ground filter capacitors are connected to the output end, and the power supply rejection characteristics of the output voltage are greatly improved. 1.5 Startup 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 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 consists of Mp1~Mp6 and Mn1~Mn4. Its working principle is that the inverter composed of Mp1~Mp4 and Mn1 drives Mn2 and Mn3, making Mn2 and Mn3 conductive, thereby indirectly providing 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 results are 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 due to 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 is not very good at low frequencies, at about -10 dB, and needs to be improved; the rejection at high frequencies is very good, basically stable at about -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 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 good performance. The noise at the power supply voltage is about 9.6 nv/Rt.