Design of chopper op amp circuit in class D audio system

1 Introduction

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Based on the 0.35 micron N-well process, a single-supply fully differential chopper op amp circuit is designed. At the same time, in order to reduce the residual voltage offset, T/H (track-and-hold) demodulation technology is adopted. When the circuit is operated at a chopping frequency of 150KHz, the input equivalent noise reaches 31.12nV/Hz.

In the design of op amp circuits for Class D audio amplifiers, the signal's Total Harmonic distortion and noise pose challenges to the design of the op amp. For audio signals in the 20-20KHz range, the op amp's distortion is mainly caused by voltage offset and low frequency 1/f noise. This problem is particularly acute with the relatively high 1/f noise and voltage offset of the CMOS process. When the required offset voltage of the circuit is lower than 1mV and the input equivalent noise is lower than 100nV/Hz. Ordinary CMOS op amps are difficult to meet. Common static offset zero-zero techniques, such as trimming, can well eliminate the effects of voltage offsets, but they do not reduce 1/f noise. The best way to solve this problem is to use dynamic offset-cancellation techniques such as auto-zero and chopping techniques. Auto zero tiechnique is used to reduce the offset and noise by sampling the low frequency noise and offset and then subtracting them from the instantaneous value of the signal at the input or output of the op amp. Since the auto-zero technique uses the principle of capacitive sampling, it is easy to fold the broadband thermal noise into the baseband frequency during circuit operation, and the wider the bandwidth of the op amp, the more noise on the sampling capacitor, usually up to 70nV. /Hz. Chopper Technique uses the principle of modulation and demodulation to move low-frequency noise and offset to the high-frequency part and filter it with low-pass filtering. Since there is no aliasing of thermal noise, the noise-to-voltage ratio of the op amp is automatically stabilized. Zero technology is lower. However, the effects of chopping switch charge injection and charge feedthrough effects can still produce residual voltage offset of around 100uV. Moreover, the use of chopper switches will increase the thermal noise level of the device.

2 How does the chopper op amp work?

The principle of the chopper op amp is shown in Figure 1. Vin is the input audio signal, modulated by a chopping switch with a frequency of fch and amplitude 1. According to the Nyquist sampling principle, in order to avoid aliasing of the input signal, Fch must be much larger than 2 times the signal bandwidth.

The principle of chopper op amp

Figure 1 Principle of chopper op amp

After modulation, the signal is moved to the odd harmonic frequency of the chopped square wave. This signal is amplified by the op amp with gain of Av. At the same time, the input noise and input offset voltage of the op amp are also amplified by the op amp. The output of the op amp is modulated by a chopping switch with amplitude 1 and frequency fch. The output signal is:


It can be seen from equation (1) that after the second chopping, the input audio signal is demodulated to the low frequency band, and the voltage offset of the operational amplifier and the low frequency noise are shifted to the chopping square wave after only one modulation. On the frequency odd harmonics, after low-pass filtering, the high-frequency components in the output signal are filtered out, and the low-frequency components are reduced to audio signals, thereby achieving accurate amplification of the audio signal.

Fourier analysis of the output signal yields the final input noise spectral density (PSD) of the op amp as:


The coefficient K is related to the noise parameter of the process.

3 design of the op amp circuit

The hybrid amplifier designed in this paper is a CMOS fully differential circuit structure. It consists of a chopper switch, a main operational amplifier circuit, an output stage and a common mode feedback circuit. The operating voltage range of the circuit is 2.5V to 5.5V. The circuit structure of the chopper op amp is shown in Figure 2.

Chopper op amp circuit structure

Figure 2 Circuit structure of chopper op amp

The input chopping switch completes the modulation of the audio signal. The chopping switch introduces residual voltage offset on both the upper and lower edges of the clock. Figure 3 shows the waveform of the residual offset voltage at zero input.

(a) residual offset voltage (b) chopping signal

Figure 3 (a) residual offset voltage (b) chopping signal

T/H demodulation and control timing

Figure 4T/H demodulation and control timing

By analyzing the characteristics of the CMOS switch, it can be concluded that the equivalent input residual offset voltage is Vos, rmts = 2Vspiketfch, where t is the time constant of the MOS switch. From this equation, it can be seen that there are three ways to eliminate the residual voltage offset:

1. Reduce the chopping frequency:

2. Reduce the input resistance;

3. Reduce the charge injection effect of the chopper switch.

Since the corner frequency of 1/f noise of MOS tube is generally above tens of KHz, the reduction of chopping frequency does not modulate 1/f noise well, and the input resistance is only related to the internal resistance of the signal source, which is difficult in design. The input resistance is reduced, so only the charge injection effect of the switch can be considered. For this purpose, the input chopper switch adopts a complementary clock structure, and uses a minimum line width in size, which can reduce the on-resistance of the transmission and provide a large voltage swing; on the other hand, the charge injection and the feedthrough are reduced. The effect is to reduce the residual voltage offset. Considering that the PMOS transistor has better 1/f noise characteristics than the NMOS transistor, the input transistors MP1 and MP2 use a large-area PMOS transistor, which can reduce the voltage offset caused by the device mismatch and reduce the transistor 1/f noise. The corner frequency improves the noise characteristics of the op amp.

In order to reduce the residual voltage offset evenly, the output of the fold-cascode op amp adopts T/H demodulation technology, and the circuit structure and timing are shown in Fig. 4. The working principle of the circuit: K1~K4 is closed when the signal is tracked, K5~K8 is disconnected, and the output signal is kept on the capacitors C1 and C2. When the circuit outputs, K1~K4 are disconnected, K5~K8 is closed, C1 and C2 are closed. The voltage values ​​are loaded onto the load capacitor C3 and summed. Since the voltage on C2 is reversed when superimposed on the load capacitance, the residual voltage offset of the amplifier can be effectively cancelled. Because the demodulator uses high-impedance junction chopping. Therefore, a smaller area of ​​the NMOS transistor switch can be used to reduce the influence on the output pole.

The main op amp adopts a fully differential folded cascode structure. In the structure of Class-D, due to the frequent opening of the output power MOSFET, the generated electromagnetic interference (EMI) will form a strong ripple on the power supply. It is found that when the chip is operated at a supply voltage of 5V, the power fluctuation caused by EMI can reach ±2V. The fully differential structure can improve the power supply rejection ratio and common mode rejection ratio of the operational amplifier, and weaken the influence of power supply noise and common mode noise. Moreover, the mirror pole is avoided, and thus stable characteristics can be exhibited for a larger bandwidth.

To provide higher gain and voltage output swing, add a common source op amp output stage after fold-cascode. After using a secondary op amp. Analyze the frequency stability of the op amp. For the time being, the influence of the chopper switch is not considered. It can be inferred that the circuit has at least three LHP poles, which are the main pole Wp1 introduced by the miller compensation capacitor and the output pole Wpout generated by the output filter capacitor. For the first non-primary pole, and the non-pole Wp3 introduced by folded-cascode (the drain of MN1, the source of MN3), the relationship between the three is Wp1.
The common mode feedback circuit is composed of MN7~MN10 and MP10-MP12. The input end is connected to the reference voltage of VDD/2, and the other end is connected to the common mode output of the main operational amplifier. The common mode detection circuit is composed of resistor and capacitor. The bias current of the main operational amplifier is adjusted after error amplification.

4 Simulation results and layout design

At SMIC O. Under the 35 micron N-well process. The cadence spectre tool was used to simulate the circuit designed in this paper. Among them, the process parameters of each device are typical, the power supply voltage is 5V, the input signal is the amplitude 10uV, the frequency is 1KHz standard sine wave, the chopping frequency fch=150K, the simulation waveform is shown in Figure 5 and Figure 6.

Op amp's amplitude frequency

Figure 5 Amplitude-to-phase frequency characteristic curve of the op amp

Chopping output waveform

Figure 6 chopping output waveform

As can be seen from Figure 5, in the typical case, the main pole of the op amp is within 10HZ, and the phase margin is about 75 degrees. It can fully guarantee the stability of the operation under various comer conditions. From the transmission fn waveform, the residual voltage spike caused by chopping has also been significantly improved. Table 1 shows the open-loop simulation results of the op amp.

Table 1 Operational open loop simulation results

Op amp open loop simulation results

The layout of the circuit is designed and optimized by SMIC 0.35um process rule. The grounding of the substrate is fully enclosed double gardring, which effectively reduces the coupling noise of the substrate. The differential pair uses dumb gate common centroid matching to reduce the input voltage offset. In addition, in order to reduce the interference of the peripheral circuit to the operational amplifier, the filter capacitor of the latter stage is dispersed around the operational amplifier circuit, and the optimized layout area is 0.24 mm x 0.34 mm.

Layout layout

Figure 7 layout layout

5 Conclusion

The 1/f noise and voltage offset of Class D audio amplifiers have a direct impact on signal distortion and noise performance, especially when the input signal is zero, the most significant background noise is achieved by using a fully differential chopper op amp circuit and T/ H demodulation technology effectively reduces the low frequency noise and voltage fire of the system. Tests on the chip after the tape slice show that the circuit greatly improves the noise performance of the Class-D.

The author of this paper innovates: the use of fully differential chopper op amp circuit and T / H demodulation technology, effectively reduce the low frequency noise and voltage offset of the class D audio system.

Project Economic Benefits: This project has been successfully released. According to Forward Concepts lnc data, the total output value of global Class D audio amplifiers in 2008 is as high as 800 million US dollars.

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