Noise Analysis and Design of the pre-operational amplifier

Noise Analysis and Design of the pre-operational amplifier

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Source: China Power Grid Author: Huang linghua, Wang Xinan, and Liu Wei

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In Class D Audio Power Amplifiers, a pre-operational amplifier is an important module located in front of the entire topology to process the input signal source or set the amplification gain, or achieve the purpose of Impedance Transformation, so that it matches the input sensitivity of the latter power amplification level; the front amplifier obtains and stably inputs the audio signal, and ensures the differential signal, during the design, we need to minimize the blinking noise and thermal noise of the equivalent input, reduce the output resistance, and increase parameters such as PSRR, CMRR, SNR, band width, and conversion efficiency.

In general, the Flashing Noise of bipolar transistor has a lower angle frequency (the intersection of flickering noise and thermal noise), which is lower than the Flashing Noise of MOS transistor. In the design system of low frequency such as audio, the Design of bipolar transistor is conducive to noise reduction. However, in the design of the hybrid signal circuit, the substrate noise has a great impact on the bipolar transistor. Therefore, in the design of the hybrid signal circuit, more MOS transistor is used, so the op-amp mentioned here adopts the CMOS process to complete the corresponding design.

1. Features of the pre-operational amplifier in the Audio Amplifier

As shown in 1, Class D Audio Power Amplifier consists of the following modules: pre-operational amplifier, modulation level, offset level, control level, driver level and output power transistor level (BTL ); the pre-operational amplifier is located at the beginning of the entire structure. In this design, the pre-operational amplifier must have two working modes: Play and mute, in normal working mode, the op amp receives the signal source, works normally, and completes modulation and signal regeneration at subsequent levels. In noise suppression mode, the op amp stops receiving the input signal source, the differential output end is clamped at a fixed voltage, and other modules work normally. The BTL output end is the same output square wave. On the load, the signal regeneration and reproduction are not visible, and it is in the mute state, the main function of static noise state is to suppress the pop noise when the host is switched on. The internal structure of the implemented circuit is shown in 2.

2. Noise Characteristics of the pre-operational amplifier

There are five noise sources in the operational amplifier circuit: shot noise, thermal noise, flicker noise, and burst noise) avalanche noise (avalanche noise), for the CMOS process, granular noise, burst noise and avalanche noise are usually not significantly affected in the operational amplifier circuit, even if there is, can also be eliminated, it is not considered in noise analysis.

2.1 Noise Model

Resistance Noise is mainly thermal noise. This noise can be equivalent to an ideal noise-free resistor connected to a voltage source, or a current source is connected in parallel as its noise model. Its Equivalent Noise current and voltage are:

The noise indicators provided by the Operational Amplifier manufacturer usually refer to the noise tested at the operational amplifier input end, including hot noise and flashing noise. The noise inside the operational amplifier is described by an internal equivalent. The operational amplifier can be viewed as an ideal Noisless OpAmp ), A noise voltage source is connected at the same-phase input end of the ideal noise-free operation amplifier, and a noise current source is connected to the same-phase and reverse-phase input ends to the ground to characterize internal noise, for single-tube NMOS or PMOS, their equivalent noise current and noise voltage are:

In the above various formulas and the following formula, K is the Boltzmann constant, T is the thermodynamic temperature, and gm is the cross-guide of the transistor. K is the flicker noise coefficient of the MOS transistor. W and L are the effective gate width and length of the MOS transistor, respectively. Cox is the gate oxide capacitor per unit area.

2.2 Noise Analysis of the pre-operational amplifier

The noise model of the front Op Amplifier in the audio power amplifier can be shown in 4. R1 and R2 are input resistors, R3 and R4 are feedback resistors, and R3 and R4 are adjustable resistors, it is used to set the gain of the entire power amplifier. e1, e2, e3, and e4 are the thermal noise voltages of four resistors respectively, and the voltage of the four Resistors on the input noise is:

The noise of the pre-operational amplifier is the sum of the resistance noise and the internal noise of the op-amp. The following analyzes the internal noise of the op-amp.

2.3 Internal Noise Analysis of Fully Differential op amp

We know that the key to noise design is the low-noise design at the input level. Therefore, in most operational amplifier designs, the first level is not the gain design, this level of noise directly determines the noise characteristics of the entire op amp. The PMOS tube has lower noise coefficient than the NMOS tube, which is conducive to reducing the input noise voltage. Therefore, the input stage usually adopts the differential Input Structure of the PMOS tube. Figure 5 shows the noise analysis at the input level of the Operational Amplifier. The input tube is PMOS.

When the source pole of the differential branch is connected to the same point, the noise of the current source load is the noise source. The noise equivalent to the Mp1 and Mp2 can be offset by the differential effect, this reduces the noise of the circuit. Mp1 and Mp2 are input differential tubes. In addition, for Mn3 tubes, the effect of noise voltage on input can also be ignored.

3 circuit design and Physical Layer Design

From the analysis of the above noise characteristics, we can see that to improve the noise of the operational amplifier, We need to select an appropriate resistor and a proper gate-width-length ratio of the mos tube. This paper uses Winbond 0.5 μcmos as a typical process, analyzed the op-amp noise, 6 and 7, with L1 <L2 <L3.

As shown in figure 6 and Figure 7, the larger the input tube and Load tube L, the better the noise characteristic. However, due to the layout and stability requirements, it is impossible to use an excessively large L value. Through the same simulation, we can also draw a similar conclusion on the input width-to-length ratio. Therefore, the op-amp in this article selects the proper resistance and the input-level and load-pipe width-to-length ratio, and completes a good design, figure 8 shows the detailed circuit diagram and table 1 shows the basic simulation results of the design.

From the simulation results in Table 1, we can see that the op amp adopts a low static current design to achieve lower noise characteristics, higher power supply rejection ratio, and faster conversion rate.

Figure 9 shows the complete layout of the pre-operational amplifier in the Power Amplifier. Using the Winbond 0.5 μm CMOS process, the process itself suppresses the noise of the substrate, the design of the audio power amplifier provides a good premise. The three frames are the external feedback resistor, the internal structure of the operational amplifier, and the internal zero resistance adjustment, in addition, the matching of resistance, capacitance, and transistor is well realized.

4 Conclusion

Noise is a very important parameter of operational amplifier, which determines the sensitivity of the entire system. This paper analyzes the noise characteristics of the front-Op Amplifier from the noise parameter, A Method to Improve noise is provided, and the related design is completed using winbond 0.5 μm CMOS process.