Bandwidth and large swing CMOS power amplifier 3

Low-power, high-performance audio amplifiers have been pursuing. Greatly in recent years the development of CMOS power amplifier, using this process will effectively reduce the power consumption, but the ensuing question is how to obtain an effective gain bandwidth, reducing power supply noise, how to effectively reduce the harmonic distortion , at low power supply was almost full scale output voltage to obtain a valid output.

This article describes the two-stage amplifier with Miller compensation structure, simple structure, satisfy the gain bandwidth and output swing requirements.

Amplifier structure

The power amplifier uses three amplification structure. Input stage for the folded architecture, the input tube p tube, to reduce flicker noise. Folding the input stage of the output directly as the output level in the p channel transistors in the drive.

Second-class non-inverting amplifier stage, which consists of a common-source p channel input tube, a current mirror and a current of 80μA constant-current source component. The important role of the level circuit is to control the output level n provide reasonable bias voltage, so that the output stage bias voltage of two separate tubes to achieve the AB class output.

Output stage is push-pull of the AB class structure, the structure is characterized by alternating tube p tube and n conduction, so that only the loss of output voltage over-drive voltage of a tube, effectively increase the level of the output voltage to meet the low power voltage signal output drive requirements. The design of the circuit output stage quiescent current is 1mA. Amplifier output stage and the first and third two-level between the two Miller-type compensation circuit, Miller compensation using a zero-resistance compensation method, the method of compensation capacitor CC through a resistor in series RZ, eliminated only by compensation capacitance caused by the right half plane (RHP) zero effect, increasing the stability of the circuit.

npower2 power control line for the province, when its value is low potential, the entire circuit does not work, at the provincial power state. Provincial power control for the portable system is very necessary.

Specific structure shown in Figure 1, in which Vp1, Vp2, Vn2 and Vn2 the bias voltage.

Bandwidth and large swing CMOS power amplifier 3
Amplifier circuit structure

Amplifier stability analysis

Not received compensation in the case of capacitors and resistors, open-loop frequency response of the pole

f1 = 1 / (2πR1C1); f2 = 1 / (2πR2C2); f3 = 1 / (2πR3C3),

Type in: Ri and Ci (i = 1,2,3) are the first i-level equivalent input resistance and capacitance, which is the output load resistor R3.

Power amplifier with compensation of three conditions for the stability of the design is the main pole f1 ≤ fT

Figure 2 is not added with a 32Ω load compensation in three amplifier phase characteristics of the simulation results of Porter, this map can be seen, no additional compensation before the unity gain frequency range there are two poles, the phase margin is only 36 °, the circuit is not stable enough.

Bandwidth and large swing CMOS power amplifier 3

Figure 2 does not increase compensation Bode phase diagram

Figure 3 shows the three amplifiers 32Ω load compensation increases after the simulation results, f1, f2 pole compensation is through the effective separation, f1 move to low-frequency points, moving to a lower frequency, and f2 are moved to outside the frequency fT to achieve very high frequency. The f1 ≤ fT

Bandwidth and the three large swing CMOS power amplifier

Figure 3 Bode phase diagram after compensation

Using two zero-resistance compensation method will introduce two zeros, one for the right half plane (RHP) zero, the other for the left half plane (LHP) zero. Their values were: ZRHP = 1 / (CC2 / gm3-RZ2CC2); ZLHP =- 1 / (RZ1CC1), which CC1, RZ1, CC2, RZ2 were the first and second levels of compensation capacitors and resistors, gm3 transconductance of the third grade.

Right-half plane zero amplitude there will slow down, making the intersection of the extrapolated gain more from the origin, and thus greatly reduce the stability of the circuit, it is necessary to eliminate it. Formula from the right-half plane zero, in theory, can be introduced: Select a value of zero resistance RZ2, to meet the RZ2 = 1/gm3, the right half plane zero is moved to infinity, and thus no longer produce the stability of the circuit impact. But in the actual work output control circuit transconductance because of the compensation resistors respectively of the current work, temperature, process and other factors change, will deviate from the theoretical value, so both requirements are not equal satisfaction. In the actual design is also no such stringent requirements, in general, as long as the zero position to meet the unity gain frequency of 10 times, zero stability of the circuit can be ignored.

The circuit can be seen in the open-loop phase margin cases up to 86.6 °, then the gain bandwidth of 100MHz, the circuit's stability are greatly improved.

The following discussion will be left half plane zero and the right half plane zero in the design of how to meet the requirements.

From the preceding analysis and Figure 3 we can see the reason why the circuit bandwidth of up to 100MHz, mainly the lower left half plane zero value, slowing the unity gain frequency band amplitude and phase of decline. Although the design of circuits that zero is harmless, but according to the requirements of headphone audio circuits, 100MHz bandwidth is not necessary, it can be the basis of this circuit slightly improved, the resistance RZ1 into 100Ω, compensation capacitor Cc1 size change, then the left half plane zero of the value of 350MHz, this time a unity gain frequency of 32MHz, the left half plane zero size of the unit gain bandwidth is no longer influential.

For the right-half plane zero, the design chosen compensation capacitor Cc2 as 2pF, compensation resistor RZ2 to 1kΩ, the formula by the right half plane zero and should satisfy the conditions (ZRHP ≥ 10 times the gain bandwidth) can be introduced (1/gm3 -RZ2) have great range. Transconductance and the resistance that can actually work with the current, temperature, process and other conditions change, and allow a greater range of variation exists.

The circuit's output load is 32Ω resistor, because this design uses a single power supply, the output offset at 1.65V, so the actual circuit in the series required a large output coupling capacitor, to prevent DC current flow through the headset caused power loss, serious damage may even drive headphones or a headset. This circuit uses a 220μF capacitor. Join this low-frequency response of capacitance on the circuit has a certain influence, but still satisfy the human ear's hearing range of requirements.

Output voltage swing

For portable power amplifier, due to its reduced supply voltage, in order to be effective requires that the output voltage as close as possible full scale output swing requirements. The circuit design uses a class AB output structure, so when the only loss in the output voltage of a p n tube or pipe over the driving voltage, the output voltage range has been improved. The following AB output stage in the study based on how to reduce n and p control over tube voltage in order to further improve the voltage swing. To n tube, for example, saturated conduction in n tube flow through it when the quiescent current 1mA, then the current, if the output tube to obtain a larger width to length ratio, then the over drive voltage VDSAT = VGS-VTn can have smaller value, which will reduce the minimum output voltage, the output will be bigger. P tube similar to the larger aspect ratio can also increase the maximum output voltage level. The design of the circuit selected p tube aspect ratio is 3072 / 1, n tube width to length ratio of 768 / 1, of course the larger width to length ratio be at the expense of area, but the circuit output stage of the width to length ratio much smaller compared to other circuits.

Maximum output swing of the circuit up to 2.7V, output voltage efficiency of 81.8%, belong to a large output voltage swing, then the corresponding maximum output power of 29mW.

Amplifier Distortion

PA is an important performance indicator is the total harmonic distortion plus noise. Total harmonic distortion is mainly caused by the nonlinear elements. Completely symmetric in the circuit elements and does not exist does not match the case of even harmonic distortion can be eliminated, but in fact, such a strict circuit does not exist, but also for audio circuit, for a greater impact on sound quality odd harmonic distortion. Because the human ear more sensitive to the odd harmonic distortion, while the even harmonics to be more poor. Reduce the total harmonic approach generally is to increase the amount of internal feedback loop, and good use linear devices to achieve. This study used a three-level amplifier, there are two internal feedback loop, compared with two amplifier harmonic distortion effectively suppressed.

Measured at 3.3V supply voltage, 1kHz input signal frequency, 0dB gain, and full-swing output voltage is 2.7V, the amplifier circuit of the three Total harmonic plus distortion-68dB.