Improved LC filter in class D. audio power amplifier using. simulated inductor *

Transcription

1 CHAPTER 6 Improved LC filter in class D audio power amplifier using simulated inductor * * Partial contents of this Chapter has been published in D.Susan, S.Jayalalitha, Improved LC filter in Class D amplifier using simulated inductor, International Conference on Electrical Sciences, SASTRA UNIVERSITY, Sep

2 6. Improved LC filter in class D audio power amplifier using simulated inductor 6.1 Introduction Class D audio power amplifier is used to amplify the audio signal as this amplifier has the main advantage of low power dissipation. It makes use of the low pass passive filter at the output stage to remove the high frequency components of the modulated output and to retrieve the amplified audio signal. This passive filter makes use of inductors and capacitors. The size of the inductor becomes exceedingly large due to more number of turns at audio frequencies [82-83]. In this chapter, the passive component namely the inductor used in the filter is replaced by the simulated L which makes use of the active devices along with resistors and capacitors. It eliminates most of the disadvantages of passive filter and gives the improved performance of the filter as well as faithful amplification from class D audio power amplifier. 6.2 Working of Class D audio power amplifier The complete block diagram of the class D audio power amplifier is given in the Figure 6.1.These class D audio power amplifiers are used in the audio frequency range of 20 Hz to 20 khz compared to other types of amplifiers because, the power dissipation is less as its output consists of train of positive and negative pulses due to pulse width modulation. The frequency content of the pulses at the output includes not only the desired audio signal but also the unwanted high frequency components. A low pass LC filter is usually inserted between the 75

3 output stage and the load namely the loud speaker to retrieve the audio signal. So the filter performance is very important as it decides the overall efficiency [84], reliability and the audio performance. Figure 6.1 Block diagram of class D audio power amplifier The audio input signal which is to be amplified is modulated using the modulation technique namely the Pulse width Modulation (PWM) [85]. PWM compares the input audio signal with the triangular wave of required frequency. This produces a train of pulses and the duty ratio is proportional to the amplitude of audio signal in PWM. The number of pulses in a given time is proportional to the average value of the input signal in PWM. This modulated signal is then used to drive the driver circuit which produces the amplified output. The output consists of the modulated output which has train of positive and negative pulse which contains the fundamental frequency as well as the other frequency contents. In order to extract the desired amplified audio output, LC filter is used at the output stage [86, 87] as 76

4 shown in Figure 6.1. The output is shifted with the reference to the input and it is compensated by introducing a phase shifter at the output. 6.3 Disadvantages of using LC filter in Class D audio power amplifier The implementation of class D audio power amplifier has many challenges and one among them is the design of filter at the output stage. The conventional method uses inductors along with the capacitors. The ideal inductor used in the LC filter is an iron core. The size and the number of turns used for the inductor makes it impractical for class D audio power amplifier and it produces Electro Magnetic Interference (EMI). The use of inductor core results in heating and so cooling problem crops up. Hence, an alternate and an efficient solution of replacing the ideal inductor is by a simulated inductor whose performance is same as that of the ideal inductor over the entire audio frequency range of operation is proposed here. In this chapter, the simulated L used as an alternate method of implementing L and its application in class D audio power amplifier along with the design is presented. 6.4 Construction of class D audio power amplifier using simulated L The simulated L is obtained from the basic Generalized Impedance Converter (GIC) which consists of two operational amplifiers and five impedances. The GIC provides high Q and wide range of frequency operation [88, 89]. By properly choosing the components of the impedances, the impedance of the GIC circuit is given by V I 2 Z in scr (6.1) 77

5 which represents an inductor and whose value is given by 2 L CR.The expression is obtained with the basic assumptions of operational amplifier. This simulated L is used for the implementation of the LC filter at the output of the Class D audio power amplifier. The complete circuit diagram of the class D audio power amplifier which consist of a triangular wave generator, MOSFET driver, LC filter designed with the simulated L and the phase shifter are shown in Figure 6.2. Triangular wave generator Sine wave input C 1 A4 R 1 A5 A6 A R 2 PWM output R 3 MOSFET Switching stage Low pass filter using simulated inductor Phase shifter circuit A1 R 2= 10K A R1 R2 R3 C4 I R5 C C=0.01uF R 1= 10K A3 R=156K R 3= 10K Output A2 Figure 6.2 Complete circuit diagram of Class D audio power amplifier with simulated L 78

6 6.5 Design of the complete circuit of class D audio power amplifier using simulated L Triangular wave: For f 0 4KHz V0 PP 4V R1 330, R2 5.6K, R3 1K and C 1F Low pass filter: 1 f 0 For Hz 2 LC f 100 and C 1F L 2. 53H 0 Simulated inductor: 2 L CR where R1 R2 R3 R5 R and C4 C For L 2. 53H and C 1F R K Phase shifter: 0 Phase shift 90 R1 R2 R3 10K, R 159K and C 0.01F Experimental results and discussions A sine wave of input 1V, 500Hz is given as one of the inputs to the comparator which is to be amplified, while the other is the triangular wave of 4V, 4 khz as shown in Figure 6.3. The output obtained is the PWM wave which produces the train of pulses as shown in Figure 6.4. This is given to a MOSFET switching stage via a driver circuit whose output is an amplified PWM wave. The amplified PWM wave is given to an LC low pass filter stage where the low frequency components are allowed to pass through and the high frequency components are restricted [90]. The output stage contains the LC filter where L is replaced by the simulated L before being given to the audio load, loud speaker. The filter consists of the simulated L designed for the inductor value of 100H with C=25.33nF. The output is an amplified version of the input along with the phase shift as shown in Figure 6.5.The phase shift is compensated by a proper phase shifter and the phase shifted output is shown in Figure 6.6. The experimental set up is given in Figure

7 Figure 6.3. Triangular wave input having the amplitude of 4 V, 4 KHz Figure 6.4. PWM output at the output of the comparator 80

8 Figure 6.5. Input and Output waveform of class D audio power amplifier with some phase difference Figure 6.6. Input and amplified Output waveform of class D audio power amplifier with no phase difference 81

9 Figure 6.7. Experimental set up of class D audio power amplifier The use of simulated inductor in the class D audio power amplifier provides the following advantages Eliminates the use of heavy size and more number of turns of inductor Avoids EMI problem[91] Removes heat dissipation which is caused by the insulation resistance of the wire of the inductor Makes the fabrication of inductor using simulated L compatible with today s IC technology. The advantages of a Class D audio power amplifier are reduction in power loss and increased efficiency [92, 93]. 6.7 Conclusion The major shortcoming of using class D audio amplifier lies in the design of low pass filter used at the last stage. This can be avoided by using simulated L which has the main 82

10 advantage of reduction in the size of inductor. The simulated L and the class D audio power amplifier are designed and the results are presented 83