Class D amplifiers

Jan 19 • Notes • 1592 Views • 4 Comments on Class D amplifiers

Ques 1. What are class D amplifiers???

Ans. CLASS D amplifiers, also known as switching amplifiers are audio amplifiers that work in the digital domain. Class D amplifier generates the equivalent analog output for the speakers by using pulse width modulation (PWM) or pulse density modulation (PDM) rather than the traditional digital-to-analog conversion.

Ques 2. Give the circuit diagram of class D amplifier.

Ans

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Ques 3. How do class D amplifiers work?

Ans. Class D amplifier is a switching or PWM amplifier. This class of amplifier is the main focus of this application note. In this type of amplifier, the switches are either fully on or fully off, significantly reducing the power losses in the output devices. Efficiencies of 90-95% are possible. The audio signal is used to modulate a PWM carrier signal which drives the output devices, with the last stage being a low pass filter to remove the high frequency PWM carrier frequency.

Ques 4. What are the advantages of class D amplifiers?

Ans 4. In a conventional transistor amplifier, the output stage contains transistors that supply the instantaneous continuous output current. The many possible implementations for audio systems include Classes A, AB, and B. Compared with Class D designs, the output-stage power dissipation is large in even the most efficient linear output stages. This difference gives Class D significant advantages in many applications because the lower power dissipation produces less heat, saves circuit board space and cost, and extends battery life in portable systems.

Ques 5. Compare linear and class D amplifiers.

Ans. The primary and main difference between linear and Class D amplifiers is the efficiency. This is the whole reason for the invention of Class D amplifiers. The Linear amplifier is inherently very linear in terms of its performance, but it is also very inefficient at about 50% typically for a Class AB amplifier, whereas a Class D amplifier is more efficient, with values in the order of 90% in practical designs. Figure 3 below shows typical efficiency curves for linear and Class D amplifiers.

Gain – With Linear amplifiers the gain is constant irrespective of bus voltage variations, however with Class D amplifiers the gain is proportional to the bus voltage. This means that the power supply rejection ratio (PSRR) of a Class D amplifier is 0dB, whereas the PSRR of a linear amplifier is very good. It is common in Class D amplifiers to use feedback to compensate for the bus voltage variations.

Energy Flow – In linear amplifiers the energy flow is always from supply to the load, and in Full bridge Class D amplifiers this is also true. A half-bridge Class D amplifier however is different, as the energy flow can be bi-directional, which leads to the “Bus pumping” phenomena, which causes the bus capacitors to be charged up by the energy flow from the load back to the supply. This occurs mainly at the low audio frequencies i.e. below 100Hz.

Ques 6. What are the major drawbackc of class D amplifiers?

Ans.

1. Nonlinearity in the PWM signal from modulator to switching stage due to limited resolution and/or jitter in timing

2. Timing errors added by the gate drivers, such as dead-time, ton/toff, and tr/tf

3. Unwanted characteristics in the switching devices, such as finite ON resistance, finite switching speed or body diode characteristics.
4. Parasitic components that cause ringing on transient edges

5. Power supply voltage fluctuations due to its finite output impedance and reactive power flowing through the DC bus
6. Non-linearity in the output LPF.

Ques 7. Discuss EMI in class D amplifiers.

Ans.  EMI (Electro-Magnetic Interference) in Class D amplifier design is troublesome like in other switching applications. One of the major sources of EMI comes from the reverse recovery charge of the MOSFET body diode flowing from the top rail to the bottom, similar to the shoot-through current. During the dead-time inserted to prevent shoot through current, the inductor current in the output LPF turns on the body diode. In the next phase when the other side of the MOSFET starts to turn on at the end of the dead-time, the body diode stays in a conducting state unless the stored minority carrier is fully discharged. This reverse recovery current tends to have a sharp spiky shape and leads to unwanted ringing from stray inductances in PCB traces and the package. Therefore, PCB layout is crucial for both ruggedness of the design and reduction of EMI.

Ques 8. Explain choice of transistor sizing in class D amplifiers.

Ans. The output transistor size is chosen to optimize power dissipation over a wide range of signal conditions. Ensuring that VDS stays small when conducting large IDS requires the on resistance (RON) of the output transistors to be small (typically 0.1 V to 0.2 V). But this requires large transistors with significant gate capacitance (CG). The gate-drive circuitry that switches the capacitance consumes power—CV2f, where C is the capacitance, V is the voltage change during charging, and f is the switching frequency. This “switching loss” becomes excessive if the capacitance or frequency is too high, so practical upper limits exist. The choice of transistor size is therefore a trade-off between minimizing IDS 3 VDS losses during conduction vs. minimizing switching losses. Conductive losses will dominate power dissipation and efficiency at high output power levels, while dissipation is dominated by switching losses at low output levels. Power transistor manufacturers try to minimize the RON 3 CG product of their devices to reduce overall power dissipation in switching applications, and to provide flexibility in the choice of switching frequency

advantages of class d amplifierDiagram of class d amplifierDrawbacks of class d amplifierEMI in class d amplifier

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