The Class-D Amplifier. (From the book Introduction to Electroacoustics and Audio Amplifier Design, Second. Edition – Revised Printing, by W. Marshall Leach, Jr., …

(From the book Introduction to Electroacoustics and Audio Ampli?er Design, Second Edition – Revised Printing, by W. Marshall Leach, Jr., published by Kendall/Hunt, c ° 2001.)

A class-D ampli?er is one in which the output transistors are operated as switches. When a transistor is o?, the current through it is zero. When it is on, the voltage across it is small, ideally zero. In each case, the power dissipation is very low. This increases the e?ciency, thus requiring less power from the power supply and smaller heat sinks for the ampli?er. These are important advantages in portable battery-powered equipment. The “D” in class-D is sometimes said to stand for “digital.” This is not correct because the operation of the class-D ampli?er is based on analog principles. There is no digital coding of the signal. Before the advent of the class-D ampli?er, the standard classes were class-A, class-AB, class-B, and class-C. The “D” is simply the next letter in the alphabet after “C.” Indeed, the earliest work on class-D ampli?ers involved vacuum tubes and can be traced to the early 1950s.
Fig. 1 shows the basic simpli?ed circuit of a class-D ampli?er. We assume a bipolar power supply so that V ? = ?V + . The ampli?er consists of a comparator driving two MOSFET transistors which operate as switches. The comparator has two inputs. One is a triangle wave, the other is the audio signal. The frequency of the triangle wave must be much higher than that of the audio input. The voltage output of the comparator can be written

This voltage is applied to the input of a complementary common-source MOSFET output stage. Each transistor operates as a switch. For vC = ?V1 , M1 is on and M2 is o?. If the voltage drop across M1 is negligible, then vO = V + . Similarly, for vC = +V1 , M2 is on, M1 is o?, and vO = V ? . In practice, there is a small voltage drop across the on MOSFET switch so that the peak output voltage is less than the power supply voltage. For the case vS = 0, vO is a symmetrical square wave. The low-pass ?lter consisting of L1 and C1 passes the average value of the square wave to the loudspeaker, which is zero. Thus vO = 0 for vS = 0. The network consisting of R1 and C2 compensates for the inductive impedance of the loudspeaker voice coil so that the ?lter sees a resistive load at high frequencies.

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