Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Patent
1997-12-11
1999-10-26
Ngo, Ngan V.
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
257329, 257337, 257384, 330251, 330277, 381120, H01L 2976, H01L 2994, H01L 31062, H01L 31113
Patent
active
059733683
DESCRIPTION:
BRIEF SUMMARY
INTRODUCTION
A convention push-pull (class AB) linear amplifier (FIG. 1A) modulates load power by continuously varying conduction through its pass elements during most, if not all, of the conduction cycle. Q.sub.1 conducts during the positive half-cycle, and Q.sub.2 conducts during the negative. During each half-cycle, the conducting transistor operates in its linear region. The transistor must supply the required current to the load while reducing the voltage between the supply and the load. The power dissipated in the transistor, which equals (V.sub.BUS -V.sub.LOAD).times.I.sub.LOAD, is wasted in the form of heat. Q.sub.1 and Q.sub.2 have large heat sinks to prevent the output stage from overheating.
The class AB amplifier uses a bleeder circuit to reduce crossover distortion, which occurs during the zero-crossing of the input signal when neither transistor is on (when the input signal is below the V.sub.BE of either transistor). See FIG. 1B. The bleeder circuit biases both transistors on during crossover, but the circuit draws current that further reduces the efficiency of the amplifier.
Ballast resistors in class AB designs prevent the transistors from going into thermal runaway. Bipolar transistors are at risk because their V.sub.BE s have negative temperature coefficients. Usually, the diodes and transistors are mounted on the same heat sink to ensure that the V.sub.BE s track. The common heat sink helps minimize crossover distortion over temperature.
Other conventional class A, B, and C amplifiers also use switching elements in their linear made of conduction for a large percentage of each cycle of an audio input signal. This linear operation reduces efficiency to about 60% in the typical class AB amplifier and requires the use of large heat similar to dissipate the other 40% of the power.
Alternatively, the switching elements of class D amplifier are either cut off or in saturation most of the time, allowing high efficiencies. The high efficiency translates into reduced heat sinking, smaller size, and lighter weight. Also, class D amplifiers do not suffer from crossover distortion within the audio bandwidth.
The concept of a class D switching amplifier has been known for about 50 years. Early attempts to develop switching amplifiers with vacuum tubes were limited by the tubes' large voltage drops and low current capabilities, which reduce the amplifiers' efficiencies and limited their output power. In the late 1960s, bipolar transistors became a practical alternative to vacuum tubes and allowed the implementation of switching amplifiers with very high efficiencies at low frequencies.
However, an audio switching amplifier requires high-frequency operation, which is generally equal to at least four or five times the bandwidth of the 20-kHz audio spectrum. Higher frequency operation makes it easier to design a filter that removes the carrier frequency before the audio signal drives the speaker. Using bipolar transistors at the required frequency of 80 KHz or greater results in excessive switching losses that eliminate the class D amplifiers' efficiency advantages.
In the 1980's MOSFETs became available that could meet both the switching-speed and conduction-loss requirements to effectively implement class D amplifiers. The first switching amplifiers using MOSFETs incorporated electrically isolated drivers to allow the use of N-channel devices. N-channel MOSFETs yield more efficient designs; these MOSFETs have approximately one-third the conduction losses of their P-channel counterparts. However, the isolated drive circuits were complex and limited the use of switching amplifiers.
Class D amplifiers convert the audio signal into high-frequency pulses that switch the output in accordance with the audio input signal. Some class D amplifier use pulse width modulators to generate a series of conditioning pulses that vary in width with the audio signal's amplitude. The varying-width pulses switch the power-output transistors at a fixed frequency. Other class D amplifier rely upon pulse density modulators.
REFERENCES:
patent: 4439738 (1984-03-01), Atherton
patent: 5160896 (1992-11-01), McCorkle
patent: 5194821 (1993-03-01), Brambilla et al.
patent: 5510753 (1996-04-01), French
patent: 5629616 (1997-05-01), Weggel
patent: 5805020 (1998-09-01), Danz et al.
Hemmenway Donald F.
Pearce Lawrence G.
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