![]() ![]() This issue was covered in Section 1.5 of my MOSFET amplifier paper. Pavel, this is a good question, but I was not really oversimplifying. This is pretty easy, however, since today's power MOSFETs from the same tube are usually like peas in a pod. Keep in mind that you should match power MOSFETs if you are going to parallel them. There is less need for this with power MOSFETs. Secondly, it is to build up a larger total output stage Safe Operating Area so that less-intrusive protection circuits can be used. ![]() First, it is to mitigate the beta droop problem at high currents. People typically parallel numerous pairs of bipolar output transistors. Of course, if you're building a big honking power amplifier, there is nothing stopping you from running three or four pairs, each at 200 mA, to greatly reduce static crossover distortion while having a very nice Class AAB design. Note that this results in an output stage idle power dissipation of about 24 watts with +/- 60V rails. They work quite well with a standing bias current of, say, 200 mA, which will result in a source-follower output impedance on the order of one ohm per device, or 0.5 ohm for the complementary pair. Power MOSFETs still make very good output stages even without error correction, but they need to be biased somewhat hotter than a bipolar stage to keep the static crossover distortion small. This is one reason that they benefit so much from the application of error correction. This leads to higher output impedance in a source-follower arrangement, which can lead to higher static crossover distortion. The most notable is their lower transconductance at a given current as compared to bipolars. They have far superior thermal bias stability to bipolars, even though at normal bias current levels they also have a modest but positive temperature coefficient of current as a function of gate voltage. They can source incredible current, and don't need a driver capable of sourcing high current. As a result, they produce very little dynamic crossover distortion. Their equivalent ft is on the order of 300 MHz. I like MOSFETs because they are fast and immune to secondary breakdown. ![]() Admittedly, this is a crude approximation, but it helps draw the lines of distinction between the two devices. Make its current as a function of bias voltage about ten times less sensitive to temperature than a normal bipolar transistor. Give it a peak current capability in excess of 50 amps. ![]() Make it have no secondary breakdown, with a failure mechanism that is purely thermal in nature. Make it 5 times as fast as a fast ring-emitter (sometimes called a perforated emitter) transistor. Give it about one-tenth the transconductance of a normal bipolar transistor at a given current. Take a bipolar transistor and give it infinite beta at all currents and give it a Vbe of about 3.5V instead of the usual 0.7V. The best amplifiers are made by those who know best how to deal with the limitations of the devices they choose. It is possible to make an outstanding amplifier with either technology, and it is also possible to make a terrible amplifier with either technology. Each technology has its advantages and disadvantages. Although this paper is perhaps best known for its application of error correction to MOSFET output stages, a very big part of the paper is about the application and advantages of power MOSFETs to audio amplifier output stages.Īs in most engineering decisions, there is a tradeoff between choosing MOSFETs versus bipolars for an output stage. Secondly, look in Section 1 of my paper, "A MOSFET Power Amplifier with Error Correction". First, look under Power Amplifier Design/Why I Prefer Power MOSFETs. I won't be able to go into all the details here, but there are two places on my web site at where most of the answers to this question lie. I'll begin here by dicussing my preference for power MOSFETs over bipolar output transistors. Both of these are likely to spark some discussion and maybe even some controversy, so I'll answer them in two separate posts. ![]()
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