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Old 22nd October 2006, 06:51 PM   #1
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Default fast transistors

What does it mean?

Gain bandwidth product, or

rise time, settling time, fall time, storage?

And why faster the better?
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Old 22nd October 2006, 06:55 PM   #2
jleaman is offline jleaman  Belgium
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The speed of the transistor / fet has a huge difference. Rise time is how fast the transistor can react to the function it is being used in. I believe that when talking in a amplifier rise time if it is slow you can lean into distortion, also the sound would not come out cleaner.
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Old 22nd October 2006, 07:00 PM   #3
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Generally it refers to the transistion frequency, fT. Rise and fall times are fow switching and would never come into play in an audio amplifier unless it was clipping extremely hard, harder that anyone would be able to bear!
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Old 22nd October 2006, 07:52 PM   #4
AndrewT is offline AndrewT  Scotland
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many of the transistors used in an audio amplifier are fast types.

Often hFE in the range 200MHz to 300Mhz are used at or near the front end. Some designers use devices upto 500MHz in audio amplifiers.
Some discrete opamps uses these fast types all the way through.

Power transistors are inherently slower.

Drivers are often 50MHz to 150MHz and output devices are in the range 700kHz to 60MHz.
Part of the reason for the popularity of FETs in the output stage is that they were very much faster than the old and slower BJTs that were available upto 1980 (I am sure there were some fast BJTs then, but at what cost/availability?).

Since then much faster BJTs have been developed at economic cost and have taken back some of the power amp market.

An amplifier that uses fast devices can have less phase shift between input and output (it depends on the design). This lower level of phase shift allows a greater stability margin with the same level of feedback and bandwidth and this can lead to more accuracy in the output signal. This measured improvement in accuracy may translate into better sound quality.

I suspect that a few manufacturers have deliberately used FAST in their advertising because their research has proved the speed/accuracy hypothesis. Others (like myself) have just jumped on the bandwagon.
regards Andrew T.
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