What is the Ft of a compound transistor
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 21st January 2014, 06:58 PM #1 diyAudio Member   Join Date: Mar 2011 What is the Ft of a compound transistor Can it be derived from the characteritics of the driver and output BJTs ? __________________ Transistor junction temperature is not transistor case temperature.
 24th January 2014, 07:55 PM #2 diyAudio Member   Join Date: Sep 2006 Yes, basically. You can construct Bode plot for both and multiply. Generally a high frequency driver plus lower fT output will be dominated by the output device. John
 24th January 2014, 09:54 PM #3 diyAudio Member   Join Date: Jul 2003 Location: Bath, UK It can be worse than that; for example, the classic darlington pair requires some method of pull-down from the OP device base in order to ensure turn-off. Without enough base-charge suckout the composite can be rather worse than either device used. (For anything you care about it's worth calculating how much you need, to optimise turn-off vs loading on the driver. Choose amongst several intersecting curves...) Last edited by martin clark; 24th January 2014 at 09:56 PM.
 25th January 2014, 09:06 AM #4 diyAudio Member   Join Date: Jun 2005 Location: Kent, UK You could have a go at measuring fT Fake transistors?. My own efforts weren't terribly accurate but you can easily tell if fT has changed.
 25th January 2014, 03:04 PM #5 diyAudio Member   Join Date: Sep 2006 Hi Martin Your comments address two separate issues. fT is defined by current gain, so it is collector current out for base current in. This means high impedance. You can get a higher fT for a Darlington than for a single transistor - even though the power transistor may have a gain less than 1 the driver which may have a high fT could still have a high gain, so the pair have greater than unity gain. If you have a low impedance drive, as you suggest, then it is no longer "fT" but effectively common collector. In practice, in an audio amplifier or indeed in most practical circuits I agree we want the transistors to switch on and off as fast as possible. Providing a low impedance helps to turn them on and off faster. In fact, in one of Doug Self's early articles he cited the good old 2N3055 as having a rather alarming characteristic. In typical amplifiers at the time, this was that the supply current begins to rise at frequencies above 10kHz or so. This is related to the 1 MHz fT of those old transistors. One solution I used was to replace the usual 68 or 100 ohm base resistors by 10 ohm resistors. But the drivers get overworked a bit then. John
 25th January 2014, 07:22 PM #6 diyAudio Member   Join Date: Nov 2004 Location: Germany IMO fT is not as useful as parameter for compounds as it is for single transistors, simply because we are looking at a 2nd-order response. You cannot really infer hFE(f) from it (you can for a single transistor if DC hFE is known).
 25th January 2014, 08:19 PM #7 diyAudio Member   Join Date: Sep 2006 fT is the frequency where the gain is unity, so you can apply that to one or more transistors, even if there is a second order roll off. Yes, you would have to make two or more measurements in the gain-frequency response to find the value. It is easier to determine fT for a single transistor, but the question was asked if you can obtain it from the individual responses of the transistors, which you can. John
 27th January 2014, 09:37 PM #8 diyAudio Member   Join Date: Mar 2011 I followed the Bode plot route. One can construct the Bode plot of a transistor from the Ft and the DC current gain, Beta0 given in the data sheets. Using Beta = Beta0 / ( 1 + j * ( f / Ft ) ) The cut off frequency is Ft / Beta0 Before roll off, the amplitude of Beta is Beta0 After roll off the amplitude of Beta decreases 20dB per decade, intersecting the 0dB line at f = Ft So it is easy to construct the Bode plot from Ft and Beta0. For a coumpound BJT Output transistor Ft1 Beta1 Driver transistor Ft2 Beta2 Ft2 > Ft1 I finally found the following result: The Ft of the compound transistor is The geometric mean of Ft1, Ft2 , in other words: Sqrt( Ft1 * Ft2 ) or Ft1 * Beta2 Whichever is smaller. __________________ Transistor junction temperature is not transistor case temperature. Last edited by mchambin; 27th January 2014 at 09:40 PM.

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