From: Kevin McMurtrie
Subject: Re: type of NPN transistor for minimul voltage drop C to E
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Date: Sun, 24 Nov 2002 09:02:25 GMT
NNTP-Posting-Date: Sun, 24 Nov 2002 01:02:25 PST
In article ,
"John S. Dyson" wrote:
>"Spehro Pefhany" wrote in message
>> On Fri, 22 Nov 2002 23:44:02 GMT, the renowned "Brill Pappin"
>> >Any ideas an what type of transistor I should look at to get the minimum
>> >voltage drop when on (NPN)?
>> >A typical diode (1N4001 you can expect about .6V (rule of thumb) a
>> >transistor will be about the same (give or take) is there a particular type
>> >that will have significantly less voltage drop?
>> No, a saturated silicon BJT (transistor) will be < 100mV from
>> collector to emitter, in most cases, for a small signal transistor.
>Given the same BJT geometry, you want a transistor with the
>highest beta in the direction opposite that you plan to use (for
>lowest offset voltage.)
>So, if you use a normal transistor, in a non-inverted circuit, and
>need the high beta, then you also need a transistor with a high
>reverse beta. The high forward beta will keep your base
>current drive minimal, but the high reverse beta will tend to keep
>the voltage drop across the part minimized.
>Normal transistors tend to have a BF in the range of 20-2000,
>while the BR tends to be in the range of 2-10. In cases where
>you can deal with high base current drive, but need to absolutely
>minimize the voltage drop, then you might try connecting the
>transistor in reverse. The bad news is that some of the breakdown
>voltages will tend to be much less.
>If you only need a slow transistor, with high gain and moderately
>high current, then the zetex line of parts (ztx688-ztx696 for
>NPN or ztx788-ztx796 for PNP.) The forward beta on these parts
>tends to the 400-2000 range, and the reverse beta can be as high
>as a couple of hundred. Additionally, the junctions are large, with
>high Is, so the associated drops at 1ma (for example) are very low.
>If you can stand a low breakdown voltage, a ztx688 or ztx689 can
>do some things that are uncommon WRT normal 2n3906 or 2n2222
I'll have to remember that trick. I could have used it in the self
biasing circuit of an audio amp I built. It would have eliminated a
voltage divider on the base of the power transistors and increased the
loaded output voltage swing by a volt or two.