> On Sat, 15 Mar 2008 01:34:17 +1300, Apptech wrote:
>> > The BS170 n-ch is quite common in circuits.

>> Yep - right down there with the horrid 2N700x.

> What have you got against these?

I'll speak of the 2N700x which is similar to the BS170

First the good:

-They are very cheap - much lower cost than almost any 
alternative (except the equally cheap 2N700x :-) ).

- They are available in TO92 through hole, which makes them 
easy to use for experimenters.

- They are very widely available.

- 60V rating is good.

Here is the data sheet for the highest current version that 
Digikey reports - 75% up on most.
Note that it CLAIMS a "low threshold voltage" of 3V max - 
but it will support a few 10's of mA drain current at that 
gate voltage at any decent Vdson.

    http://www.st.com/stonline/products/literature/ds/10202/2N7000.pdf


The bad.

Compared to the vast majority of MOSFETS they have a low 
current rating, below that of may bipolar transistors, which 
is bearable.

But they have horrendously high on resistance (Rdson) - 
almost 2 ohms at 'decent' current and 10 volts gate drive.

        http://others.servebeer.com/misc/2N7002.JPG

- They don't have the redeeming grace for experimenters of 
having good performance at 5V. Many other FETs also need 
higher gate voltages, but they usually have something good 
to offer in exchange.

With 5v gate drive they have about 1 volt on voltage at 250 
mA (= 4 ohm Rdson)

To get Vsat (Vgs) down to 0.2V at 5v gate drive you can 
handle only about 50 mA (again about 4 ohms Rdson).

It takes 7 volts gate drive to get it into a "decent" mode 
and even then it drops almost a volt at 300 mA !

- Maximum continuous current varies with supplier but is 350 
MA max (ST), 200 mA typical (many)
Higher than some TO92 bipolar but lower than many.


        Russell






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