FET selection

[rubberlips]

When selecting FETs, other than normal voltage limits and current limits what's the next characteristic to compare FETs when substituting FETs??
The reason I ask is that the circuit has BF245's in it, however the only FETs I have on hand are J201, MPF102, 2N5485, 2N5459, 2N7000. The closest looks like 2N5485

Pete

[brett]

Hi.
You can count the 2N7000 out because it's a MOS fet.  The main guide is the gate-source votage (Vgs) that turns the fet on.  Unfortunately, they are variable from fet to fet and for a type, they often state a range or a minimum value.  The high values (say -1V) are J201 types, then 2N5484 and 2N5457.  Mid-range values (say 2 to 3V) are typical of 2N5485 and 2N5486.  Then comes the old faithful MPF102 at 4 or 5V.  Note thet these are typical vales and minimum values are often quoted as a volt or two lower.

I'm sorry, but I'm not familiar with the BF245, but a check on the net for "BF245 datasheet" should turn something up.

cheers

[Rob Strand]

It actually depends of the circuit want BF245A, BF245B, or BF245C.   You should be safe with the 2N5485 or the MPF102, go for the 2N5485.

The most important parameter depends on the circuit.

[rubberlips]

Thanks guys,
The BF245 Vgs was between -1.6 to -3.8V, didn't have a typcal value on the datasheet. I'll try the ones you suggested Rob, it probably won't make that much difference is the circuit

Pete

[Albert Kreuzer]

Hi,

the BF245 has a Vgs(off) range from -0.5 to -8 V :shock:
Usually, in a battery operated circuit, you rather want -0.5 than -8, so you select them accordingly (that's what I did for my bass preamps). Usually those with -2 to -3.5 Volts are the ones to take.
No matter which one you use, you have to tweak the bias in your circuit
Anyhow, 2N5457 or similar should work w/o problems.

Cheers,
Albert