Low power BJT's

[gareth]

Hi,

Just a quick question regarding low power BJT's. A couple of years ago I read somewhere that you can parallel low power devices to get higher output currents.

Is this true ? If so what are the pro's and con's of doing this ? And is it, on the whole, a worthwhile route to take ?

Thanks
Gareth

 

[CBS240]

Hi

The obvious con is more parts involved. Maybe better Ft from the small signal devices?? It depends on application, but IMO if more power is the goal, it is better to use a larger device. Although, it is common to parallel many small signal J-fets for better noise performance. Theoretically, I suppose you could parallel enough small signal J-fets to make a power amp, but that would take a lot of resources and effort.

BTW, anybody actually ever tried that???

 

[myhrrhleine]

It's quite common to parallel transistors.
Commonly power outputs because as you spread out the power/currect/etc. you can get better performance (consistent current gain (less beta droop) iow more linearity) and greater reliability.

It'll work with smaller xistors too.
Input xistors are paralleled to reduce overall noise.

 

[gareth]

.
Commonly power outputs because as you spread out the power/currect/etc. you can get better performance (consistent current gain (less beta droop) iow more linearity) and greater reliability.

It'll work with smaller xistors too.
Input xistors are paralleled to reduce overall noise. [/B][/QUOTE]


Hi,

So would the electrical characteristics of these devices work out the same as other electrical devices, series and parallel, or are they going to react differently because of the nature of the component ?

Also would the better current gain less beta droop and more linearity make it easier in matching npn and pnp transistors ?

Thanks
Gareth

 

[myhrrhleine]

.
Hi,

So would the electrical characteristics of these devices work out the same as other electrical devices, series and parallel, or are they going to react differently because of the nature of the component ?

Also would the better current gain less beta droop and more linearity make it easier in matching npn and pnp transistors ?

Thanks
Gareth

Yep.
even internal divice capacitance.
So beware

 

[gareth]

Yep.
even internal divice capacitance.
So beware [/B][/QUOTE]



Hi,
Ok, so will this bring down the gain of the transistors or something ? Or am I on the wrong track ?

Thanks
Gareth

 

[fotios]

Hi,

Just a quick question regarding low power BJT's. A couple of years ago I read somewhere that you can parallel low power devices to get higher output currents.

Is this true ? If so what are the pro's and con's of doing this ? And is it, on the whole, a worthwhile route to take ?

Thanks
Gareth

Hi Gareth

If a Vce of 60 to 80 Volts is enough for you; Check this type of ON Semi

D44H - D45H

Also from two pairs and above there is need for matching them as for their beta (hFE).

Fotios

 

[gareth]

Hi,
So how would I be able to effectively test and match the gain of the transistors ?

How critical is it as I would like to make a low(ish) power Class A amp for my first build and I thought that I could use parallelled low power BJT's in the VA stage.

Thanks
Gareth

 

[jacco vermeulen]

I thought that I could use parallelled low power BJT's in the VA stage.

Correct

 

[myhrrhleine]

Yep.
even internal device capacitance.
So beware

Hi,
Ok, so will this bring down the gain of the transistors or something ? Or am I on the wrong track ?
[/B][/QUOTE]

It effects frequency response and can modulate gain at diff voltages.
IOW, increased distortion.