Low power BJT's

gareth · 29th March 2008, 12:40 AM

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 · 29th March 2008, 03:17 AM

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 · 29th March 2008, 04:07 PM

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 · 29th March 2008, 07:45 PM

.
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 · 29th March 2008, 08:29 PM

Quote:

Originally posted by gareth
.
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 · 29th March 2008, 09:32 PM

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 · 30th March 2008, 05:19 AM

Quote:

Originally posted by 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

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 · 30th March 2008, 02:43 PM

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 · 30th March 2008, 02:50 PM

Quote:

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

Correct

myhrrhleine · 30th March 2008, 09:14 PM

Quote:

Originally posted by gareth
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.

29th March 2008, 12:40 AM	#1
gareth diyAudio Member Join Date: Jan 2008	Low power BJT's 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

29th March 2008, 03:17 AM	#2
CBS240 diyAudio Member Join Date: Aug 2005 Location: K-town	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??? __________________ All the trouble I've ever been in started out as fun......

29th March 2008, 04:07 PM	#3
myhrrhleine diyAudio Member Join Date: Jan 2006 Location: Avalon Island	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. __________________ Just because you can't hear it doesn't mean no one can.

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29th March 2008, 07:45 PM	#4
gareth diyAudio Member Join Date: Jan 2008	. 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

29th March 2008, 09:32 PM	#6
gareth diyAudio Member Join Date: Jan 2008	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

30th March 2008, 02:43 PM	#8
gareth diyAudio Member Join Date: Jan 2008	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

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