low noise replacement for BC547, BC557, BD135, BD136

[k.junold]

.

 

[blmn]

k.junold,

I don´t have Douglas Self´s book with me now, but, you can find at Douglas Self´s site (www.dself.demon.co.uk) articles about power amps using japanese series of low power transistores with low noise and high Ft. (2SA970 etc.)

regards

 

[peranders]

What are your application? Transistors don't have to have low noise. It depends where they are. Normally only the input transistors are important. BC546, 547 548 549 550 556 557 558 559 560 etc are normal and OK in most situations. BD135 136 137 138 139 140 don't have to have low noise in most situations.

The speed is also OK 100-300 Mhz for the BC types and aorund 100 MHz for the BD types.

 

[Pedja]

Hi P-A, nice, quick answer.

 

[peranders]

Very low noise transistors have drawbacks (not so fast, higher capacitances, not so high Hfe in some cases) so that means (as allways) that you must choose transistor type with taste. Each transistor must be chosen considering what it should do.

 

[Pjotr]

Besides that Per, ultra low noise only makes sense in low signal apps AND low source resistance. I.e. MM pre amps and even then … The BC446 – BC 560 small signal transistors can be considered low noise for nearly all audio apps.

Cheers

 

[Pedja]

…and besides all that, the question was asked 3 years ago.

 

[peranders]

…and besides all that, the question was asked 3 years ago.

Hm, but why was the thread so high up....?

 

[Pedja]

No idea. It was high before you have answered?

As about the thread topic, since the question was what is lower noise replacement for BC547, the answer can be BC549/550 (549/550 are lower noise than 546/547/548).

Pedja

 

[pjacobi]

Anyway, just imagine the pure guy asking the original question:

Sitting in front of the PC, soldering iron switched on, desperately pressing 'refresh'.

And no answer for three years.

And to be not 100% off topic: 3 BC547 would have lower noise than one BC547, but you must have well matched devices to get fair current shating without further measures.

Regards,
Peter Jacobi

 

[xslavic]

Can sc1815 be considered a direct replacement for BC547 in audio applications,without changing the B base resistor?

 

[Mooly]

Can sc1815 be considered a direct replacement for BC547 in audio applications,without changing the B base resistor?

In general yes. It's a "cooking grade" NPN like the BC547. Just make sure that you don't exceed the ratings in the data sheet.

No components should need altering to use it.

 

[xslavic]

ok

I ment in this circuit,thanks!

 

[Mooly]

Yes, they would definitely be 100% OK for that.

[The circuit is a little unusual just seeing it in isolation. The inputs are very very low impedance (a virtual ground). Is it used as an I/V convertor from a DAC ?]

 

[xslavic]

Yes,I'll use it as I/v converter for dac , in dual mono ,and I'll need two of these as a result i think I get xlrafter the transistors at 3.16v if I'm not wrong but was afraid of the base resistor and the difference in freq 80 vs 300mhz

 

[xslavic]

Yes low impedance but I changed the 320 ohm Res to 195 to compensate for the higher output

 

[Mooly]

The base resistors are OK left at 51 ohm. They isolate the opamp output from the B-C and B-E junction capacitance of the transistor and prevent any stability issues. The absolute value won't be very critical.

Absolutely no problem with the different ft of the transistors either.