Choice of small signal BJTs

[autir]

Hello ppl

 

[audioguru2]

I used to use BC547B and C and BC557B and C transistors when I worked for Philips, but they are hard to get in Canada.
Now I use 2N3904 and 2N3906 transistors that are made by many manufacturers.

The BCxxx  transistors have the advantage of selected current gain in 3 groups. The 2Nxxx transistors can have just about any amount of gain. ;D

 

[walid1]

There are many general purpose transistors like what audioguru said and 2n2222, 2n2907
i 'll tell you what i do;
i get some electronic board from say radio or any other not used device and get all the transistors from it then get their datasheets and put them in boxes for future use. this makes me familiar with many of these transistors.

 

[audioguru2]

I don't use transistors from radios and things. They have too many different types and I can't purchase them if I need more.

 

[autir]

i get some electronic board from say radio or any other not used device and get all the transistors from it then get their datasheets and put them in boxes for future use. this makes me familiar with many of these transistors.

Exactly what I used to do.

But if I design a circuit and put it on the Internet, I cannot recommend a possibly cheapo, hard to find transistor type.

So, audioguru, I take it as you recommend me to stick just with the BC546-550 family?

 

[audioguru2]

So, audioguru, I take it as you recommend me to stick just with the BC546-550 family?

Yes, and BC557 for PNP. The BC549 NPN and BC559 PNP are especially low-noise types.

 

[Alun]

I don't use transistors from radios and things. They have too many different types and I can't purchase them if I need more.

I disagree, I encourage using transistors from old appliances providing you can find the datasheets.

For most applications the transistor type isn't critical, I  typically use  BC547, 2N2222A, BC337, 2N2369, BC557 ect.

I would be more worried about separating them into groups like NPN and PNP, for the latter I normally use BC560, BC227, 2N2905 ect.

 

[audioguru2]

I can buy only one 2N3904 for $.08CAN at Newarkinone and they cost less if I buy more.
I know it, I like it and I use it. I wouldn't touch a cheap Chinese 2SCxxxx from a radio.

 

[autir]

Hold on a second people!

As we know, the flock of characteristic curves in the Vce - Ic level signify the relation between the incoming (Ib) and the outgoing (Ic) signal. In order for the output signal to be an exact replica of the input (though amplified), the curves must be as parallel as possible.

I attach two pictures. The first is the flock of the BC548, and the second of 2SA1015 from UTC.

As we can clearly see, the cheapo SA1015 I have scavenged from an old radio outperforms the famous BC548. In fact, it makes the perfect amplifier.

I know that this seems too good to be true. Especially the fact that the A1015's curves are COMPLETELY parallel to the x axis. I believe that this is mathematically impossible, but I am not certain.

Your thoughts?

View attachment 37721

View attachment 37722

 

[Staigen1]

Hi there Autir

I think the 2SA1015 curves are a fake! ;D
I will check out on the net if i can get something

//Staigen

 

[Staigen1]

Hi again

I was right, here is the Toshiba version:

//Staigen



2SA1015.pdf

 

[audioguru2]

Do you honestly have a pure current source as an input to a transistor? Don't you also feed it voltage swings? Then look what happens to the distortion of any transistor with a pure voltage input and the transistor circuit doesn't use any negative feedback:

View attachment 37725

View attachment 37726

 

[autir]

I think the 2SA1015 curves are a fake!

I found the section describing the transistor curves in my theory book.
Yes they are fake. Without any doubt.  >

Then look what happens to the distortion of any transistor with a pure voltage input and the transistor circuit doesn't use any negative feedback:

What I understand from your reply is that it is impossible to replicate with 100% accuracy the input signal? Even with class A?!?

 

[audioguru2]

[What I understand from your reply is that it is impossible to replicate with 100% accuracy the input signal? Even with class A?!?

Yes, that is why opamps use so many transistors. Class-A transistor stages produce even-harmonics distortion like I showed before with a voltage input.
Transistors are non-linear, so push-pull stages are used in opamps to eliminate even-harmonics distortion. Current sources are used instead of collector resistors, or bootstrapping of the collector resistor is used. Of course, plenty of negative feedback is used along with voltage gain of up to one-million in good opamps results in linearity that is just about perfect.

Have a look at this duscussion about simple transistor amplifier stages and ways to improve them. Note that they have plenty of negative feedback which reduces their voltage gain to 3. With less negative feedback resulting in more gain, the distortion is much worse.
http://www.dself.dsl.pipex.com/ampins/discrete/discrete.htm

 

[autir]

Yes, that is why opamps use so many transistors. Class-A transistor stages produce even-harmonics distortion like I showed before with a voltage input.

For the second time you shock me completely.
What about all these people who scorn at op.amps and ICs and design these very expensive class-A amps claiming to be Hi-Fi?

 

[audioguru2]

Class-A transistor stages produce even-harmonics distortion like I showed before with a voltage input.

For the second time you shock me completely.
What about all these people who scorn at op.amps and ICs and design these very expensive class-A amps claiming to be Hi-Fi?

Then those people are making their own opamps from separate transistors.
They use class-A only to warm their homes and to avoid the crossover distortion caused by a poorly designed push-pull circuit.
Maybe they are the same people who still use vacuum tubes (valves) in audio amplifiers.

Some opamps have a 0.000008% distortion that certainly can't be heard and is extremely difficult to measure. Their output operates in class-AB, not class-A.

National Semi and others make excellent class-AB IC audio power amplifiers.

 

[autir]

I used to use BC547B and C and BC557B and C transistors when I worked for Philips, but they are hard to get in Canada.

They cost 0.05 Euros each down here.
Want me to send you a hundred or two?  ;D

 

[audioguru2]

Please don't send me any transistors, they distort too much. I have been using low distortion opamps for most of my audio projects.
With my FM transmitter project I had a hard time getting any gain from its preamp transistor and still have inaudible distortion. I added negative feedback to a transistor preamp circuit that sounded awful.