Why select a BC547 over BC337 etc?

[KamalS]

bc337 bc547

I was having a look at the "Amplified Ear" project over at https://www.redcircuits.com (https://www.redcircuits.com/Page38.htm) and noticed that 3 of the 4 transistors were BC547 and only one was a BC337.

To this effect I have two questions:

1. AFAIK, the BC337 seems to be a better choice in every way over the BC547, so why did the author use the BC547 at all?

Would replacing the BC547 with BC337 in the circuit affect the system performance in anyway?

2. I would be using the circuit to drive a 32Ohm impedance headphone. This would mean a current of (1.5 - 0.7) / 32 = 25mA through the transistor at saturation and EVEN LESS when it is at active region.

In that case WHY DO I NEED to use the BC337 at all (BC337 seems to drive more current and have better hFE stability of 100 till 100mA compared to 547's hFE going rapidly below 200 at ~25mA)?

Would replacing the BC337 with BC547 in the circuit affect the system performance in anyway for such a scenario?

 

[millwood]

the bc-337

the 547 is a beefier transistor (800ma vs. 100ma) and it is used as a power amp in the last section.

a 337 will most likely work most of the time but runs a much higher risk of being blown.

 

[Audioguru]

bc547

Some headphones are 8 ohms per ear and are 4 ohms when in parallel. Half the 1.5V battery across 4 ohms results in a current in the output transistor of 188mA. A BC547 has a max allowed current of only 100mA so the higher current BC337 is used..

 

[KamalS]

elektor tun

@millwood - are you really sure of what you said? Infact are you sure about anything you have been saying?

I am not confronting you - I just want you to cool down and look/verify once more whatever you say.

Instead of rattling at me for this reply, just calm down and have a look at the datasheet.

Please.

Added after 5 minutes:

Some headphones are 8 ohms per ear and are 4 ohms when in parallel. Half the 1.5V battery across 4 ohms results in a current in the output transistor of 188mA. A BC547 has a max allowed current of only 100mA so the higher current BC337 is used..

My point exactly! I was asking if I could:

1. Replace all the BC547 with BC337 instead

or

2. Put a limiting resitor to limit current to sat 75mA and replace the BC337 with BC547 instead.

Do let me know the pros and cons.

The reason is very simple - unified BOM.

Also do you know a better circuit for the same?

I would actually use two o fthese and finally replace the BC337 power stage" with a TIP127 so that I can run a small speaker

What do you think about this?

 

[Audioguru]

darlington hfe variation

I think I simulated the circuit and saw its severe distortion.
The BC547 transistors have such a wide range in current gain that BC337 transistors can replace them and some circuits will work well and others won't, the same as if BC547 transistors were used.

A TIP127 is a high voltage PNP power darlington transistor that is completely different from the NPN BC337 transistor.


If you modify the circuit so that it can drive a speaker then you will probably have severe acoustic feedback howling.

 

[FvM]

replace bc547 with bc337

I don't see much sense in discussing BC547 versus BC337 for this very basic circuit. In the given dimensioning (that seems rather arbitrary in my opinion) there's no danger of overloading a BC547 but you can use all BC337 as well - or any other small signal NPN BJT that can be found when sweeping your lab. The profound analog design magazine Elektor once invented the type specification TUN for this kind of transistor.

As said, parameter variation (particularly of current gain) doesn't allow a precise operation point prediction anyway.

 

[millwood]

what is a bc547?

I think I simulated the circuit and saw its severe distortion.

it was supposed too: the last stage is agc (with the 1st stage).

 

[vsmGuy]

darlington bc337

The profound analog design magazine Elektor once invented the type specification TUN for this kind of transistor.

I always used to wonder what TUN and TUP meant.

Could you tell a little more?

AFIC, it won't matter technically if you used BC337's all over the place - a simple look at the DS would have sufficed I would say.

This design/choice of components would make sense if you were to sell these things as there are significant differences in pricing between BC547 and BC337 tapes/reels.

In short, for a hobbyist, just buy BC337's and save the guys at Fry's hassle.

 

[FvM]

bc337 vs bc547

I'm not aware anymore of the exact designation, I think it has been transistor universal npn respectively pnp. But any (small signal) npn is basically the same.