About VBE transistor spec in a amp

[Jean Parent]

Hi

In a audio amp like the one I've put in that image shack link;

http://img246.imageshack.us/img246/5204/ppampan6.jpg

For the VAS transistor, if we replace the BD139 by a transistor with a
very different VBE specification, like a 2SC3503, what result it would do
in the circuit voltages and result ?

The BD139 have; VBE=1 V at a VCE of 2 V and a iC of 500 ma.The 2sc3503
have; VBE=.8v at a VCE of 5 V and a iC of 10 ma.

As you can see the BD139 do have his VBE at 50 time more amp the the 2SC3503

I want to understand about VBE specs and how it change the fonction of a
amp circuit in a case like this.

Thank

Jean

 

[Jean Parent]

First, I'd change both the BD139 and the BD140 if you are going to use
substitutions ... selecting a complementary pair to maintain the balance.

I'd not worry as much about the Vbe specs.

Hi

I forgot to say, for this amp, wen a transistor like the 2sc3503 do have
his VBE at 50 time lower amperage than the BD139, is it mean that the
2sc3503 will need more voltage from the input pair transistors to work ok ?

Thank

Jean

 

[Eeyore]

Hi

In a audio amp like the one I've put in that image shack link;

http://img246.imageshack.us/img246/5204/ppampan6.jpg

For the VAS transistor, if we replace the BD139 by a transistor with a
very different VBE specification, like a 2SC3503, what result it would do
in the circuit voltages and result ?

The Vbe WON'T be very different in reality.

Graham

 

[Eeyore]

I forgot to say, for this amp, wen a transistor like the 2sc3503 do have
his VBE at 50 time lower amperage than the BD139, is it mean that the
2sc3503 will need more voltage from the input pair transistors to work ok ?

No.

ALL bipolar silicon transistors are pretty similar as regards basic specs like
Vbe within certain constaints such as power rating and application.

For 'fun' I once even replaced a BC108 with a 2N3055 in a preamp circuit and it
still worked.

Worrying about the different ways manufacturers spec these things is pointless.
The basic numbers are largely defined by semiconductor physics. And
characteristics where Vce is only 2V are 'saturation' characteristics which are
quite different to normal operation. What WOULD be more relevant would be to
check for fT (speed) differences that might affect stability.

Graham

 

[Jean Parent]

Jean Parent wrote in message ...

Those figures for Vbe are rather high for a transistor operating in it's
linear region. They were most likely generated for switching conditions or
something like that. If the substitute is a recommended transistor, I
wouldn't worry about it. The amp has unity gain at DC. Any offset at the
output can be trimmed out with the 20k pot.
BTW, I thought the MJE2955 and 3055 were discontinued about 20 years ago.
bg

Hello

Mouser still sell those old transistors.

Thank to all who reply

Jean