I see many many diy amps with BD139 as Vbe multuplier.
Id like to know what is desirable in a VBE transistor,
Nice High Ft, low c, low cost, gain?
I have BD139-10 low gain and BD139-16 higher gain,
which would be better, or does it not matter with temp to vbe change?
thanks
Id like to know what is desirable in a VBE transistor,
Nice High Ft, low c, low cost, gain?
I have BD139-10 low gain and BD139-16 higher gain,
which would be better, or does it not matter with temp to vbe change?
thanks
May be cost. The Vbe multplier doesn't amplify nothing, so gain isn't a key. In my opinion, what there must be is a low thermal resistance to quickly respond to runaway in the output stage.
Not the same nice easy to mount hole and small device outline as the TO-126 package BD139 though.
It's a voltage source so it really doesnt matter. TO-92 devices work too, but a TO-126 is easier to mount. A TO-220's metal tab would probably create too much thermal lag to make it useful as a vbe device.
I'm not an expert, but I experienced that Hfe is important. During my last amp I made, I have tried a lot of VBE multiplier configurations to find one that has +- the same tempco as the OPS.
For example: see the datasheets of 2SC4495 / 2SC3964. These are specially made for this amplication. Both are having a Hfe of minimum 500.
All you ever wanted to know about Vbe multipliers..............
Simplified Analysis of the VBE multiplier Bias Circuit for CHAPTER 1 Analysis and Design of Biasing Circuits
Trying to find out if hfe has much to do with selection or is it response time to a change in temp.
Jam
Simplified Analysis of the VBE multiplier Bias Circuit for CHAPTER 1 Analysis and Design of Biasing Circuits
Trying to find out if hfe has much to do with selection or is it response time to a change in temp.
Jam
How much Hfe do you think is needed? The requirement here is minimal and the previous posts have given the reasons
that have applied to the choice of BD139 for decades, no doubt.
1. Its a BJT 2. It's cheap 3.It's easily mounted. - end of story!
The tempco of BJT silicon B-E junctions are all similar. The tempco. of the circuit will be different to that of the
multiplier's and is set by the circuit gain or it's operating slope in -mV/degree as determined by the operating voltages,
component selection, adjustment setting and the integrity of the thermal coupling to the device it is controlling.
Fortunately, nothing like Hfe 500 is needed from this T0126 transistor!
that have applied to the choice of BD139 for decades, no doubt.
1. Its a BJT 2. It's cheap 3.It's easily mounted. - end of story!
The tempco of BJT silicon B-E junctions are all similar. The tempco. of the circuit will be different to that of the
multiplier's and is set by the circuit gain or it's operating slope in -mV/degree as determined by the operating voltages,
component selection, adjustment setting and the integrity of the thermal coupling to the device it is controlling.
Fortunately, nothing like Hfe 500 is needed from this T0126 transistor!
If the Hfe has an excessive influence on the tempco, this means that the circuit around the multiplier (resistors) is poorly designed.
In this respect, a higher gain eases things and smooths out differences.
Normally, the tempco is dependent mainly on the current density, which is why it is important to choose a large enough device.
The best solution being of course to dispense completely with this kind of outdated technique....
Hi,
That is because its cheap, with the BD140 its a very popular driver
transistor, so easy to get hold of, and the cheapest package type
you can thermally couple to something. Being a driver it also
matches driver Vbe voltages well, so its simply very common.
rgds, sreten.
Thanks guys, I get it,
BD139/140 have been awesome for years,
and they werent to bad in their day, and still are pretty nice today.
I've used them as drivers in "cheap" amps.
Was wondering about high/low hfe, but now my minds put at ease.
Learning lots from DiyAudio, Its members Rock!
BD139/140 have been awesome for years,
and they werent to bad in their day, and still are pretty nice today.
I've used them as drivers in "cheap" amps.
Was wondering about high/low hfe, but now my minds put at ease.
Learning lots from DiyAudio, Its members Rock!
The hfe has a influence on temp, the higher the hfe the more temperature sensitive the transistor is, so ideal youd use a super beta or a darlington. The high hfe also helps the vbe have very low impedance which makes it more resistant to vas current fluctuations due to high load from output stage and powersupply fluctuations. A lower voltage transistor also has higher sensitivity with regard to temperature, even a 20 to 30 volt part is fine.
Good parts for the job has been mentioned, the bd works but its not so good when you start using triple EF outputstages.
The best part for the job is KTC3114, if you can get it, so far Ive only been able to purchase factory direct. The low noise, low Cob, very hig hfe and ft make it ideal, it comes in To126 package as well.
Good parts for the job has been mentioned, the bd works but its not so good when you start using triple EF outputstages.
The best part for the job is KTC3114, if you can get it, so far Ive only been able to purchase factory direct. The low noise, low Cob, very hig hfe and ft make it ideal, it comes in To126 package as well.
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