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Old 6th July 2012, 04:56 PM   #1
Vostro is offline Vostro  South Africa
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Default Why bd139 Vbe multiplier?

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
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Old 6th July 2012, 05:01 PM   #2
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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.
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Old 6th July 2012, 05:07 PM   #3
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The simple answer is that you have a hole in the transistor. Easy to mount on the heatsink.
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Old 6th July 2012, 05:09 PM   #4
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Quote:
Originally Posted by peranders View Post
The simple answer is that you have a hole in the transistor. Easy to mount on the heatsink.
TIP transistors also has a hole in it.
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Old 6th July 2012, 05:26 PM   #5
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Not the same nice easy to mount hole and small device outline as the TO-126 package BD139 though.
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Old 6th July 2012, 05:58 PM   #6
jaycee is offline jaycee  United Kingdom
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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.
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Old 6th July 2012, 06:03 PM   #7
Bensen is offline Bensen  Belgium
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Quote:
Originally Posted by Vostro View Post
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
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.
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Old 6th July 2012, 06:14 PM   #8
jaycee is offline jaycee  United Kingdom
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I guess it depends how large an output stage you are driving?
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Old 6th July 2012, 06:18 PM   #9
jam is offline jam  United States
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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
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Old 6th July 2012, 07:04 PM   #10
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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!
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