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Old 22nd December 2016, 01:45 PM   #1
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Default On Vbe multipliers

Hi,

I'd like to know the important parameters to choose the active device of a Vbe multiplier, aka bias spreader. Most important thing seems to be ease of mounting it in very close thearmal contact to the output devices of a power amp. So we nowadays choose SOT32/TO126 devices, as they have become rather cheap. Formerly TO92 transistors, plugged into a 5 mm hole by using thermal glue, were more common.
Next would be the collector current rating, which has to exceed the current through the Vas or predriver stage.
Transition frequency, Vce and Ptot don't seem to matter at all, as the spreader uses to be in parallel with a large capacitor and bias voltage commonly is some volts only (and collector current in the range of some few mA's).
But what about hfe? Can I, e.g., use those ancient video transistors with their rather modest hfe, such as BF457, that I have laying around in large quantities?

Best regards!
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Old 22nd December 2016, 01:52 PM   #2
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The selected device needs to change it's current flow at the same proportional rate as the output devices.
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Old 22nd December 2016, 02:13 PM   #3
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Thanks, but I don't see the point, as e.g. in the Honey Badger there's a 22F capacitor, that can easily carry AC, across the bias spreader?!?

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Old 22nd December 2016, 02:22 PM   #4
AndrewT is offline AndrewT  Scotland
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Originally Posted by jwilhelm View Post
The selected device needs to change it's current flow at the same proportional rate as the output devices.
I don't think that is the way the Multiplier passes current.

The main current through the Vbe multiplier transistor is near constant DC.
The low AC impedances in parallel with the transistor suck most of the AC current rather than passing all the AC through the transistor.

If you have a 10mA VAS and allow for upto 10mApk as the AC signal, then you would probably find that the maximum current peak through the Vbe multiplier transistor never exceeds 10mApk (110% of the 9mA transistor quiescent current).
If the Multiplier voltage is 3.6Vdc to drive a triple EF output stage, then the highest transient dissipation is likely to be around 36mW. Well within the capability of a 225mW sot23.
That sot23 can be glued to the collector lead for quickest response time and highest response temperature to Tj of that output device.

Is there an LTspice expert out there willing to simulate the worst case dissipation in the Vbe multiplier transistor?
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Last edited by AndrewT; 22nd December 2016 at 02:27 PM.
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Old 22nd December 2016, 02:25 PM   #5
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The main point of a bais spreader is to create a voltage difference between the positive and negative drivers to create a bit of class a operation. The second main function is to keep this class a operation constant as the output stage changes temperature. The transistor(s) sensing the temperature must change their current flow the same amount proportionally as the output devices and drivers to achieve this.
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Old 22nd December 2016, 02:29 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by jwilhelm View Post
The main point of a bais spreader is to create a voltage difference between the positive and negative drivers to create a bit of class a operation. The second main function is to keep this class a operation constant as the output stage changes temperature. The transistor(s) sensing the temperature must change their current flow the same amount proportionally as the output devices and drivers to achieve this.
The main function of the Vbe Multiplier is as a Shunt Voltage Regulator. It tries to keep a constant DC voltage across the following stages two inputs. Not necessarily maintaining ClassA. The next stage could be switching off on alternate waveform halves.
The secondary purpose is to add a temperature compensation to the Vbias so that output bias current does not change significantly as temperatures around the output stage change.
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Last edited by AndrewT; 22nd December 2016 at 02:32 PM.
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Old 22nd December 2016, 02:32 PM   #7
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Originally Posted by AndrewT View Post
The main function of the Vbe Multiplier is as a Shunt Voltage Regulator. It tries to keep a constant DC voltage across the following stages two inputs. Not necessarily maintaining ClassA. The next stage could be switching off on alternate waveform halves.
The secondary purpose is to add a temperature compensation to the Vbias so that output bias does not change significantly as temperartures around the output stage change.
So we agree then
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Old 22nd December 2016, 02:33 PM   #8
AndrewT is offline AndrewT  Scotland
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No.
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Old 22nd December 2016, 02:35 PM   #9
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Originally Posted by AndrewT View Post
No.
Okay. Feel free to nit pick about some ridiculous miniscule point then if it makes you feel better.
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Old 22nd December 2016, 03:03 PM   #10
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Thanks. To my understanding a Vbe multiplier equals an adjustable Zener diode, and shares it's low dynamic impedance.

And now, what about hfe? Is it really necessary e.g. to use a 2SC3503C (hfe=40, ~40 €ct), or a MJE340 (hfe=30, ~50 €ct), or a BD139 (hfe=25, ~50 €ct), or can I take my BF457 (hfe>25) that don't cost me anything? Oh, I see, hfe of these don't differ all too much, so there's no need to worry about?

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