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Old 6th December 2004, 06:45 PM   #1
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Default Vbe Multiplier Help Please

I have been trying to understand PMA’s amplifier introduced here: New error correction amp

I have a problem with a Vbe multiplier disclosed by Nelson Pass in his patent: US3,995,228. To achieve bias voltage Vb = 9 V, with 2*Vbe = 1.4 V, the equation to be solved is:

(R1+R2+R3)/R2 = Vb/2*Vbe

Setting R1=R3= 600 Ohm, R2 = 543 Ohm

Looking at the schematics: web.telecom.cz/macura/pm_ab_er1.gif , the resistor R7 - corresponding to R2 - is 10 kOhm.

Now, interestingly enough, simulation confirms the value 10kOhm.

Any help would be appreciated.

M
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Old 6th December 2004, 06:55 PM   #2
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By your own formula, you would expect the middle resistor R2
to be about 200 ohms, this of course reflecting a simple
Vbe multiplier with two complementary devices and three resistors
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Old 6th December 2004, 07:14 PM   #3
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Nelson wrote:

"By your own formula, you would expect the middle resistor R2
to be about 200 ohms, this of course reflecting a simple
Vbe multiplier with two complementary devices and three resistors."

I have made a mistake, the value of 2*Vbe = 2.8 V ( I have a BE junction in series with a diode).

However, it still does not explain the discrepancy.

M
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