AB outputstage

klingest · 4th May 2009, 09:45 PM

hi ...

i am studying electronics and have been given the objective by my lector, to design am amplifier with an class AB output stage to give about 150W at 8 ohm, with a quiescent current of 50 mA.

I want to use a Vbe multiplier and because of my output transistors i need a Vbb of at least 2.8V

so how should i do that... i just cant figure it out...

i thought i came to a solution with R2 = 3.3K, R1= 1k, and Iref = 2.5 but something is wrong... the Quiescent current i not at all 50mA... !!

What am i doing wrong ?

kenpeter · 4th May 2009, 11:30 PM

Four diodes (or diode strapped transistors) in series?
Laugh, but its the same parts count as the resistors....

Or you can diode strap ONE mosfet gate to drain,
and VGS-On might have about the right voltage.
But MOSFET temperature coefficient may bend
the wrong way for this application.

Maybe there is an LED with your voltage?

AndrewT · 5th May 2009, 07:37 AM

Hi,
is that 50mA of quiescent current, the total for the whole amplifier? i.e. front end quiescent current + driver bias + output bias?

4th May 2009, 09:45 PM	#1
klingest diyAudio Member Join Date: Apr 2009	AB outputstage hi ... i am studying electronics and have been given the objective by my lector, to design am amplifier with an class AB output stage to give about 150W at 8 ohm, with a quiescent current of 50 mA. I want to use a Vbe multiplier and because of my output transistors i need a Vbb of at least 2.8V so how should i do that... i just cant figure it out... i thought i came to a solution with R2 = 3.3K, R1= 1k, and Iref = 2.5 but something is wrong... the Quiescent current i not at all 50mA... !! What am i doing wrong ?

5th May 2009, 07:37 AM	#3
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, is that 50mA of quiescent current, the total for the whole amplifier? i.e. front end quiescent current + driver bias + output bias? __________________ regards Andrew T.

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4th May 2009, 11:30 PM	#2
kenpeter diyAudio Member Join Date: Nov 2007 Location: Dallas	Four diodes (or diode strapped transistors) in series? Laugh, but its the same parts count as the resistors.... Or you can diode strap ONE mosfet gate to drain, and VGS-On might have about the right voltage. But MOSFET temperature coefficient may bend the wrong way for this application. Maybe there is an LED with your voltage?

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