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Old 10th February 2016, 04:05 AM   #1
diyAudio Member
Join Date: Sep 2012
Default An intersting idea for efficient simple 8-transistor amplifier?

I was thinking of modifying an H-Bridge to have darlington transistors (almost removing the need of biasing resistors. maybe increasing efficiency)

And then using that to drive a speaker (Without PWM input. and just variable AC input from a line-out from a headphone output from a PC. connected to the 4 base's in a simple way)

Could this work? Or would I need to modify a lot more of the circuitry?
From what I know. It looks close to two push-pull circuits combined. but with the load in between both sets of "emitters" of each push/pull circuit and decoupling capacitor if required

I know it'd cause more voltage drop. but wouldn't it still be more efficient by using more than half the power supply's voltage? at 12v you'd get about 1.4v drop (depending on transistors used)

rather than 0.7v drop from just a push-pull amp of a split 12v with virtual ground

Is this an interesting concept idea or could it actually work?

Last edited by realflow100; 10th February 2016 at 04:24 AM.
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Old 10th February 2016, 06:30 AM   #2
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Join Date: Nov 2007
Location: Dallas
I would not abuse an H-Bridge intended for PWM.
Crossing zero at audio frequency will be seriously
rough, class B or C, or perhaps burn up...

Biasing resistors serve a purpose, and removing them
will not increase efficiency. Lack of current control
resistors again why PWM H-bridge is not well suited.
PWM operate on the principal that inductance will
block the high frequency and control the current.

You propose to operate at audio frequency, where
inductance is not sufficient to control current.
So, you will need bias and current control resistors
everywhere, lots of them... Get used to it.

You describe an output stage, not an entire amplifier.
The closest workable circuit to what you describe is
called quasi-complimentary. While you are at it, you
might as well also look up "Sziklai Pair".

Yes, you can bridge. But doing so is a lot more
transistors than just 8... Maybe half a bridge you
could manage with 8, and DC blocking cap to GND
on the other "missing" side.

Last edited by kenpeter; 10th February 2016 at 06:53 AM.
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Old 10th February 2016, 07:49 AM   #3
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Join Date: Sep 2012
H-bridge requires 4 transistors. But looks like it switches the volts between +12v to -12v and 0v would be neither side of the bridge on if it was a 12v power supply (average not accounting for voltage drop)
Creating darlington with 4 more transistors to pair each transistor for more gain.
Why exactly would an HBridge not work with normal AC frequencies? is it just not setup for that? biasing resistor values massively need to increase in ohms with a darlington transistor vs single transistor right? just simply touching the + and base lead with my fingers on opposite hands would cause the transistor to output significant power for darlington pairs

my idea was taking the base leads and wiring them in a simple circuit to protect and attempt to make it work on AC signals. as well as biasing if neccessary. (darlington transistor pair may need to reduce gain with resistor in series with input signal but potentially need less biasing)

Last edited by realflow100; 10th February 2016 at 07:58 AM.
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Old Yesterday, 12:43 AM   #4
diyAudio Member
Join Date: Nov 2007
Location: Dallas
PWM H-Bridge is just not set up for that (current control through the zero crossings).
You will cross with no current (sounds bad), or too much current (burns up). These
can only be used above audio frequency, where an inductance controls the current.

Darlington pairs need 1.4V base bias to begin to conduct, and current control resistor
in series with each emitter to prevent conducting more than intended. A half bridge of
complimentary Darlington's will need at least 2.8V bias from base to base. And if the
base drive signal is not capable of slightly exceeding the power rails, you will lose the
bias voltage worth of output swing. And twice that much lost in a bridge...

Siziklai pair on the other hand, only needs 0.7V, and the usual thing is to replace one
of the Darlington's with a Siziklai, for 2.1V spread base to base... The reason we don't
replace both Darlington pairs with Siziklai pairs for 1.4V spread, is replacing only one
allows final output devices to be of identical type and easily matched. Only the helper
driving transistors need to be complimentary. We call that quasi-complimentary.

Will additionally need a resistor in series with each base. Prevents disaster, just do it...
Doesn't reduce gain substantially, it prevents the transistor from doing weird parasitic
stuff that you probably havn't read about yet.

Bias is the art and science of surrounding parts that provide optimal operating voltage
and current, and prevent unintended modes of operation. Without proper use of them,
your project will not behave as intended.

Last edited by kenpeter; Yesterday at 12:59 AM.
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