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Old 10th February 2016, 03:05 AM   #1
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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 03:24 AM.
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Old 10th February 2016, 05:30 AM   #2
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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 05:53 AM.
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Old 10th February 2016, 06:49 AM   #3
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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 06:58 AM.
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Old 10th February 2016, 11:43 PM   #4
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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; 10th February 2016 at 11:59 PM.
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Old 13th February 2016, 05:11 PM   #5
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OK scrapping that idea I've come up with a fairly efficient but simplified push-pull amplifier design from experimentation
its an odd design but it works on nearly any voltage from 3V to 19V

its a normal push-pull amplifier design with normal resistors and diodes but the interesting thing is i have the capacitor hooked from negative power rail to "in-between" the speaker and the transistors emitters
so the capacitor charges to half the power supply voltage. and the transistors allow the speaker to conduct more or less in either direction
and it also seems to be fine without an input coupling capacitor (allows very low input frequencies)
Output capacitance is 35V 11,000uF for more output power

it seems to be capable of driving a 75W 4ohm 6.5" subwoofer very well with plenty loud enough sound.

It uses two normal diodes and two 1K resistors and a PNP and NPN power transistor (40W each max rating)
if the resistors are 0.5K the sound is cleaner at louder volumes but wastes more energy and reduces the maximum actual power that can get to the speaker

Last edited by realflow100; 13th February 2016 at 05:18 PM.
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Old 13th February 2016, 09:22 PM   #6
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By your description, the capacitor not only charges up to the -ve rail voltage but also shorts the AC output of the associated device. Efficient? Can you not sketch and scan the circuit with component values or enter it into a schematic drawing or sim. program and post the image file?

Last edited by Ian Finch; 13th February 2016 at 09:25 PM.
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Old 14th February 2016, 07:26 AM   #7
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it doesnt get as warm as the previous design that i tried (after following a diagram that i had the components for)

now im going to try to describe how my own design works

I put the speaker between 0v and vOUT with a decoupling capacitor (speaker makes a small noise when being powered up or powered off)

The negative input wire is at vOUT for more maximum gain and positive at where it says "small bias"
This circuit works well for me and is plenty loud enough for a 75W 6.5" subwoofer
i have large heatsinks for both transistors so they can breathe and barely get warm
it seems to run the best between 7.5V and 15V
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Old 14th February 2016, 03:43 PM   #8
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Originally Posted by realflow100 View Post
.....i have the capacitor hooked from negative power rail to "in-between" the speaker and the transistors emitters
so the capacitor charges to half the power supply voltage....
Ok, so what is this capacitor? It can't be the output coupling capacitor because that should connect the output (from "in between") to the speaker +. The speaker return (-) is to ground.

BTW, using a simplified illustration of an amplifier as a working schematic for a power amplifier is not a good idea. The author at least, would be aghast at reading this thread by now . I assume that there is some voltage amplification going on somewhere else and you do have an actual amplifier where there is more going on than a little current buffering in a pair of output transistors.

If you want something that is elementary/simple and likely works efficiently, have a look at this 15W/3 transistor design and start with something that functions - then play with it and see if you can measurably improve it. That shouldn't be too hard, as you'll be starting with something that is really too simple: 15W audio Amplifier (using 3 transistors) ~ Easy Electronics
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Old 15th February 2016, 01:24 AM   #9
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Its a 11,000uF DC coupling capacitor between the middle outputs of the transistors. speaker in series with the capacitor and speaker to negative of the voltage supply. with positive going to capacitor from the speakers positive.
i wanted to try to cancel the need for split-rail power supply and this seems to work fairly well. Also I dont have my extra transistors yet. so i'm going to have to wait till upcoming thursday.
getting 20 PNP and 20 NPN's but they are low power and low voltage. so only good for increasing gain or making into a darlington/sziklai pair
or pitifully inefficient op-amp/preamp

I only have the two transistors at the moment.

Last edited by realflow100; 15th February 2016 at 01:28 AM.
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Old 15th February 2016, 02:51 AM   #10
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It seems like your wiring description was a bit confused - that's why posting accurate circuit diagrams or schematics is essential for meaningful discussion. In other words you have just the business end of a single supply amplifier output stage, like you find in most amplifiers from back in the early years of solid state. There are still lots of these around in the form of smaller, old amplifiers, discrete parts hobby kits and low power, class AB utility audio. You are in well covered territory there.

My question for your 2 transistor output stage is; where is the amplifier? There is no amplification in those transistors, just current gain. If you look at my link, the design is basically similar but adds voltage gain (claimed 40dB), making it a real amplifier. The BC548 shown is a low power, low voltage NPN anyway so no problem using one of your NPN transistors, if they are a known audio signal type.
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