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Class AB push-pull amplifier design help

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rgv

New Member
Hello guys, I'm a newbie here

Could you give me some tips or advices on how to design a Class AB push-pull amplifier? It is required that Vcc = 12 V, load resistance is 8 ohms, and power delivered to load could range from 1~5 Watts.... I just don't know what's the most suited complementary transistors to use.. also I don't know what kind of diodes to use to match those to the transistors( to produce current mirror) and what values of resistors should be used.

any help would be appreciated.. thanks :D
 

RCinFLA

Well-Known Member
There are plenty of schematic examples on web.

Simplest uses NPN-PNP emitter follower output stage but this gives up slightly more then two diode drops of output voltage swing.

More difficult to bias but enabling rail to rail output swing uses NPN-PNP collectors feeding output.
 

audioguru

Well-Known Member
Most Helpful Member
With a 12V supply, a simple amplifier will have an output power of 1.5W into 8 ohms at clipping.
 

rgv

New Member
^ thanks for the schematic, but that design looks too complicated for me..

as much as possible I want to design my own simple circuit, with all values of resistor, diodes to use and transistor to use coming from my own wit... I just need some guide on how to choose the values guys

..the design mentioned by RCinFLA was what I am talking about: A single supply push pull amplifier using two complementary transistors, two diodes to match the transistors, and two resistors with same value in order to produce current mirror.

..I am actually planning to use BC546/BC556 complementary transistors, but what type diodes should I use? as for the resistors, I am planning to use a high value of R1 and R2 to at least increase the input resistance a bit because I will connect this power amp to a multistage preamp (two cascaded CE amps, which requires high output resistance).. also is there a chance of increasing the power delivered to the load if I will couple the power amp to a CC amp (to minimize loading effects)? power delivered to the load greater than 1 watt is enough for me, but 2 watts is better..


I need your tips guys.. I don't know if what am I thinking is correct
 
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Jony130

Active Member
Look at this simple schematics
 

audioguru

Well-Known Member
Most Helpful Member
Why didn't you calculate the peak current in the transistors before selecting BC546 and BC556 transistors that will blow up with that much current?

With a 12V supply, the peak current into an 8 ohm speaker is about 4.5V/8 ohms= 563mA.

Then look at the datasheets for the BC546/BC556 to see that their max allowed current is only 100mA.
Also look at the datasheets for the BC327 and BC337 to see that their max allowed current is 500mA to 800mA for some manufacturers.

The very simple amplifier posted by Jony will produce about 1.5W at clipping into 8 ohms if its supply is 12V. If you want more power then you must replace the BC327/BC337 transistors with power darlington transistors (or make them yourself) and increase the supply voltage.
 

BrownOut

Banned
If I were building a discrete AB amplifier, I would take Jony130's design and replace the two base voltage setting diodes with a single VBE multiplier with a variable resistor to set the voltage. That allows precision setting of the quiscent current of the output transistors. I've built a number of these amplifiers, and that method works nicely.
 
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