# class ab amplifier, 30 watts rms output

Discussion in 'The Projects Forum' started by yhandz, Feb 24, 2009.

Feb 24, 2009
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2. ### mik3 Senior Member

Feb 4, 2008
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It would be better to use an op amp for the input differential stage since it is more precise than two discrete transistors.

3. ### Audioguru Expert

Dec 20, 2007
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With a 12V supply, the amplifier's output into 8 ohms at clipping is only 1.3 Watts, not 30W.
With a 25V supply, the amplifier's output into 4 ohms at clipping is only 13 Watts, not 30W.

For an output of 30W at clipping into 4 ohms the power supply must be about 36V and the transistors must have very high current gain.

4. ### yhandz Thread Starter Member

Feb 24, 2009
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actually.. our professor wants us to get the 30 watts output

5. ### studiot AAC Fanatic!

Nov 9, 2007
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Your configuration is sometimes called half-bridge.

With this type of configuration a rule of thumb is

Power out (watts) = V$^{2}$ / 8R

Where V is the power rail voltage and R is the load impedance.

You need higher voltage rails or a full bridge configuration to achieve the power levels you seek.

6. ### Audioguru Expert

Dec 20, 2007
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The very simple amplifier circuit doesn't even have bootstrapping so its output will have a total loss of at least 5V from the rails.
The TIP41 and TIP42 output transistors have a minimum current gain of only 12. So at 4.25A peak into 4 ohms (30W RMS) the base current is a whopping 354mA and driver transistors are needed.

Maybe a 48V supply can be used and then it will produce 30W into 8 ohms and the base current will be less than half.

I wonder if the supply is actually plus and minus 25V for a total of 50V?

Feb 24, 2009
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8. ### Audioguru Expert

Dec 20, 2007
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Philips used that circuit in a portable radio 54 years ago. You won't find suitable transformers for your circuit today.
It has no negative feedback so it will be very distorted.
The transformers will limit the high frequencies so it will sound muffled like an AM radio.

9. ### yhandz Thread Starter Member

Feb 24, 2009
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so can you make a schematic diagram for me?

10. ### studiot AAC Fanatic!

Nov 9, 2007
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This is a big subject.

Since you are studying you should be practised in reading other peoples's text.

So read my last post again. I gave you the solution.

If you think about it, the maximum power you can extract from a 12 volt rail into 8 ohms directly is 144/8 = 18 watts at DC.

I expect you prof wants you to review and study amplifier types. You will perhaps realise class D can approach this.

There is one type of amp which can do better. This is known as a full bridge amp. With this you can switch the whole 12 volts alternately back and forward (in polarity) across the 8 ohm load, effectively doubling the supply rail.

A quick look at Google shows plenty of long articlkes availbale about this subject.

11. ### yhandz Thread Starter Member

Feb 24, 2009
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i need class ab amplifier

12. ### Audioguru Expert

Dec 20, 2007
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Did you read about the classes of amplifiers?
Class-AB is when the output transistors always conduct a little so they do not create crossover distortion.

The first circuit you posted operates in class-AB if the diodes match the output transistors. But its supply voltage is too low for an output as high as 30W and it does not have enough transistors for the output current required for 30W.

13. ### eblc1388 AAC Fanatic!

Nov 28, 2008
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I always thought that the output transistors has to conduct a lot in order to eliminate crossover distortion.

But this will make them "Class A" by definition.

14. ### Audioguru Expert

Dec 20, 2007
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With class-B the output transistors are turned off when the output voltage is at idle and from +0.7V to -0.7V. So the resulting missing 1.4V of signal is crossover distortion.
The transistors need from 10mA to 50mA of idle current so that the output signal can shift smoothly from one transistor to the other.

Class-A has many Amps of idle current.

15. ### eblc1388 AAC Fanatic!

Nov 28, 2008
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What I meant is amplifier with class-AB operation, even with 10-50mA idle current, still suffer from crossover distortion.

The small output idle current will help to reduce cross-over distortion but not eliminating it.

16. ### Audioguru Expert

Dec 20, 2007
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Rod Elliot of Elliot Sound Products designed many amplifiers. His simple 60W amplifier has a few mA of idle current and its distortion is 0.05%.

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Feb 24, 2009
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18. ### thingmaker3 Retired Moderator

May 16, 2005
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There are several free software packages with which you can make a PCB layout from a schematic.

19. ### kuracha19 New Member

Jan 10, 2009
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Hey, Guyz can u give me a booster for a 100 watts power amplifier plz. with the schematic diagram and the pcb lay-out if its awkie with u!?
Thank u very Much for the Help!!