BC547 Amp help

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Sulvek

Joined Aug 27, 2022
20
Hello all! I made the amp in the schematics attached. I'm aware it is a bad design and am only doing it for learning.

First thing I need help understanding is if I replace the 10k resistor with a jumper there is no music. I thought it would be much louder and probably blow the speaker since I'm not getting enough resistance. Why is there no music?

I know I should be looking at other designs but I really enjoy the simplicity of this design and am curious how I can upgrade it. I would love to improve the sound quality just a tad but keeping the transistor cooler is my main concern. I don't imagine the transistor would last to long if left on due to the heat. More than anything I would like this circuit to work indefinitely without self destructing.

Any other info would be great. I have seen some other designs that use two BC547's that I'm going to try out shortly just to see the difference in performance and then move up the latter. Would be great to have a place that shows the latter up in amps starting from simple circuits like this and up to professional designs you would see in an actual product. Thanks for any help!
 

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AlbertHall

Joined Jun 4, 2014
12,157
Welcome to AAC, @Sulvek
With the 10k shorted the transistor is fully the turned on so there van be no music.
To protect the transistor the 19k should be increased to at least 47k. It will be quieter but qill last longer.
 

WBahn

Joined Mar 31, 2012
27,395
When you short the resistor, you turn the transistor into a diode resulting in max DC current through the speaker with no way for the audio signal to modulate it in any meaningful way.

As you say, this is not a very good design. But there are some things you can learn from it. Adjust the resistor size until, with no music, the voltage across the transistor is about 4.5 V. That will give you the largest potential signal swing across the speaker. There will be a significant DC current flowing, which will not only waste power and drain your battery, but possibly damage the speaker if it's not up to it. Ideally, you want the speaker to see no DC current, only an AC signal, but that's not much of an option here. Also, with that DC current, the transistor is going to heat up and may require heatsinking, depending on how stiff the 9 V supply is and what the DC resistance of the speaker coil is.
 

Audioguru again

Joined Oct 21, 2019
5,166
If you have a speaker with an 8 ohms impedance then its resistance is about 7.2 ohms.
For the maximum output signal then the collector of the transistor should be about 4.5V (half the supply voltage) then the idling DC current in the transistor is (9V - 4.5V)/7.2 ohms= 625mA and will be 1250mA on output peaks.
The datasheet of the BC547 transistor shows that its maximum allowed current is only 100mA and it works poorly above 50mA.
You need an audio amplifier circuit or IC not a single weak little transistor.
 

MisterBill2

Joined Jan 23, 2018
13,128
The circuit is what is known as a "Class A " amplifier.
Following a recipe that provides no explanation is not a way to learn how electronics works.
Both Bertus and A.G. provide correct information, but not quite enough of it.
 
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