Today we are going to learn electronics by designing a 3 transistors amplifier circuit in a simple way. You will be able to design your own amplifier circuit with a driving power of 1W to 7W. But if you want a higher driving power amplifier, you can do that without much inconvenience.
The objective of this article is just to learn, in the simplest way possible, about designing a power amplifier circuit. As a result, we will not be looking into in-depth calculations like power loss, component deficiency, and so on.
However, we hope that you find this article useful and that we can learn more together in the future.
Designing the Circuit
Next, let’s see the general circuit design we are going to use this time. As you might have noticed, the electronic circuit is quite similar to the Small Signal Amplifier Circuit. The Q1 is configured as a Class A amplifier, which has high gain but low working power.
So we add two more power transistors, Q2 and Q3. These two transistors work in a push-pull configuration, a Class AB amplifier.
Class AB amplifiers have an interesting point in that D1 and D2 need to be properly configured to bias Q2 and Q3.
Picking Transistors
We can roughly assume that if we want the power of 10W at a voltage of 20V, we should go with the power transistors in the range of TIP41 (NPN) and TIP42 (PNP) in the package of TO-220.
As for Q1, we pick small transistor, a small-signal transistor is used for voltage gain, followed by complementary power transistors for current drive.
Our goal is to find the values of R1, R2, R3, and R4. We can do that through the principle that states that its resistance is equal to the voltage drop across it divided by the current flowing over it.
The preamplifier
Although this stage looks simple, it largely determines how the whole amplifier behaves.
Small choices here have a noticeable impact on stability and sound.
The detailed biasing choices and measurements are documented separately.
→ Further notes are available on Patreon.
Finding the Value of Resistors
We will find the value of each individual resistor. Resistor values were selected to establish stable operating points across stages.
About Capacitors
In the design of the circuit, we use the same train of thought as in the last article. So for the capacitors, we will not look too much into the way of finding their capacitance.
The C1 and C2, which are both coupling capacitors, needs to respond well to low frequencies and have a low XC value to minimize the current loss in our AC signal.
The C3 is a decoupling capacitor that increases the gain in an AC signal and functions as if it were a very low-resistance resistor.
C4 is a noise killer. A transistor amplifier circuit larger than 1W might require C4 to lower the chance of our circuit turing into a high-frequency generator or an oscillated circuit.
C5 will reduce voltage spikes or noise from the power supply. Normally, the noises have a very high frequency, more than 1MHz. The noise will think that C5 is a very low-resistance resistor, so it will flow through C5 to the GND.
C6 will keep the current smooth during high power usage. Like when we play very loud music abruptly and our amplifier circuit requires a high amount of electricity in a short period of time, causing the sound level to drop.
The C6 will function similarly to a tank, storing electricity and improving the efficiency of the circuit.
This article reflects how I currently think about simple power amplifier design. I keep ongoing notes and experiments separately, for readers who enjoy following the process rather than just the result.
Testing
Basic DC checks and signal testing were performed to confirm proper operation.
If the input signal is lower than this, the sound output will be inaudiable, and we would need to run it through a preamplifier first.
We tested it with a 1KHz sine wave signal, and the sound output sounds great.
But if the input signal is too high, it will cause the output signal to have a distorted waveform. So when actually listening to the real music, it would be audiable but severely distorted. From our testing, the highest value without distortion is 15Vp-p.
Conclusion
We tested this circuit throughout, and it is quite usable. Proving that we can actually make a transistor amplifier circuit in a simple way, even though it is only suitable for a low-power output.
But if we want more power, we can switch out the transistors to the ones that can withstand higher power, increase the power supply, and increase the bias current of the transistors.
And we see that the quality of the transistors is very important, especially Q2 and Q3. They need to have as similar electrical characteristics (namely, hFE and IC) as possible.
Also, when we set a higher current and voltage target, the power loss within the circuit also increases. Currently, just the bare circuit, even without load, already requires around 100mA to operate.
If you are interested in this type of circuit or want us to go into a more complicated subject, please let us know.
👉 Continue Reading
Prefer reading offline or exploring the full design process?
The complete notes — including expanded schematics, design decisions, and experimental observations — are available in the eBook on Patreon.
Self-taught and driven by curiosity since childhood. I solve electronic challenges by getting my hands dirty at the workbench. My approach is simple: experiment, learn, and repeat.
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This is wonderful. I just can’t wait to put my hand on and go through every step of the calculation relating to this design.
Thanks very much ???? ????.
Hi, John
Thanks for your feedback. I am happy you like this post. If you finished it please it for us.
Have a great day.
Boa tarde .
Grato pelas iniciativas culturais até aqui .
Peço e agradeço , seja mostrado com igual simplicidade , o circuito que reproduziria dele o som 2.1 .
Como se fossem 3 circuitos desse sintonizados .
Seria Estéreo com reforço independente dos Graves .
Grato pela atenção !
Hi,
Thank you for your opinion. Next time, we will experiment with a bass-treble tuning circuit using RC. It was interesting learning about XC calculations. We think it will be useful for learning electronics. However, if the content is not what you expected, we apologize.
Why this? Arbitrary?
ID is 10mA; The ID is the current flowing through R3, D1, D3, C-E of Q1, and R4.
And this? Arbitrary too?
VE (VR4) is 0.3V; Voltage drops across R4 or VR4
How to choose if so?
Hi Kuwer,
First of all, I thank you for your question.
The concept for designing this circuit. We use experiments and make predictions based on our prior knowledge. ID initialization is important and VE has a lot to do with circuit performance.
Of course, this circuit may be more efficient. Suppose using the most appropriate equipment value. This will be the next step of our learning.
In conclusion, we test to meet the results (various voltage points) and the device does not get too hot according to available resources.
Thanks again.
Apichet
Ok. Thank you.
Bom dia Apichet !
Peço e agradeço , se oportuno , voltar ao meu questionamento de 9 de maio sobre o áudio 2.1 .
Só você ou a sua ordem com tanta informação , pode a mim e a outros ajudar a esclarecer essa esquecida arquitetura .
Será que é porque não tem expressão comercial ?
Good evening
First of all, I apologize that I may not be able to answer all your questions because I am not an expert on amplifiers. I made very little of it. Because of past experiences, My family (my mother) doesn’t like high-wattage amplifiers. because of the loud noise So I don’t have much experience with it. But I like learning about electronic circuits.
As for the 2.1-channel amplifier, I haven’t seriously tried building one yet. For now, we will focus on learning simple electronic circuits.
Is this a post? https://www.eleccircuit.com/simple-delay-speakers/#comment-359706
Thank you for visiting the website and your support.
God bless you.
Hey,
I’m finding your simple circuits really useful and informative to learn about electronics, keep up the good work!
I’ve built this amplifier on a breadboard and it sounds pretty good. However, I have a serious problem with it as it is burning out the output coupling capacitor (C2): literally, they’re hot, the tops are starting to bulge and one even went pop!
Any ideas or suggestions? The test voltages are pretty close. I measured the hFE of Q2 & 3 (TIP41/42) and they aren’t very close: 98 & 124. Is that a problem?
Any help would be appreciated, I’d like to sort it out so I can get it on a permanent strip board for use.
Hello,
First of all, thank you for your interest in building this circuit. It is very suitable for learning simple audio amplifier circuits.
But I’m a bit saddened that you can see the results of capacitors like this.
I would like to share my experience with Electrolytic capacitors. It will not withstand voltage levels higher than specified and if connected to the wrong polarity, it will be damaged as well.
In this article, I admit my mistake. There is no component list: because I thought we had suggested this. There are many articles such as: https://www.eleccircuit.com/what-is-capacitor-principle-working-types/#Electrolytic_Capacitors
Hope this experience Might be useful for you.
I sincerely apologize for wasting your time.
Thanks for your quick reply.
You’re not wasting my time ????: I’m learning and blowing up the odd capacitor is part of the fun ???? (always wear safety glasses !)
I think I may have spotted the problem? On the 2 schematics the polarity of C2 has negative connected to the output of Q2/3 and positive to the speaker+, Is this an error?
The first simpler diagram has it the other way round, as it is also on the TIP41/42 version of your 4 transistor amp: https://www.eleccircuit.com/4-transistor-audio-amplifier-circuit/
Hi,
Great! My kids don’t really like wearing eye protection. It may be dangerous from those accidents.
That’s right. Current will always flow through the transistor, through the capacitor, to ground through the loudspeaker (load), so it will always receive positive voltage first.
Have a good day,
Hi again,
I’ve built this amplifier up on stripboard, attached a sizable heatsink and put it in a nice box. I’m running a Bluetooth card into it and driving an old Aurotone speaker. It sounds great, I love it’s sound.
The only problem I have is that if I leave it on, but not playing music, it gets very hot. I can play music quite loudly for hours, the heatsink is warm but the outer casing remains cool to touch; if I leave it idle for 10-20 minutes the casing is very warm and, upon opening it, the heatsink is too hot to touch!
Any insights or suggestions would be appreciated.
Still loving this little project ????
Hi,
I am glad you had a great experience. I can see the atmosphere of listening to soft music in the room and sipping hot tea in the afternoon, it is one of the joys of us electronic amateurs, right? 🙂
God bless you,