Three transistor audio amplifier

This is the circuit diagram of a simple three transistor audio amplifier that can deliver around 100mW power to a 25 Ohm speaker. The diodes D1 and D2 provides a constant bias voltage for the transistors Q1 and Q2.The transistor Q1 works as a preamplifier. Transistors Q2 and Q3 drives the speaker. The type no of the transistor is not very crucial here. You can use any NPN transistor for Q1, Q3 and any PNP transistor for Q2.Any way the minimum collector current capacity of the transistors must not be less than 100mA.The circuit will work well with an 8 ohm speaker too, but the volume will be a little less.

Circuit diagram.



  • The circuit can be assembled on a Vero board.
  • A 9V PP3 battery can be used to power the circuit.
  • Loud speaker K1 can be a 25 Ohms one.
  • Capacitor C1 and C2 must be rated at least 15V.
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45 thoughts on “Three transistor audio amplifier

  1. farzan

    sir i’m using a 8ohm speaker, the output from the speaker is extremely low u cant even notice that its even working. what should i do?

  2. I have two question.....(1) can 16v rated capacitor replace the C1, C2. Because you said c1, c2 must be rated 15V. Then, secondly can we connect a speaker to the input of this circuit to make it a microphone?

    Pls can i use Electric Mic instead of condenser mic for the input of this Amp circuit?

  3. Seetharaman

    Hi the minimum voltage requirement is 15volt, above this voltage can be used comfortably. You can connect a condenser microphone not a speaker. speaker can be used with suitable matching circuit and pre amplifier.

  4. I have two question.....(1) can 16v rated capacitor replace the C1, C2. Because you said c1, c2 must be rated 15V. Then, secondly can we connect a speaker to the input of this circuit to make it a microphone?

    You said C1, and C2 must be rated 15v. But can i use 16v rated 470uf and 47uf… Then, can i connect speaker the input to use the circuit as a microphone?

  5. allan

    hi can i ask if how much capacity of an amplifier would i use in a 400W speaker??? and should i install a capacitor on my speaker??? how much capacity should i put on the speaker??? pls i need your advice…tnx

  6. seetharaman

    Hi Kiran being at Bangalore you will get it from Texonic. you can use BEL100P for Q2 and BEL100N for Q1. You can use BD135/136 2N2905/2N1711. etc etc even BEL187/188 also can be used.

  7. kiran

    can i use the same transistor 2n2905 insted of 2n3053 for my
    circuit?because i did not get it in sp road ,bangalore.
    please can you reply me fast.

    from kiran

  8. meydey

    Can i ask a question..
    our assignment is to make a combinational circuit of BJT and FET.. at first we are to submit an amplier circuit with three stages.. and we pass the circuit above.. ^^,
    now me must make that circuit a combination of BJT and FET..
    atlease one of the BJT transistor becomes FET..

  9. Bk Kwon

    hi i want to ask one question.
    in upper circuit, i wanna make sound controller
    i think that circuit need Potentiometer.
    so what do i have to do…?

  10. Seetharaman

    Hi RKH you can use a 10K potentiometer for volume control connect C2 negative terminal to slider of the potentiometer. one end of the potentiometer to common (emitter of Q3. now connect the input to common and and other (free) end of the potentiometer.

  11. RKH

    Hi, Seetharaman, is there any way to control volume? What resistor should I change? or is potentiameter? if so, where should it be positioned?

  12. Seetharaman

    Hi Raj, No major change is required.keep the same circuit assemble and change R1 to 390 ohms and change the Q3 base bias reistance R2 100K to 100k preset resistance. feed 4.5 volt (3 X AA cells) supply without any input signal adjust R2 till the Q1 Q2 emitter voltage with respect to common negative reaches 2.25 volt. now the amp is ready for use.

  13. Raj

    Hi Seetharaman,
    Thanks for the cct and your support to the questions.

    How could I alter this amp to work with 8ohm spk and with supply of 3AA batteries?
    Thank you.


  14. ruben

    Something must be screwy lol. Theres still no difference with a 100PF capacitor between Q3’s base and collector.

  15. seetharaman

    Hi Ruben Sorry 100PF should be between collector and base of Q3, it will definitely solve your hf oscillation problem.

  16. ruben

    Thanks for replying. The diode polarities are correct. I put a 100PF capacitor between the collector and emitter of Q3, but i didn’t notice a difference. for C2 I was using a 3.3uF capacitor instead of 4.7, but I don’t think it makes a difference. and I did hook up a disc capacitor (not quite 470PF) between the base if Q3 and ground. I was guessing that ‘common’ meant ground. Correct me if I’m wrong. It still is basically the same as before. Do you have any other ideas as to what I am doing wrong? Thank you again for your help.

  17. seetharaman

    Hi Ruben Check the polarity of D1 and D2, fix a 100PF ceramic capacitor between collector and emitter of Q3. connect 470 PF between base Q3 and common. C2 is 4.7uF

  18. ruben

    Hi and thanks for posting the circuit here. I am having a bit of trouble with it tho, and I was hoping you might be able to help me. I’m pretty sure I’ve got the circuit wired properly but when I turn the power on I get a lot of crackle and static so that you cant here the song properly. I can tell that the amp is trying to play the song, but there is so much interference or something that is hindering it. I’m wondering if you know the reason why. I am using transistors that I pulled off a random circuit board, but they are the right type. I’ve switched a couple of the transisors, but I still get the same noise, even tho it does sound a bit different. I was using an 8 ohm speaker, and I changed that to 25ohm speaker (three 8 ohms wired together) and I didn’t see a difference. From my litle experience with building circuits before, it sounds like I’m missing a connection or two,but I’ve double checked the circuit and I don’t see an error. Do you have an idea of what I’m doing wrong or how I could fix this. Thanks so much for all your help.

  19. seetharaman

    Hi thomas. your output requirement is 10 watts into 8 ohms. v squ = Z X power = 80. V = 9 volt rms hence pk is 12.6 pk to pk is 25.2 volt drop in both the output transistors say 5 volt you require 30 volt DC or plus minus 15 volt. may be an operational amplifier TL071 with two complimentary power transistors like TIP122/127 you can achieve the requirement

  20. Thomas Djunke

    Hi I’m a designing an audio amplifier that has a voltage gain of 100, provides an output of 10 W and is connected to an 8 ohm speaker.

    Would you be so kind to help me with the values of the components please?

    Best Regards

  21. seetharaman

    Hi Ziad it is a low power head phone amplifier (general purpose micro power amplifier) nearer to class AB (almost tending towards class A)

  22. seetharaman

    Hi Mina R2 is the bias resistance for Q3, as well half supply stabiliser at the emitters of Q1 & Q2.

    Yes. but you have to add a capacitor of 220pf between collector and base of Q3 and 1Kpf between Q3 base and ground. Ensure that the amplifier is properly screened so that the RF from the mobile does not pierce in.

  23. seetharaman

    Hi Joanna it is a simple amplifier for general purpose use. in fact it is a head phone amplifier with good quality and enough power.

  24. seetharaman

    Hi Islam the amplifier is designed for 100mw power. that is with 25ohms load it requires a drive current of around 70mA. The out put trasistors will be cool running. If you want to use anyother impedance speaker you have to provide heat sink/emitter resistance to output trasistor. Matching transformers are not recomended for these simple circuits, the indutive load may kill the output transistors. Two numbers 8ohms speaker can be connected in series as load the current will raise to 150mA. heat sink for the output transistors are a must. the power output will be 300mw

  25. islam

    sry.i want to asking about something

    how can i do matching with speaker to the circuit?
    by transformer or how???????
    plz.answer me

  26. seetharaman

    Hi Esther Function of R2 is to get half the supply voltage at the junction of Q1 & Q2 emitter. Yes you can replace with a potentiometer to adjuct half supply voltage and seal it.
    If you want a volume control connect 22K or 10K pot with slider to C1 negative end. cold end of pot to common and input signal between common and hot end of the pot. Without input signal if the out put current is high use 100 to 200 ohms resistance in parallel with D2 to bring it down to 5 to 7mA

  27. Esther

    can we replace the 100K resistor with a 100K potentiometer, and what are the power ratings for the resistors


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