Three transistor audio amplifier

john April 7, 2010 50 Comments

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.
  • December 2, 2015

    can i use tip41c in place of 2n3053???

  • hina
    December 2, 2015

    can i use tip41 transistor in place of 2n3053???
    plzz reply

  • farzan
    October 24, 2014

    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?

  • Az
    September 15, 2014

    I see 9V+ but not 9V- ???

  • May 19, 2012

    sir i need calculation of this circuit

  • Seetharaman
    April 13, 2012

    Hi Please read my comments you can understand. hope it is in simple english

  • 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?
    April 11, 2012

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

    • bijay pandeya
      June 30, 2015

      yes you can connect any capacitor having volatge range greater than 15 v.

  • Seetharaman
    April 4, 2012

    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.

  • 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?
    April 3, 2012

    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?

  • wilbox
    February 24, 2012

    how does this system operate.? can you please explain how currents and voltage run.??

  • Leonardo Cimacio
    February 19, 2012

    Sir,may i ask something??if u connect a usb reader to this circuit may i know how to connect on this sir?

  • allan
    September 30, 2011

    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

  • seetharaman
    July 27, 2011

    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.

  • vysakh
    July 25, 2011

    i think
    you can use 2906 instead of 2905

  • vysakh
    July 25, 2011

    no,you can’t use as a replace. one is a pnp and another is npn transistor.

  • kiran
    July 25, 2011

    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

  • Darexs
    April 17, 2011

    pls i need fm transmitter circuit diagrams thanks

  • meydey
    March 24, 2011

    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..

  • hager
    March 19, 2011

    please can u give me more details obout circuit explaining more ,thanks .

  • Seetharaman
    December 4, 2010

    Hi BK Kwon please refer my comment dated 30th nov. use 10k pot.

  • Bk Kwon
    December 3, 2010

    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…?

  • Seetharaman
    November 30, 2010

    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.

  • RKH
    November 30, 2010

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

  • Seetharaman
    October 15, 2010

    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.

  • Raj
    October 14, 2010

    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.


  • seetharaman
    August 2, 2010

    Hi Ruben your transistors may be at fault, my prototype is quite good, try by replacing them.

  • ruben
    August 1, 2010

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

  • seetharaman
    August 1, 2010

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

  • ruben
    July 31, 2010

    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.

  • seetharaman
    July 30, 2010

    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

  • ruben
    July 30, 2010

    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.

  • seetharaman
    June 6, 2010

    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

  • Thomas Djunke
    June 5, 2010

    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

  • seetharaman
    May 19, 2010

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

  • Ziad
    May 18, 2010 that a power amplifier ? … if it’s a power amplifier is it class B or class AB ?

  • seetharaman
    May 18, 2010

    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.

  • mina
    May 18, 2010

    can i use this circuit with input of the mobile?

    • junaid
      June 11, 2015


    • bijay pandeya
      June 30, 2015 nan connect.there is no matter you can use any audio sound

  • mina
    May 18, 2010

    why we use r1 , r2 ?

  • seetharaman
    May 17, 2010

    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.

  • Joanna
    May 16, 2010

    what is the solution for designing this circuit?

  • seetharaman
    April 18, 2010

    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

  • islam
    April 18, 2010

    sry.i want to asking about something

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

  • seetharaman
    March 14, 2010

    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

  • Esther
    March 12, 2010

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

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