This article is a list of various types of amplifier circuits built using Transistors alone.This includes a Class AB amplifier using 4 transistors, then we have a headphone amplifier circuit and finally a low power amplifier using transistor. This article is solely dedicated to publish more transistor amplifier circuits. So you may keep visiting this post in future for more updates.

List of  Transistor Amplifiers

1. Class AB 4 Transistor Amplifier

2. Headphone Amplifier using Transistors

3. A Low cost amplifier circuit  using Transistor

These are circuits that can be used in a lot of low scale applications. The main feature of these circuits is that they all are just basic designs and the components used can be easily obtained from your junk box. More over the value of the components are not very critical and slight variations in it will not affect the performance.

1. 4 Transistor Class AB amplifier.


A very simple and easy to build class AB audio amplifier using four transistors is shown here. In class AB operation each output device conducts more than half of the input signal cycle. Up to 78% efficiency is possible with class AB designs and cross over distortion is reduced. The circuit shown here is suitable for small radio receivers, audio players, intercom, telephone etc.

Transistor Q1 with its associated components is wired as a pre amplifier stage. The audio input is coupled to the base of Q1 through resistor R1 and capacitor C1. Resistor R3 provides collector to base bias for Q1 and C3 is an AC by pass capacitor for the collector resistor R4. Collector to base biasing is a good method of biasing for circuits like this as it provides enough negative feedback, prevents thermal runway and stabilizes the operating point. The second stage is the driver stage for the push pull pair. Q2 with its associated components perform this job. This stage is also collector to base biased and its input is coupled to the output of the preamplifier stage using capacitor C2. Resistor R8 limits the collector current of Q2.The third stage is the class AB push pull section comprising of transistors Q3 and Q4. Diodes D1 and D2 provides the bias voltage for the push pull stage. The output of the amplifier is coupled to the loud speaker through the capacitor C4. C5 and C6 are power supply filter capacitors.

Circuit diagram of 4 transistor amplifier.


  • The circuit can be assembled on a vero board.
  • K1 can be an 8 ohm/5W speaker.
  • C6 must be grounded near to the Q1 and C5 must be grounded near to the loud speaker ground. This reduces noise.
  • Use 5V DC for powering the circuit.

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2. Class A Headphone amplifier.


This is the circuit diagram of a Headphone amplifier operating in the class A push pull mode. In class A mode the output device (transistors) conduct over the entire input signal cycle. The maximum possible efficiency for Class A operation is 50% and it further reduces when capacitive coupling is used. But the advantages of Class AB amplifier are no cross over distortion, high fidelity and low harmonic distortion. These amplifiers are most suitable for low power applications.

In the circuit transistor Q1 works as the preamplifier. Resistors R6 and R7 provides potential divider biasing for Q1. Audio input is coupled to the base od Q1 through capacitor C2, resistor R9 and POT R10. Emitter of Q1 is coupled to the base of Q2 through resistor R3. Diodes D1 and D2 provides bias voltage for Q2. Base of Q3 is directly coupled to the collector of Q1. Resistor R5 limits the collector current of Q2 and Q3. C4 and C5 are power supply filter capacitors. Output of the amplifier is coupled to the head phone using the capacitor C3.

Circuit diagram of head phone amplifier.

head phone amplifier
Head phone amplifier Class A



  • The circuit can be assembled on a vero/perf board.
  • Power supply can be anything between 6 to 24V DC.
  • I used 12V DC for powering the circuit.
  • Z1 can be a 100 ohms or higher head phone.
  • Electrolytic capacitors voltage rating must be according to the supply voltage you use.

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3.Low power transistor amplifier circuit


A very simple low power three stage transistor amplifier circuit is shown below. First and second stages are pre amplifiers and third is the output stage. Collector to base biasing is employed for each stages and it is enough for circuits like this.

Collector to base biasing: Consider the first stage.R1 is the collector resistor and R2 is the base resistor. This type of biasing provides some negative feedback, avoids thermal runaway and stabilizes the operating point. When the temperature increases the collector current also increases. This increases the voltage drop across the collector resistance R1 and this decreases the voltage drop across base resistor R2. As a result, base current drops which in turn drops the collector current and thermal runaway is avoided. Stabilization of operating point is achieved by the prevention of variation in collector current with temperature.

When a transistor amplifier is operating, the collector voltage will be phase opposite to the input signal. Since base resistor R2 is connected between collector and base, some fraction of the phase opposite collector voltage is fed back to the input (base) through the base resistor R2 and thus negative feedback is achieved. Negative feed back reduces the voltage gain of the amplifier but it improves the stability.

In the circuit collector of Q1 is coupled to the base of Q2 using the capacitor C2. R3 is the biasing resistor for Q2 and R4 is its collector resistor.Collector of Q2 is coupled to the base of Q3 using the capacitor C3. R5 is the biasing resistor for Q3 and speaker is connected as the collector load. Capacitors C4 and C5 are filters. C1 is the input DC decoupling capacitor.

Circuit diagram of low power transistor amplifier.

transistor amplifier
Low power transistor amplifier



  • The circuit can be assembled on vero or perf board.
  • Use 9V DC for powering the circuit. A 9V PP3 battery is fine.
  • All electrolytic capacitors must be rated 10 or 15V.
  • An optional protection diode can be added in series to the positive supply line. This protects the circuit from accidental polarity reversals.
  • Type number of the transistors are not very critical. Replacement will not affect the performance to a great extend.
  • Powering the circuit using a mains adapter will induce some noise.
  • Speaker impedance can be 64 ohms or more.

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  1. Ivan Law

    How volts are the capacitors used for the 1st circuit?

  2. where is the volume knob for the AB amp

  3. hii,, bro,,, what is function of diode in Circuit diagram of 4 transistor amplifier? and what if its without those 2 diode?

  4. Hi sir! Can I get the manual calculation of the class ab 4 transistor amplifier? Please? Thanks! ASAP!

  5. the 4 transistor amp is a bad design.
    it clips with an input more than 1mV
    with 1mV the output is 30mW … with 7.5% THD
    it is easy to do much better.

  6. Hi,
    Does the Transistor headphone amp work?
    Do I need to change any values to use 32ohm headphones?
    I need to use a 50k vol.pot preferably directly on the input (my USB-DAC has Edcor signal transformers instead of opamps on the output). Do I need to adjust anything to do this?
    I have a bunch of 12-0-12VAC transformers (1500mA), if I build a headamp with this schematic…should I set Vregs (LM317) to 10Vdc? They work better with a bit of headroom right?

  7. Has anyone got the AB amplifier to work? I have designed it about 3 different times and had no luck. Some people said they did, others said they did not.

  8. i make the crkt 1 but it doest work,,,i have an output of stable 4Vac,,what should i do???and Q3 have higher input about 2vdc rather than the other transistor,,pls reply..

  9. phillip sinyinza

    am glad i finally found this site for my education and knowledge acquiring.thanks to you developers.

    • Use The Transistors That are Listed 2N2222 and 2N2907 are Small signal Transistors While The 2N3055 are High Powered Transistors And Must Be heat sinked. The configuration layout for the 2N2222 is To-92 and the configuration layout for the 2N3055 is TO-3

  10. Hi everyone
    I constructed the circuit of the 4 transistor class AB amplifier and it worked well. I just have a small question;
    Why is C3 used to bypass R4? I mean why do we need to bypass it? and why to bypass it with such a small capacitor?
    Thank u in advance

    • hi parsa……
      are you sure that 1st circuit work correctly?

  11. the class AB circuit doesn’t work, te currents are correct but the load isn’t. I don’t understand why people will publish this when it ain’t working.

    • I was build 4Transsistors amp at first time and it works well.
      the power output should be around 3W cause of 2N222 can’t amplifier over 600mA through Ic-e
      I connect the power source by using old USB cable with my laptop sometimes by battery.
      Just add it with tone control by my own RC frequency cut-off circuit that can control around 20Hz-16KHz

      I found problem that is
      C4 has block the output so I bring it out.And Short the circuit it works.
      In my country doesn’t have 15uF electrolyte caps( C2 ) so i use 10uF instead.
      C3 i use my old fans caps. D1-2 i use my old radio diodes.
      it can drive through 8OHM speaker better than 6,4,3.5 and 2 OHM
      i got some noise when i raise up BASS.the other are great!

  12. Hi , I just constructed Circuit diagram of low power transistor amplifier
    the last one from this page…I don,t know why , my speaker pulsing without any signal…when I apply signal from my guitar pickup to see if the amp working, the sound coming out very noisy…after the vibration of the string going down a little, the amplifying effect stops and the rhythmical pulsing going further ( like a heart)…

    Why this happening? Please give me an advice where to look for problem in this schematic?


  13. Hi all, I have a hearing impairment which makes listening to tv frustrating for my wife and I as I need it louder than normal, but, when the volume alters from soft speech to loud music etc we are constantly battling with the remote, is there such thing as an auto volume adjuster amplifier to keep the tv sound on a constant level to bring relief to my wife and I.
    Many thanks in anticipation.

  14. where should I connect c3? across r3 and at the diode? or across r3 to r7?

  15. In the first ckt, C3 should be across R7 and about 100pF, C2 can be 1 mF, and R9,R10 should be 1/2 to 1 ohm. K1 will only see about 1/4 watt at 5V supply.

  16. Peltier cooler. Some surplus dealers have them, but they ain’t cheap. You might find one junked somewhere. If the cool ing unit doesn’t work, take it apart; sometimes if one of the little (1/8th inch long) elements is corroded; you can jump across it with a wire.


    An eny one have a idea about light witch is giving by sun?O K,I will give a idea but i dont know how it get,enyway derect sun light can it get u r room ?Yes i think using 2 pc of magnificent glass or one reflector tyoe glass and sutable fiber optics cable.Yes may be im pool but sumone who knos about transfering light withen fiber optic cable and to be manufact the device to joint magnificent glass and fiber optic cable.
    Do u think it will work? why not think about how satalite antenna working.Out side audio vedio signal come to TV withen electric cable.Cannot wefollow this theory for this matter.I hope some day some one will do this type of lighting system,Im sure.what u think?Plese write u r comments.Im inviting to every body who intrusted about this. Thank u circuits today.


    Hi,Wish u a good day every one.I need to know what is the construction of portable 12v dc powered cooler.And what is the name of tha device which is giving cool & heat,when we give the power.

  19. In the first circuit the capacitor should be placed across R3 not R4.

    • Jackson Elfers

      Why would you need a bypass capacitor across R3? It wouldn’t be doing any good, right?

      • At high frequency the capacitor shunts the resistor and decrease the load impedance of the transistor, reducing the gait. It prevents oscillations at high frequencies.