Class AB Audio Amplifier

This week we’ll take a look at another type of audio amplifier, the class AB amp. As we saw in Hack #19 the class A amp has a transistor that is biased with a voltage that makes it essentially stay turned on all the time. This single transistor handles the entire waveform both positive and negative.
The class B amplifier uses two transistors. One handles the positive side of the waveform and the other handles the negative side of the waveform. Each transistor is in an off state until the waveform crosses over the zero point and then it must turn on. This takes about 0.7 volts so there is a small amount of time that the transistor is not amplifying the signal. This lag time at the crossover point can cause distortion. There is a way around this however, the class AB amplifier.
In the class AB amp the transistors are biased in such a way so as to never fully turn off. They are kept on by two biasing diodes which allow a small amount of collector current to flow even when there is no signal present. This means then that the transistor will be “ON” for more than half a cycle of the waveform but much less than a full cycle giving a conduction angle of between 180 to 360o or 50 to 100% of the input signal depending upon the amount of additional biasing used. The amount of diode biasing voltage present at the base terminal of the transistor can be increased in multiples by adding additional diodes in series.
There’s a great tutorial on class AB amps on Electronics-Tutorials.ws. I encourage you to visit this site where you’ll find a wealth of educational information on electronics.

This project is fairly easy to build and all the parts are readily available. You can use just about any NPN and PNP transistors, just make sure they’re within similar specs to each other.

Keep on hackin!

Parts list:

2 – 2N3904 NPN transistors
1 – 2N3906 PNP transistor
1 – 47 uF electrolytic capacitor
1 – 470 uF electrolytic capacitor
1 – 100K ohm resistor
1 – 1K ohm resistor
2 – 1N4148 diodes

Schematic:

20 thoughts on “Class AB Audio Amplifier

  1. Great stuff as always! I noticed on your whiteboard in the video you labelled the speaker at 25 ohms. Would any changes need to be made with speakers or headphones of higher impedance?

  2. Thanks for the tut!
    How do we reduce harmonic distortion? How do we increase power?

  3. Hi , im new to electronics , im looking to build a headphone amp so i can use my mobile phone headphone output to go into the AUX input on my car stereo. Currently the volume is very low and i have to have the stereo turned up high. Would it be possible to use this amp as a pre-amp by connecting the speak outputs to a component left and right as inputs?

    Regards

    Stuart

  4. Hi, I’m new in electronics and I would like to know the WV value for both electrolytic capacitor. Is it 10, 16, 25,35….? Thanks

  5. Hello Dino,
    First let me say that this is a great blog. I love what you are doing here and you have a bunch of great hacks. Some of them are over my head right now but others are right up my ally.
    I am interested in the audio amp hacks you have posted and I had a few questions. I am trying to amplify a signal from a 4.5v 10 sec recordable sound module. It is not very loud compared to a 20-sec rec. SM I have. I would like to go with the simplest, most cost effective circuit that will give me the best ‘bang for the buck’.
    One question I have is on the input voltage. You have listed a couple of your amps with V input range but you used 9v on your hacks (except the preamp hack). So does this circuit (analog amplifier) also have a variable input voltage? Does the input voltage directly effect the gain?
    I am a little fuzzy on the function of the preamp vs the regular amp. They both seem to amplify the signal. I only need a little gain for my project (I just need to turn the volume up a little). The preamp hack is the simplest circuit with the fewest components. Would this in itself amplify the audio signal or should I build the regular amplifier instead? If I need to build a regular amp then which would you recommend for my application the LM386 amp or this analog amp?
    Thanks for your help.

    • Thanks for the comment and I’m glad you like the blog. For what you want, I would recommend building the LM386 amp. It has a volume control and it runs for quite some time on a 9 volt battery. The preamp is designed to boost a very low level signal before running it through a higher powered amp.

      • Thank you for the quick response. I won’t be using a 9v power source though. I will be incorporating the amp circuit into the sound module circuit so it will be running of of 4.5v. I don’t really need a volume control as this is for a prop. I could use a trim pot but once I get the right volume level it will not be adjusted again. I am not aiming for a specific volume, I just want it to be a little louder.
        So the preamp circuit wont boost the volume at all at 4.5v?

    • Minimal. Probably less than 1/2 watt. More power would require bigger transistors and another circuit design.

  6. I’m using speakers that do not have an external volume control built in like the speaker shown in the video. Is there a simple way to have a gain adjustment incorporated in this circuit that would not require an op amp? Perhaps a potentiometer somewhere?

  7. Heya ,,, this is a cool blog having cool stuff and simple circuitry and cost efficient too,,,, but i wanna ask that can it drive a Sony 8 ohm 6inch speaker mean how much watt it generate sir ? Thanks

  8. I built the amp and it works great. I replaced the 1K with a 222Ohm because it gives a better sound with less distortion. Great work!

    • I added an additional 9v battery to the circuit and it raises the volume about 3 times. Really loud and crystal clear. The problem is that both the NPN and PNP transistor become extremely hot. Any suggestions how this can be prevented? How could we achieve more volume with just one 9 volt battery or is it impossible? How can we adapt the circuit to work with 18 volt without overheating? Thank you so much.

      • They get hot because you are pushing them to their limit with 18 volts. They’ll eventually fail getting that hot. They need a heat sink.

  9. Hi,
    I built your circuit but I get clipping when I send 100 Hz sine wave but then I noticed I am using an 8 ohm speaker. What would I need to do to hear 100 Hz sine wave coming out of an 8 ohm speaker with your circuit?

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