Three transistor audio amplifier
Description.
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.
Notes.
- 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.
can we replace the 100K resistor with a 100K potentiometer, and what are the power ratings for the resistors
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
sry.i want to asking about something
how can i do matching with speaker to the circuit?
by transformer or how???????
plz.answer me
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
what is the solution for designing this circuit?
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.
why we use r1 , r2 ?
can i use this circuit with input of the mobile?
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.
hi..is that a power amplifier ? … if it’s a power amplifier is it class B or class AB ?
Hi Ziad it is a low power head phone amplifier (general purpose micro power amplifier) nearer to class AB (almost tending towards class A)
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
Tommy
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
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.
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
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.
Hi Ruben Sorry 100PF should be between collector and base of Q3, it will definitely solve your hf oscillation problem.
Something must be screwy lol. Theres still no difference with a 100PF capacitor between Q3′s base and collector.
Hi Ruben your transistors may be at fault, my prototype is quite good, try by replacing them.