I made a simple little transistor amplifier using the 2N3904 (basic class A amplifier):

enter image description here

\begin{align*} R_1 &= 2.2\, k\Omega \\ R_2 &= 1\, k\Omega \\ R_L &= 22\, \Omega \\ R_E &= \text{potentiometer} \\ C_1 &= 2.2\, \mu F \\ C_2 &= 20\, \mu F \end{align*}

It works fine, but if I leave it on for a little too long it gets really hot.

I have 2 different types of transistors that I can mount to a heatsink. TIP31A and TIP41C. But when I put them in they don't work. Could someone explain why they don't?

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  • \$\begingroup\$ tip31a and tip41c have very low current gain. Maybe that is why it doesn't work when you change the transistor. Most practical audio amplifiers (if that is what you are making) will have quite a few transistors. You can only do so much with one transistor. \$\endgroup\$ – mkeith Jun 6 '15 at 20:14
  • \$\begingroup\$ There is no reason for R1/2 to be so low. You could multiply them by 100 and still have a functioning circuit. Also you don't want to present such a low input impedance as 1k to the source. Rl could probably be doubled or tripled as well. \$\endgroup\$ – user207421 Jul 8 '15 at 23:08

Just to add some numbers: I simulated your circuit with the values given in your circuit and some sensible defaults (e.g. Vcc=9V and Vin=1V peak-to-peak).

Results are, that you operate the transistor very close to the maximum ratings.

I see a peak power dissipation of 627mW and an average power dissipation around 450mW. The maximum rating for the 2N3904 is 625mW, so no surprise that the transistor gets hot fast.

If you drop a TIP41C into the circuit you should still be able to measure a signal at the amplifier output, but it will probably a lot lower because the current gain of the TIP41C is a magnitude lower compared to the 2N3904.

As others already suggested you can run the TIP41 and the 2N3904 in darlington configuration. That'll will give you best of both worlds.

Nonetheless, from the learning experience I think your circuit is quite a success. It's running hard at the limit of the 2N3904 but otherwise it looks fine. I suggest that you take a look at differential amplifiers next. They are a very important building blocks for audio amplifiers.

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Yeah, you are dissipating too much power on the transistor. I see a few solutions here:

  • Limit your output power by limiting collector current. Be careful, do not saturate the transistor!
  • Cascade the amplifier. The output stage of the amplifier, especially class A are typically cascaded in order to have bigger power and less "dissipation per transistor"
  • Put a proper heatsink. You probably have air gape between the case and the heatsink. Use thermal paste! I usually put it on my finger and deposit a tiny amount of paste on the case of IC and then place the heatsink. You need to lower the thermal resistance as much as you can and by having air between you are making things only little better.
  • Use another power transistor(2SC5171, BC560C etc.)
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  • \$\begingroup\$ BC560C is not a power transistor, but a small-signal jellybean PNP transistor. It can handle 625mW@25°C maximum, but in any real world circuit it would handle 0.3-0.4W at best. \$\endgroup\$ – Lorenzo Donati -- Codidact.org Jun 6 '15 at 21:37
  • \$\begingroup\$ You are right, my mistake. \$\endgroup\$ – Bip Jun 6 '15 at 21:46
  • \$\begingroup\$ No my problem is with the power transistor. When I put it in the circuit not sound is amplified. I am asking why... I haven't hooked anything up to a heatsink yet because I can only get the small 2n3904 to work. \$\endgroup\$ – Trevor Jun 6 '15 at 23:51

The current gain of the two power transistors you mentioned is much less than the current gain of the 2N3904. That is probably why your circuit "doesn't work." I suggest you try a darlington configuration with the 2n3904 followed by one of the power transistors. But realistically, you cannot make a good audio amplifier with just one transistor. It would be helpful for you to describe the measurements or other evidence which cause you to think that the 2N3904 is working, and that the other transistors are not working. It would also be good to state what the resistance setting is of the potentiometer.

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  • \$\begingroup\$ The reason why I said is not "working" is because when I try to amplify done music I can barely even hear it, but with the 2n3904 I can hear it clearly. The current gain is probably my problem thanks. The resistance setting is about 15 ohms on the potentiometer. I'm also new to electronics and my goal is to make a 20w amplifier. So my idea was to start off simple and go from there. I really don't understand why you need so many transistors though could you explain why? \$\endgroup\$ – Trevor Jun 7 '15 at 14:23
  • \$\begingroup\$ For your first project, maybe you should build the Pass Zen amplifier. The schematic you linked to is not intended to be a power amplifier. It is more like a signal amplifier. firstwatt.com/pdf/art_zen_amp.pdf \$\endgroup\$ – mkeith Jun 7 '15 at 16:20
  • \$\begingroup\$ Ok so maybe I'll try that but what's the difference between a signal amplifier and a power amplifier? \$\endgroup\$ – Trevor Jun 7 '15 at 16:45
  • \$\begingroup\$ Loosely speaking, a signal amplifier boosts the voltage of a signal to get it into a better voltage range (maybe for an ADC or something). It is not intended and not capable of supplying high power. But a power amplifier boosts the signal and is capable of supplying power to a load like a speaker or transmission antenna or something like that. I don't think they are precise terms. \$\endgroup\$ – mkeith Jun 7 '15 at 17:15
  • \$\begingroup\$ @Trevor I applaud you for wanting to build a 20W amp but starting with something small, VERY WISE !!! More people should follow this concept :-) The reason for so many transistors is because more transitors = more fun :-) Seriously, in an audio amp generally the transistors at the input provide voltage gain while the output transistors are power devices providing current gain. \$\endgroup\$ – Bimpelrekkie Jun 8 '15 at 14:50

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