Class A single transistor amplifier with 2N3904

Question

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

\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?

Answer 1

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.

Answer 2

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.)

Answer 3

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.