# Why did my transistor break down? (Common Collector Amp)

I've designed a simple common collector circuit (see picture) and tested it into an 8ohm dummy load and everything seemed fine. So I connected the output to a speaker and the input to my laptop and played some tones and some music. I wanted to increase the quiescent emitter current (or at least not block it with 15 ohms), so I reduced Re to about 3 ohms. I played music and watched the increased current consumption, then the speaker started to give a big dc 'pop' about every second, then suddenly it had a large, continuous DC on it, luckily my power supply limited the current so the speaker didn't blow.

The transistor (2N3055) can take max 7 volts from emitter to base. But as far as I know my emitter voltage is always 0.65 volts less than the base voltage at all times. Unless it has something to do with current sinking in the negative cycle which I don't know much about yet. (Still a 'noob', just learning.) Max base current is 7 amps, and I'm sure it was never reached in my circuit.

The transistor was on a chunky heat sink with thermal paste and judging by the touch it never went over 60 degrees Celsius.

Currently the base-emitter junction is showing 138 ohms each direction which is messed up. Please see the circuit below:

• The power dissipation and the temperature rise killed your BJT. Your Ie current is around 1A thus, the power you dissipate in the transistor is equal to P = 1A * 15V = 15W. So you need to mount your BIT on a big heat sink to dissipate such power. Or you got a fake transistor.
– G36
Apr 14 at 14:57
• Thanks. Maybe both of those reasons. Apr 14 at 15:05
• Notice also that this circuit does not provide any voltage gain only current gain. But to get a "louder sound", you need a voltage gain as well (higher input voltage amplitude). firstwatt.com/wp-content/uploads/2023/12/art_zen_amp.pdf electronics.stackexchange.com/questions/474723/…
– G36
Apr 14 at 15:32
• Thank you, I'm planning to have a common emitter preamp with bigger voltage swing. Then this would be the output stage. Thanks for the link, I'm gonna check it out. Apr 14 at 15:39
• Yes, these Chinese fakes are all around the world these days.
– G36
Apr 14 at 15:50

The circuit makes little sense as a speaker amplifier.

When the Re was reduced to 3 ohms, the first order approximation of the circuit still tries to keep about 15V over 3 ohm resistor, passing 5A, and having 15V Vce voltage. It means it has to dissipate 75W of heat, and due to thermal resistanve of about 1.5 °C/W, the silicon junction is 112 °C higher than the case.

If you estimated about 50 °C for the heatsink, the case is higher and the silicon junction will be at about 170 degrees.

So as an amplifier, that circuit makes little sense, it wastes more than 75W of power into heat while driving 2Vrms or 0.5W into 8 ohm load.

Edit: I was so focused on the transistor that I forgot to mention that both the transistor and resistor pass 5A and both have 15V over them, so both dissipate 75W so the resistor also needs to be cooled. The whole circuit consumes 150W just sitting idle.

• Thanks for all the info. My goal is to build a single ended class A amplifier, so the wasted power is expected. As I see my two options are to either find a transistor with lower thermal resistance or a bigger/better heat sink. You mentioned that the silicone junction must be at around 170 C while the heatsink is at 50. I have an amplifier (Marantz PM 7200) which has the heatsinks at around 50-55 degrees when biased in class A without any issues. Probably lower thermal resistance. Apr 14 at 15:14
• @electroexplorer please be careful with the words silicon and silicone. Silicon is an element used in semiconductors. Silicone is one of many different compounds used for things like caulking. Apr 14 at 15:39
• Thanks for the correction, I appreciate it. Apr 14 at 15:51
• @electroexplorer No you have more sensible choises like not using so high voltages or currents to begin with. Your Maranz amp does not compare as it is likely not a Class A amplifier biased to waste 75W in a single transistor. There are likely several transistors heating it up in total, not one, and likely it also does not waste 75W when doing nothing. Apr 14 at 16:00