Transistor Amplifier GETS TO FRICKING HOT! - Page 1
Forum section: electronics / beginners

author: XMA
author_rank: Contributor
timestamp: April 27, 2018, 02:26:08 pm
content: Hi so in the pic is my circuit, I am using BC547 Transistor + 1k ohm resistor and a 9v 300ma power adapter. the problem is that the transistor is getting way-way to hot, I burn my fingers. and so my question is why is that happening. I've done tests with a 9v battery. and so they are9v battery = %50 amplifaction + distortion + slight warm9vdc 300ma power adapter = %100 amplifacation + no distortion + super hot!

author: BradC
author_rank: Super Contributor
timestamp: April 27, 2018, 02:27:30 pm
content: Pic?

author: grifftech
author_rank: Frequent Contributor
timestamp: April 27, 2018, 02:42:12 pm
content: use a different transistor (TO-220)

author: free_electron
author_rank: Super Contributor
timestamp: April 27, 2018, 02:50:16 pm
content: this is NOT how you make an amplifier ..

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 27, 2018, 03:59:11 pm
content: that is a good way to burn your transistor or your speaker ..

author: james_s
author_rank: Super Contributor
timestamp: April 27, 2018, 05:18:33 pm
content: Those are tiny little transistors driving a large speaker, what would you expect to happen? There's a reason power amplifiers use beefy power transistors mounted to a heatsink.

author: Gyro
author_rank: Super Contributor
timestamp: April 27, 2018, 05:33:14 pm
content: One transistor and 1 resistor isn't an amplifier!Even for a single transistor gain stage you need to bias the transistor properly. You have nothing to stop DC current through speaker voice-coil. All you are doing at the moment is putting a varying amount of current through the speaker on positive halves of the audio waveform only, once the signal gets above around 0.7V. Any positive DC offset and you will burn out the transistor and fry the speaker.Time to stop now and do some reading while you still have a speaker.

author: strawberry
author_rank: Super Contributor
timestamp: April 27, 2018, 07:49:15 pm
content: use suficient capacitor in series with speaker to block DC currents

author: vk6zgo
author_rank: Super Contributor
timestamp: April 28, 2018, 01:56:45 am
content: Quote from: james_s on April 27, 2018, 05:18:33 pmNot really, large speakers are more efficient than small ones.In the 1950s & '60s, most amplifiers had quite modest outputs, but were used with large speakers in full sized enclosures.These were much more efficient than the relatively tiny speaker enclosures used today.The reason modern power amplifiers use beefy transistors is because they need to produce high power to get reasonably high acoustic power out of small, inefficient, speakers.

author: Jwillis
author_rank: Super Contributor
timestamp: April 28, 2018, 02:16:37 am
content: Give this site a try.It explains how a common emitter amplifier works.https://www.electronics-tutorials.ws/amplifier/amp_2.htmlHope this helps.

author: JS
author_rank: Frequent Contributor
timestamp: April 28, 2018, 04:45:32 am
content: Quote from: vk6zgo on April 28, 2018, 01:56:45 amStill with large speakers you want higher damping factor to get good quality as they have more mass. This doesn't mean greater power but does mean better designed and bigger transistors as they can archive lower output impedance.Still, for the lab there's nothing wrong with moving a large speaker with small transistors, but you do want to have some considerations, no DC to the speaker is usually a good one. Knowing the polarization point of your transistor and choosing it correctly is another. You are using a single transistor, that's class A and you can't get away from it. Let's say an inverting NPN stage, so you can only source what the resistor in series with the speaker can take from half supply and you can sink what the speaker can take from the other half limited by the max colector current of the transistor. In general you want the transistor collector to be sitting between your rails taking the maximum current the speaker would take when the transistor is almost off. For the other side you would travel as far as getting the same signal so you get a symmetric signal. You could offset the output a little to get this just a bit better but won't change much the output power from there.So with this, 8Ω 9V (4.5V per rail). The peak current should be around 0.5A, times the 4.5V the transistor should be able to dissipate 2.5W at idle, as well as the resistor. When signal is present the dissipation would be lower as some of that power will be absorbed by the speaker, getting up to about 1.5W best case.This means that if you don't want it to be hot you need at least a small heat sink and a transistor able to handle that 0.5A without much trouble. You could go for 100mA instead, but you won't get the 9V swing any time soon, instead more like 1.6V peak to peak, at those 100mA, 80mW. This you can archive with a smallish transistor without getting too hot to touch but still warm.JS

author: Ian.M
author_rank: Super Contributor
timestamp: April 28, 2018, 08:19:33 am
content: If you want to experiment with low transistor count, fractional watt audio amps driving smallish speakers, anEagle LT700 output transformeris your best friend. Used single-ended (ignoring the center tap), its 18.7:1 turns ratio transforms the impedance of a nominally 8 ohm speaker up to an AC load of about 2.8K as seen by the transistor collector, and its primary resistance of 63 ohms allows it to be used without a collector resistor in low voltage amplifier designs with the bias point stabilised with an emitter resistor. Its nominally rated for 0.25W of power and you'll maybe achieve up to 0.1W in a single ended configuration.Drive it push-pull with Vcc fed to the center tap and a transistor driving each end of the primary, it transforms that 8 ohm speaker to about 700 ohms seen at the collectors, and you can get its full wattage out of it as there is no net DC through the primary as a whole (the DC currents in the two halves cancelling out). With careful attention to the biassing, you can operate pure class A, class AB or even class B, though that usually isn't woirth it due to crossover distortion. Use an Eagle LT44 interstage transformer for phase split base drive, and add a class A driver stage before the LT44, with a capacitively coupled 10K volume control pot in front of it and you'll have a usable three transistor single rail bench amplifier for signal tracing. With some AC negative feedback from the LT700 secondary to the driver stage to linearise it, you can make a surprisingly good quality monitor amp.

author: Audioguru
author_rank: Super Contributor
timestamp: April 28, 2018, 03:37:19 pm
content: Why not use an amplifier IC? An LM386 is inexpensive and with a 9V supply it can produce 0.45W in an 8 ohm speaker with low distortion. It gets warm when playing and is cool when not playing.

author: mikerj
author_rank: Super Contributor
timestamp: April 28, 2018, 04:53:29 pm
content: Quote from: vk6zgo on April 28, 2018, 01:56:45 amBig beefy transistors are also needed, and used, to deliver high powers into very large speakers.The OPs real problem is down a combination of a low impedance speaker and a half-assed approximation of a class A output which is putting significant levels of DC into the speaker.