I want to build simple one-transistor power amplifier for cd-player. I found 2sd605 Darlington transistor for it. I use 8 Ohm load speaker and have a lab power supply 20V. I want to connect speaker between Collector-Emitter through capacitor. I want to know how I can find collector resistor and input power-to-base and base-to-emitter resistors values.

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  • \$\begingroup\$ This is not a good approach to try and design and make your own power amplifier ! Do it like you propose and very likely your speaker will burn through. No-one makes single darlinton transistor power amplifiers. That because they can't work. Something more complex is required. Suggestion: buy a cheap power amplifier module on ebay. \$\endgroup\$ Apr 20 '16 at 9:36
  • \$\begingroup\$ @FakeMoustache I don't understand your comment. Why would it "burn through"? Do you mean that there will be a DC bias? That's what the capacitor is for, I presume. If you mean that it will sound bad, then write that, because the user does not ask about high fidelity. \$\endgroup\$
    – pipe
    Apr 20 '16 at 9:52
  • \$\begingroup\$ Previously I have been seen equal amplifier but with one transistor added to pair with my Darlington. And it’s worked fine with CD with great hearing power. And no any like speaker destroying was founded. \$\endgroup\$
    – MaxMil
    Apr 20 '16 at 9:54
  • \$\begingroup\$ There's a major flaw with your circuit. There's no speaker. \$\endgroup\$
    – pipe
    Apr 20 '16 at 9:54
  • 1
    \$\begingroup\$ It still won't work - R3 is way to high a value for any current to pass into the speaker. Take a look at sci-toys.com/scitoys/scitoys/computers/solderless/… \$\endgroup\$ Apr 20 '16 at 10:06

To protect against possible future edits, here is the circuit we are talking about:

This circuit is a joke. With no input, the transistor will be saturated and the collector voltage will be as low as it can go. The negative-going parts of the input signal will turn off the transistor, which then allows R3 to pull the collector high very weakly. You now essentially have a 20 V transient applied to a 8 Ω speaker thru a 100 kΩ resistor. You will probably be able to hear something if you put your ear up to the speaker.

Forget about this mess and move on. Trying to fix this would really be designing a proper circuit from scratch, with vague specs.

What to do next depends on your real purpose, which is not clear. In one place you way you want to use a single transistor for some reason, but in another it seems you just want a audio power amp for a "CD player", which probably means it will take normal line level audio signals in.

Decide the one thing you really want to do and ask another question about that. Include specs on what the input signal will be like, how much output power the amp should be capable of, etc. However, leave any assumed implementation out. If you knew how to do this, you wouldn't be here asking.

  • \$\begingroup\$ I want a simple Darlington-pair transistor power amplifier for CD player. Output power 5 WT, input voltage is 2V as CD output. \$\endgroup\$
    – MaxMil
    Apr 20 '16 at 11:49
  • \$\begingroup\$ @Max: Again, ask a separate question. This one is too cluttered up with your proposed circuit. Second, again, what is the real spec? It sounds like you want a 5 W power amplifier for line level audio input. OK. But then you get into implementation details like it must use a single darlington transistor. Huh? What's the real point here, to do what you can with a single darlington, or to have a real power amp? \$\endgroup\$ Apr 20 '16 at 12:35
  • \$\begingroup\$ At first I want to build what I can with one Darlington transistor and then move on to extend my circuit. \$\endgroup\$
    – MaxMil
    Apr 20 '16 at 18:03

There are many problems with your circuit.

First, the set point of the Darlington is poorly defined. You pin the base at mid voltage but ground the emitter. This sets up a pretty large base current and will turn it on hard. In turn that puts the collector near ground. Ideally your quiescent point should be mid rail.

Second, even if the set point was sensible, your response will be highly asymmetrical. When the transistor pulls, the output will follow: but when it stops conducting you need the collector resistor to do all the work. If you are driving an 8 Ohm load you would want the collector resistor to be about 8 Ohm; when your amplifier sits "quietly" at the mid point this means it draws over 1 A of current and dissipates over 10 W each through the transistor and the collector resistor.

I strongly suggest coming up a with different topology- even an emitter follower would work better (but give no gain), or simply add negative feedback to control the set point and reduce the distortion - at the expense of gain.

And make sure your collector resistor can take the heat!


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