adding power transistor to 386 amp

Discussion in 'General Electronics Chat' started by rudyauction8, Jan 27, 2012.

  1. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    I am working on a small amplifier with a 386 chip and it keeps over heating, so I want to add a NPN power transistor to the output but I have no idea how to do it. Can you directly connect it or is there something else I have to get or is it even possible?
     
  2. PaulEE

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    Dec 23, 2011
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    Can you post a schematic?

    If you're driving a speaker, is it capacitively coupled to the amplifier? If not, you might be shunting DC current to ground out of the amplifier.

    A class A common-collector can help, but a schematic is the first step in recommending how it could help...class B or AB can as well.
     
  3. rudyauction8

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    Jan 27, 2012
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  4. rudyauction8

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    Jan 27, 2012
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    I'm using r1 and c2 to filter out some of the treble and make the sound a little bit more clear. The overheating problem happens with and without them.

    Basically I want a circuit that connects the power transistor's base to the 386 output, collector to the power supply and emitter to the speaker or something similar and easy to build.
     
  5. PaulEE

    Member

    Dec 23, 2011
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    The LM386 is something called an operational amplifier, or op-amp.

    An op-amp takes the difference of two input signals at the +input and -input and amplifies this difference by a very large gain factor, typically 1 million (voltage difference of 1 micro-volt would cause an output of 1 volt). This description is accurate if all you do is hook up a power supply to Vs+ and Vs-, and signals to the inputs; no other components.

    An operational amplifier becomes more useful with the introduction of something called feedback. Simply, feedback is when you introduce some amount of output back into one or both inputs. If the feedback introduced subtracts from the input, it is called "negative" feedback; similarly, if it adds to the input, it is called "positive" feedback.

    Operational amplifiers are generally used in one of a few common configurations. You can read about these configurations here:

    http://en.wikipedia.org/wiki/Operational_amplifier_applications

    The LM386 is simply an op-amp with higher current output capability. As an example, a 741 op-amp (which I'm sure you've heard of) can supply a maximum of about 10 mA from its output terminal. What I mean is that, if you measure a voltage of 10V at the output and you have a resistor equal to 1,000 ohms connected from the output to ground, you will be able to measure 10 mA through the resistor.

    To use the LM386 correctly, you must use a configuration that allows AC amplification. A non-inverting op-amp configuration is generally used (see link above).

    The datasheet for the LM386 has an applications section with example circuits for precisely the project you're trying to do. In the PDF document, look for a section labeled "applications". Specifically, look at the diagram on page 5 of the PDF datasheet, "amplifier with gain of 20". Datasheet:

    www.national.com/ds/LM/LM386.pdf

    If it is getting hot, be sure that you are not exceeding the maximum power supply voltage. Also, double-check all connections with the schematic in the datasheet. Be sure that the input is on +input and the -input is grounded. If a capacitor is present in the schematic, make sure you use it! It makes the op-amp more stable.

    If you have any other questions, don't hesitate to ask. :)
     
    Last edited: Jan 28, 2012
  6. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    Thanks! That helps alot. So the 386 is just a powerful OP amp? Do you know how to make an OP amp with transistors? Preferably with npn driving the output? That would solve my problem.
     
  7. PaulEE

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    Dec 23, 2011
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    I think I gave you more information than you were after...but I hope it was helpful. Now that I look at your circuit, it is basically the same as the one in the datasheet. Sorry about that! Your initial comments made me think you were more of a beginner than you actually are :)

    On your schematic, you drew a resistor and capacitor on the wrong side of the output capacitor...did you mean for that resistor-capacitor chain to be tied to the LM386's output pin instead?

    Also, put a 10 K resistor from +input to ground and apply your input signal to +input (signal source) and -input (ground). This cuts down on noise from external sources.
     
  8. PaulEE

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    Dec 23, 2011
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    Yes, you can do this...but it is a fairly long conversation as to how you'd go about doing it with an LM386. The LM386 isn't quite an off-the-shelf op-amp because it has a few features inherently that regular old op-amps don't have; for example, you can use a single supply instead of a dual-polarity supply. This can complicate the procedure for designing a more powerful output stage. It also has some internal gain-setting features as opposed to a straightforward voltage-divider feedback scheme as you see in wikipedia.

    What impedance is your speaker? 8 ohm?
     
  9. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    I have the capacitor and resistor right, as they are on my circuit, they are that way because I built it by experimenting. And I messed up the input. Positive input goes to positive input source and negative input goes to ground and negative input source.
     
  10. PaulEE

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    Dec 23, 2011
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  11. PaulEE

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    Dec 23, 2011
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    That may very well solve the heating problem...but I doubt it.

    I would follow the datasheet pin-for-pin and part-for-part...
     
  12. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    it depends, the speakers range from 8-24 ohms depending on what I'm using it for.
     
  13. rudyauction8

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    Jan 27, 2012
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    The problem is that I'm running it off of a 7.4 volt LIPO battery. There isn't an easy way to simply add a transistor or 2 to the 386?
     
  14. PaulEE

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    Dec 23, 2011
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    In either case, those values are fine. Datasheet spec's outputs for 8 and 32 ohms.
     
  15. PaulEE

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    Dec 23, 2011
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    THe LM386 wasn't built to work like that. It has internal components that set the gain to 20 v/v minimum. I would have to work out on paper how to add output transistors. Here's how you'd do it with a typical op-amp:

    http://www.allaboutcircuits.com/vol_6/chpt_6/10.html
     
  16. rudyauction8

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    Jan 27, 2012
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  17. PaulEE

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    Dec 23, 2011
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    An alternative to all this transistor output stuff is to simply put a 10 ohm resistor in series with the speaker to limit the output current of the op-amp. It isn't terribly efficient, though.
     
  18. PaulEE

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    Dec 23, 2011
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    I commented on that one earlier :)
     
  19. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    So use the 555 timer to the base of the transistor? and a variable resistor? but where do I put the variable resistor? Other than that problem, I'm good
     
  20. PaulEE

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    Dec 23, 2011
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    Please post this in the 555 timer thread. I think you put it here by accident.

    --> http://forum.allaboutcircuits.com/showthread.php?t=65295
     
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