Darlington Transistor Amplifier

Discussion in 'General Electronics Chat' started by ELECTRONERD, May 28, 2009.

  1. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
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    Hey,

    I've been biasing a few transistors and only have enough gain to drive a small ceramic earphone. With a Darlington transistor, could I drive a small speaker? If I cant, how would this be possible?

    Thanks!
     
  2. bertus

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    Apr 5, 2008
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  3. radiohead

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    May 28, 2009
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    Why not use an LM386N / NTE823 DIP8 low voltage audio amp? Data sheets for semiconductors usually contain typical applications and schematics. go to http://www.datasheetcatalog.com/ for your semiconductor data sheet needs.
     
  4. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
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    Hey Bertus,

    Well, I have some 1W speakers which should be plenty. As for the "headphone or larger" thing, I'm not sure what you mean. What I had was a small electret condensor microphone connected to the transistors input. I used the common emitter configuration and had a small ceramic headphone on the output. I want more amplification (enough to drive a speaker), so i was wondering if a darlington transistor could accomplish this?
     
  5. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
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    I want to start designing my own transistor circuits, that is why... Thanks for the suggestion though! I have experimented with the LM386N.
     
  6. bertus

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  7. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
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    Okay guys, Here is the schematic I used except for Re (which I eliminated).

    [​IMG]

    How can I get more amplification or gain?

    I had two 10K resistors for R1 and R2. For Vcc I had 9V. So the voltage divider would produce 4.5V at its output. Then, the voltage on the emitter would be 3.8V. How do I calculate the value for Rc? I suppose if I had 25mA for the collector I would need a 360Ω resistor ( 9V / 25mA)?
     
    Last edited: May 28, 2009
  8. radiohead

    Active Member

    May 28, 2009
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    add another amplifier stage
     
  9. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
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    But was I correct for the collector resistor (360Ω)?

    Thanks!
     
  10. radiohead

    Active Member

    May 28, 2009
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    Why not connect a 4.7pF cap across the E-C (2N3904 would work fine), a 22pF cap (or a 6-70pF trimmer cap) from the collector to V+, an 5 to 8-turn coil (around a pencil) from the C to V+, a 470 ohm resistor from the emitter to V-, a 10K resistor from the base to V+ and your electret condenser mic from the base to V- and listen to your voice on the radio? the range will be short without an antenna which you can connect to the 2nd turn from the transistor end.
     
  11. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
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    That sounds good to me, although I want to go one level at a time. So, I want to start with small amplifiers and get gradually more difficult.
     
  12. radiohead

    Active Member

    May 28, 2009
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    draw it out, it's a simple transmitter circuit, nine components (9-12 VDC)... add a .001 uF cap from the base to ground...my bad You can even breadboard it and it will work.
     
    Last edited: May 28, 2009
  13. ELECTRONERD

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    May 26, 2009
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  14. radiohead

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    it should be, if you have enough of a signal input to the base to amplify. If you are using an electret mic, you will get some feedback.
     
  15. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
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    Could you show me how to bias this particular transistor and provide a gain of 8,000?

    I'd appreciate it!
     
  16. radiohead

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  17. Wendy

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    Mar 24, 2008
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    Another configuration to be aware of, though they aren't sold in common packages as far as I know...

    Sziklai Pair

    They make a nice addition to the Darlington Pair.
     
  18. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
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    Very Interesting! What are Darlington and Sziklai pair transistors typically used for?
     
  19. Wendy

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    Mar 24, 2008
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    Really, really high gain applications. Where you need 1µa to control several amps, which is a gain in excess of a million.

    Both Darlingtons and Sziklai Pairs have a similar problem. A conventional transistor can have less than 0.1V Collector Emitter drop, which is quite good. Both Darlingtons and Sziklai Pairs drop at least 0.6V at saturation, which means heat, and sometimes that little voltage drop is needed for other things.
     
  20. hobbyist

    Distinguished Member

    Aug 10, 2008
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    You said you want to learn how to design transistor circuits from the basic config. first.

    Lets look at your circuit and analyze the values.

    1. If you eleminated RE than voltage at the base would be 0.7v. because the VBE shunts the R2 resistor.
    2.If you eliminated RE than emitter volatage woiuld be zero volts, because emitter is connected to ground.
    2a.By using the RE than base voltage will be 4.5v.
    so lets put RE back in.

    3. since you have given 25Ma. for collector current. Then you should now calculate the emitter resistor. :3.8v. / 25mA. = approx. 150 ohms.

    from there you have a choice of how much volt. drop do you want across the transistor (VCE), or what voltage do you want at the collector with respect to ground. (VC)

    Ussually for a class A amp the voltage at the collector (VC), is around
    VCC/2. In this case 4.5V.

    4. choosing VC=4.5v. then 4.5v. is dropped across the collector resistor.

    so VCC-VC=VRC. can be also calculated as VRC=VCC/2 ALWAYS for a class A amp. And VCC/2=VC

    so now RC=VC/IC, which would be 4.5v./25mA.=180 ohms.

    There are many ways to design a simple class A amp. but what dictates a design is the parameters given known as constraints.

    Now due to the amount of base voltage gives a VE of 3.8 volts, which means that 0.7v. dropped across the transistor, so the transistor is near saturation, which would probably not work as a amplifier.

    So values for VB. would have to be redetermined so as to get a VCE that would put the transistor in it's linear region.


    Lets try to redesign this so it would work.
    Now I have not built this so this is just approximations of how I would design this project.

    1. constrraints: VCC -9v. IC=25mA.
    2. start with a VCE=2v. (arbitrarily choice)
    3. VCC/2=VC so VE=(VC-VCE) = 2.5v.
    4. RE=VE/IC = 100 ohms
    5. VB=(VE+Vbe)=3.2v.
    6.you chose 10K for R2, so IR2=VB/R2 = 320 uA.
    7.R1=(VCC-VB)/IR2= approx. 18K ohms.
    8. RC=VC/IC = 180 ohms.

    Now AV (voltage gain) = RC/RE so here AV=1.8 not much gain.
    The reason why is because IC was given as a constraint of 25mA.
    So as you could see values would need to be readjusted to get better gain.
    some times the constraints are such as to need to use different config. all together.

    But this gives you an idea of why even basic transistor designs need to be worked at from different standpoints.
    My first guesstimation of the values I calculated of this shows a lousy design. But this is according to my knowledge of this design. Other people would come up with a far better design.
     
    Last edited: May 29, 2009
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