multistage bjt amplification low output impedance problem!!!!

Discussion in 'General Electronics Chat' started by acelectr, Apr 18, 2011.

  1. acelectr

    Thread Starter Member

    Aug 28, 2010
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    Well I've nearly done with this multistage amplifier design but I'm having some troubles at the last parts. The output will be taken from a 200ohm load and the output impedance should be maximum 100 ohm. I think I've managed the output resistance issue but when I am taking the output over a 200 ohm resistance I'm getting crazy signals. But when I am exchanging the load resistance with a 20k ohm then the result is more satisfying both in its peak values and its shape. Why do I get this distortion over a small resistance load and not over a high resistance load. How one can solve this issue?????:confused::confused:

    there are two attached pics. one the general circuit and the other the last part where the 200ohm is the load which I am taking the output from. the resulting weird output signal is also attached.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    Why does your schematic drawing software use huge bright signs for voltage and current measurements that cover the schematic instead of using simple text?

    Why is your "'scope" photo a weird negative with a black background intead of as a normal positive with a white background?

    Why are your schematics saved and posted as a very fuzzy JPG file instead of as a very clear PNG file?
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Your output stage is an emitter follower with a quiescent current of 921μA. When feeding a 200Ω load, the largest possible negative-going current swing is given by multiplying than this small quiescent current by the impedance given by the 200Ω in parallel with the 4.3kΩ emitter resistor, i.e. 191Ω.

    The negative half-cycle voltage therefore limits to about 176mV. Try increasing the output stage quiescent current.
     
  4. acelectr

    Thread Starter Member

    Aug 28, 2010
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    Well than there is a serious problem here. The thing is that I cannot increase my current that high. Actually it is not possible. One need to have 13m A to get the complete output negative cycle. Since I am limited with this crappy 200 ohm small resistance load....Also I cannot decrease the last stage emitter follower's resistances because its input impedance is the output impedance of the stage before it which is the main part where I get my gain. If I decrease the resistance values than no gain and the design fails.
    Is there any way around to solve this problem?
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Use a complementary class-AB output stage to drive the low load resistance.
    It can be used to bootstrap the load resistor of the gain stage for lots of gain.
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Can you post your full content of the task that you get?
    And why you use so many voltage sources ??
    And on CC amplifier you don't need Rc resistance, and to get max output swing from CC amplifier you need to set Ve = 0.5*Vcc = 7.5V
     
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  7. acelectr

    Thread Starter Member

    Aug 28, 2010
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    Well I've made some changes at this last CC stage. The task is simply to design a multistage amplifier with 100 gain, input impedance should be more than 1M ohm, output to be less than 100 ohm and the amplifier should be non inverting. I've attached the changed CC stage. And this time I've managed to get full swing both at positive and negative cycles.yyyaaaaaaaayyy:D. I removed the Rc as you may see but the problem is that in praticle life would it be possible to built this last stage. Looking at the emitter collector current, it is nearly 50mA. I think this may cause some problems. What do you think? Are there any other suggestions for this last stage?:confused:

    Also can you explain how the <snip> I've managed to get this full swing:D I've just did couple of stuffs that you've underlined can you give some more explanations related to this??
     
    Last edited by a moderator: Apr 19, 2011
  8. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    But your last stage is supply form 30V and with on signal dissipate allot of heat.
    Ptot = 15V * 50mA = 0.7W so your 2N2222 will not survive such treatment (horror).

    Do you allow to use negative feedback ?
     
  9. acelectr

    Thread Starter Member

    Aug 28, 2010
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    Yes of course withouth negative feedback I probably would not be able to come thus far. In fact I've thought of the negative feedback at this last stage but how do you make this negative feedback at a CC amp?
     
  10. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    CC amplifier has already built-in negative feedback
     
  11. Adjuster

    Well-Known Member

    Dec 26, 2010
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    You are getting more output swing because there is more current available. You do however need to consider the transistor dissipation. At the moment this is about 15V*50mA = 750mW, too much for a very small transistor.

    Perhaps now you see why someone suggested a complementary output stage, which could offer better efficiency? This could operate in class AB with a much lower quiescent current.

    If you want to keep to a simple emitter follower. it would be advisable to substitute an output transistor rated for over 1W, the exact rating depending on how much emitter current you decide to use - how much voltage swing do you actually need to deliver into 200 ohms?
     
  12. acelectr

    Thread Starter Member

    Aug 28, 2010
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    what is a class AB? Well the output is a 5V peak to peak. Actually I've modified a little bit more and the final one is attached. As it would be seen now there is 15 mA current passing through. I think that is still not small enough but a lot better than a 50 mA one. Do you think that a 15mA current will be a problem still? I've thoguh of other configurations for the last stage but since this has to be a non inverting amplifier I think an emitter follower is a good choice. Are there any different suggestions on how I can make this better?
     
  13. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Class AB is a mode of operation for power amplifiers. http://en.wikipedia.org/wiki/Electronic_amplifier#Power_amplifier_classes

    If you think 15mA would still make life too hot for the poor little 2N2222s, like I said, use a bigger transistor. Alternatively, if you only need 5V pk-pk, why not use less than 30V total supply for the output stage?
     
  14. acelectr

    Thread Starter Member

    Aug 28, 2010
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    Hopelessly I have no athor chance but to use 2N2222. And when I am reducing the supply voltage I am getting clipping at the output. I think I have no other chance but to have high current values at the emitter and collector:(:(:mad:
     
  15. Adjuster

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    Dec 26, 2010
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    If you really don't like the complementary idea, consider using a few 2N2222s in parallel. They will not share current well with only a single emitter resistor, so they need one each, with a separate coupling capacitor to the load.

    Set the emitter resistors to pass about 10mA per transistor, say 1.5kΩ each. This would give about 150mW quiescent with 15V VCE, which is probably OK.
     
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  16. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    And why you not use JFET in the firs stage or if you limited to BJT only use bootstrap to increased input impedance
     
  17. acelectr

    Thread Starter Member

    Aug 28, 2010
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    I have no problem with the input impedance. it is the output that causes trouble. output impedance has to be 100 ohm, I gues that is also not a problem but inorder to recieve full swing at the output signal I need high current passing through the emitter collector. I've checked the datasheet of the Q2N2222. I guess 15mA wont be a problem but the high power dissipation will decrease the efficiency i think:( Also 4 bjts I think is enough:) If I will add I will have lots of difficulties during my lab work...
     
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