Mulit-stage Audio Amplifier

Discussion in 'Homework Help' started by Dannyhimself, Jan 6, 2013.

  1. Dannyhimself

    Thread Starter New Member

    Jan 6, 2013
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    Hey guys.

    I'm currently at university and am studying Audio Electronics as one of my modules. For our first task we've been asked to design a multi-stage Audio Amplifier using simulation. The specification given is:

    Voltage gain 250
    Input resistance 200 kOhm
    Output resistance 5 Ohm
    Current drain 15 mA
    Supply voltage 12 Volt


    I've been reading around the forums/internet and if i'm honest i don't understand anything at all. I'm completely new to electronics and only have the one module in it. It'd be safe to assume that i know nothing at all about the topic.

    Can anyone lend a hand? Where can i learn about this? I do have tutorials but they seem to last 20 mins out of an allocated hour session as my tutor goes through things way too fast. I also only have a week to complete this.

    Thank you for your help.

    Danny.
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    If you are allowed to use op amps, this is a very easy exercise. If it must be all discrete then it's a bit tougher but doable.
     
  3. ECC83

    New Member

    Jan 6, 2013
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    It seems a bit vague, what exactly have you been taught so far so as to tackle this? I don't think any institution teaches nothing at all on the topic and expects people to come up with a solution.

    Which university is this if you don't mind me asking?
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Do you know what the stages will need to do?

    i.e. voltage gain, impedance matching, current gain?
     
  5. Dannyhimself

    Thread Starter New Member

    Jan 6, 2013
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    Hi EEC83, i study at Huddersfield Uni.

    We have been taught Potential Dividers, Transistor Amplification using BJT transistors and the effect of capacitors in the circuit. We've also looked at buffers.

    The problem is, due to a room allocation problem, we have no seminar in which to do exercises and ask questions. We have one lecture per week. It is a good university, i probably find the task more challenging than most.

    The first part of the course was based on the practical side, building the basic circuit and recording results. Now its come to designing our own circuits. I understand the gain comes from the common emitter of the transistors but i don't understand how to manipulate the transistor values to achieve a desired gain.

    Thatoneguy - I think multiple voltage gain stages is what they are after but the specification asks also asks to comment on the AC response.

    Once again thanks for your help.
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Still haven't answered if the design can use op amps or has to be all transistors.
     
  7. Dannyhimself

    Thread Starter New Member

    Jan 6, 2013
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    Sorry, i think it has to be with transistors.
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I would use a FET input stage for the high input impedance. Second stage probably has to be a common emitter bipolar gain stage. Might need a second CE stage to get enough gain. Output could probably be a push pull discrete stage.

    Your spec is missing the most important items: distortion spec? Power delivered? Load impedance being driven?

    What signal level will be fed into the input? That is REALLY critical to know.
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Is it just me, or does it seem fishy to spec an amp using only 15mA at 12v but with 5 ohms output impedance? Will it actually be driving anything?
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    Audio amplifiers have an output impedance MUCH lower than their load. The damping factor of many audio amplifiers is 500 so with an 8 ohm load the output impedance of the amplifier is only 8/500= 0.016 ohms. Then it damps the resonances of speakers very well.

    If this amplifier has a damping factor of 100 then its typical load is 500 ohms and with a 12V supply its maximum peak output current will be 10mA.
     
  11. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    That's why I asked for the signal level: this thing looks like some kind of preamplifier to me, not a power amp. The input impedance of "line level" equipment is typically 5k - 10k Ohms so the output impedance of the pre amps does not have to be super low.
     
  12. Audioguru

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    Dec 20, 2007
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    This amplifier is supposed to have an output resistance of only 5 ohms.
    But its supply current must be 15mA.

    Preamps made with transistors never had this very low output impedance, maybe 1000 ohms or more.
     
  13. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    yep, that is a problem. To get down that low it would need some kind of class AB push pull stage or similar. Not sure off the top of my head if this is doable without giving it a little more juice, but the fact that it has no distortion spec means you could maybe leave it under biased with bad crossover distortion?
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    An LM386 little power amp has a very low output resistance, no crossover distortion and a typical idle current of only 5mA when it has a 12V supply.

    It would not be difficult to duplicate it with transistors.
     
  15. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Might be fun to try, but there's a lot of parts in there. I remember way back when I did the TDA2003 there is a lot of circuitry inside. In most cases, the schematic the manufacturer publishes is "simplified" and not representative of what's inside. Back in 1978, Fairchild actually published complete schematics. They were huge.

    The other problem is that IC circuitry relies on near perfect transistor matching to work, discrete components don't match that well.
     
    Last edited: Jan 9, 2013
  16. Audioguru

    New Member

    Dec 20, 2007
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    3 transistors make the push-pull power amplifier and two transistors make the preamp with a high input impedance. It is a simple project for school.
     
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