Amplifier circuit for 1st year project using bjt's

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prometheus82
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Homework Statement


The question states that a 3 stage amplifier needs to be designed with a 2W rms to an 8ohm load. the amplifier must consist of a pre amp stage and a power stage. (using only bjt's)


Homework Equations


Up till now we have learned about small signal model, DC biasing, ac biasing. in terms of knowledge i am still in my early stages


The Attempt at a Solution


well i started off using dc biasing in each stage whilst keeping in mind that a buffer stage had to have a high input but low output impeadance, the gain stage was the other way around. for now in the design i left out anything to remove the crossover distortion since i wanted to get it working before adding any further complications. despite this there is still something wrong with the gain stage, the simulation is showing a gain of 3 instead of the 50 aimed for and the signal is looking more like a squarewave than a sine wave.
Any helpful points as to the approach one should take to designing this circuit is appreciated, since I'm still green when it comes to this i might have missed a crucial step. also the circuit presented is the last one i calculated, if someone can see what is wrong in it, please comment.
 
Attachments
  • Hand-drawn schematic of a three-stage BJT amplifier on a whiteboard, with a 50 ohm source, 10mV input, 100k and 200k bias resistors, 600 ohm and 1k ohm resistors, 2000uF coupling capacitors, +15V and -15V rails, Av=47 and 'for beta=100' labels, and a complementary Darlington output stage with beta=10000 driving an 8 ohm load.
    036.jpg
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Discussion

Hi prometheus82! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

Those are remarkably large coupling capacitors you've chosen for stages 1 and 2!

For the first stage, you should be able to examine the voltage (DC and AC separately) and confirm that they are close to what you designed for. Then do the same for the second stage. There is no point in looking for subtle effects until you have confirmed that your DC levels are close to what you designed them to be.

Good luck!
 
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