Help on Simple Bandpass Filter

Discussion in 'Homework Help' started by BLOBY, Mar 14, 2011.

  1. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Hi,

    I hav to design a Bandpass filter (LC bandpass filter that is...) and also amplify the voltage... I searched the net but could not find easy explanations and all used op amps and stuff.. I need to design it with only one bjt (may be 2 if the gain is not sufficient from the first one.. :)) but I wanna design it with only bjts, few caps, an inductor, and some resistors... ;)

    I hav just designed a Colpitts oscillator(used as a transmitter) and the Bandpass Filter I design now should be such that the input of it should come from the receiving antenna (placed near colpitts oscillator) and the electromagnetic waves should be effectively filtered and should get amplified..

    Plz post a simple schematic or give a general idea on designing such a thing.. I hav attached a picture so that you get the idea of what im talking about.. Plz tell me how to improve it.. The filter should hav a narrow bandwidth.. And also give the maths stuff... I like maths :).. Thank you... Im waiting...
     
  2. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Sorry, I forgot to put a ground point near bottom of L and C resonant circuit...
     
  3. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Plz reply and tell me or give a link regarding bandpass filter design

    (which has its input from an antenna more preferably)...

    Also it should amplify the input voltage using bjts.. plz reply :(..

    Also, the filter should be of LC type.. not cap only or inductor only type.. :)..

    Im waitin....
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    Your transistor is not biased properly so it will not amplify the tuned signal.
    A transistor has a fairly low input resistance but the LC circuit needs to have a fairly high load resistance for it to have a narrow bandwidth.
     
  5. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Thanks Guru, I will do the biasing part.. just tell me if my idea (that of giving input from an antenna) is right or wrong? also on the improvement part of this bandpass filter...(give some schematics :))

    I will start to build my circuit if my basic understanding is right..
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    What radio frequency are you using?

    The transistor and its biasing network will load down the LC filter which will reduce its bandwidth. If you tap the coil then it can drive a transistor and still have pretty narrow bandwidth like this very simple AM radio:
     
  7. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Wooow....

    Now thats what Im talking about.. nice schematic you posted there.. thank you very much.. I am talking about the frequency of about 100 - 200 khz .. Also tell me if the colpitts oscillator working on this frequency will be able to transmit the signal(for about 2 - 3 metres ) and be picked up by this filter of mine?
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    The circuit I posted picks up local AM radio stations that transmit with hundreds of thousands of Watts on a frequency from 540kHz to 1600kHz.
    I have no idea how low is the power from your low frequency transmitter so the range might be very small since it is not a real AM radio.

    The very simple radio will probably need its coupling and filter capacitors increased in value to work at the low frequency. The output of the simple AM radio is static and low audio frequencies that modulate the transmitter.
     
  9. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    :cool: Cool.. I wil try values around 1uF and all for coupling caps...

    By the way, my colpitts oscillator is running on 9V battery.. ;)
    I hav attached the image of it.. Plz tell me if this would transmit for about 3 - 4 metres or not.. It is just a simple experiment of undertaken by me...
    The amplifier stage in oscillator is not well biased.. But im sure that it is oscillating... Thanks for your help Guru
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    Your "transmitter" has a theoretical peak output power of half of about 4.5V squared /300 ohms= almost nothing. Its range into that extremely simple AM radio will be very small.
    A real AM radio has an RF amplifier, a mixer that probably has gain and three or four IF amplifiers, not just a single transistor.
    Most transmitters have an RF amplifier to feed POWER to the antenna.
     
  11. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Oh.. Plz tel me how to improve range(also tel me how did you calculate power like that? )..
    As I said, this is just a small project for learning purpose..
    I want that transmitter to hav atleast 3-4 metre range.. So, give some tips GURU.. ;)
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    The transistor in your oscillator is badly biased because a transistor with a high DC current gain will be nearly saturated and a transistor with a low DC current gain will be nearly cutoff. Also, temperature will affect its biasing.

    Assuming that the transistor has a DC collector voltage of 4.5V then its collector current is 4.5V/300 ohms= 15mA. Therefore the heating is 4.5v x 15mA= 68mW (0.068W). The output power is less so is very, very low.

    The single transistor extremely simple AM radio I posted has low gain but is enough to pickup a very powerful nearby radio station. A real AM radio has at least 4 transistors all providing plenty of gain and good radios have more so that they can pickup weak signals from far away radio stations.

    Your frequency is very low so the antennas should be very long or maybe loop antennas can be used.

    I have no idea what the range will be. Try it and let us know.
     
  13. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Yes.. I am working on it.. Thank You very muchfor ur help..
     
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