1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

RF signal amplification

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by whiz115, Aug 6, 2009.

  1. marcbarker

    marcbarker New Member

    Joined:
    Jul 5, 2009
    Messages:
    727
    Likes:
    7
    Location:
    London, UK
    You're in good company... there's a self-taught inventor "Magic Alex", he was born in Athens, came to London UK as a student. He originally repaired televisions and became freinds with the Beatles (who looked to him as a electronics Guru), then he worked for rich and famous people doing 'technical' work for them. He always had different ideas. Magic Alex - Wikipedia, the free encyclopedia

    biased in class c http://en.wikipedia.org/wiki/Electronic_amplifier#Class_C
     
    Last edited: Aug 24, 2009
  2. whiz115

    whiz115 Member

    Joined:
    Jan 29, 2007
    Messages:
    742
    Likes:
    2
    Location:
    DotGR
    interesting... i didn't knew about him, but i know about another greek also named alex! "Sir Alex issigonis" the designer of the famous english car named Mini cooper.

    Alec Issigonis - Wikipedia, the free encyclopedia

    here in greece in many instances we don't let our people become successful, instead we take em down mostly due to underestimation of their abilities and consequently lack of support and inertia for their work... that's why they "escape" to foreign countries where they can have the potentials. As greeks We love foreign stuff, we hate what is ours and it is homemade..

    but i believe we went far too much offtopic! :D
     
  3. Damo666

    Damo666 Member

    Joined:
    Dec 3, 2008
    Messages:
    58
    Likes:
    1
    Location:
    Manchester, England. UK
    Can somebody explain why all the stages except the final need biasing? I was under the impression that class c rf amps are always off until a signal enters. So why forward biasing. I don't undertand.
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. unclejed613

    unclejed613 Well-Known Member

    Joined:
    Apr 3, 2009
    Messages:
    1,828
    Likes:
    99

    i'm thinking you may want a bypass cap from the L3-R3 junction to ground, so all of the RF voltage is developed across L3. R3 is just a current limiter for the transistor
     
  6. whiz115

    whiz115 Member

    Joined:
    Jan 29, 2007
    Messages:
    742
    Likes:
    2
    Location:
    DotGR
    if you think it's mandatory i should add it... i did the same question before
    but i didn't got any responce just tell me a possible value for that.

    i would like to ask what could be the absolute minimum input level of my amplifier so it can amplify a signal?
     
  7. Damo666

    Damo666 Member

    Joined:
    Dec 3, 2008
    Messages:
    58
    Likes:
    1
    Location:
    Manchester, England. UK
    Anyone please?
     
  8. kchriste

    kchriste New Member Forum Supporter

    Joined:
    Jul 23, 2006
    Messages:
    3,677
    Likes:
    47
    Location:
    Victoria BC, Canada
    For low level signals at the base (apx 0.7V or less), you need bias or the transistor cannot even turn on.
     
    Last edited: Aug 25, 2009
  9. unclejed613

    unclejed613 Well-Known Member

    Joined:
    Apr 3, 2009
    Messages:
    1,828
    Likes:
    99
    0.01uf (10nf) would be fine...

    the minimum input level will depend on what kind of gain you get from the first stage, and what your best impedance match between the two stages gives you at the base of the output transistor, you have to have at least 0,7V positive peak value to turn on the output transistor.. you should also use a voltage divider, rather than the "self-bias" method on the first stage. that way you can adjust the bias on the first stage. the best reason for doing this is that the gain of a transistor varies with collector current, and being able to change the bias (while keeping the transistor within it's class A operating range) will allow you to get maximum gain out of the transistor. this change in gain (Hfe) can be as wide as a 100:1 ratio, depending on the transistor, but between 10:1 and 30:1 is most common. in most transistors, the relationship between Hfe and collector current is inverse, so the closer you get to cut-off, the higher the gain.
     
  10. mneary

    mneary New Member

    Joined:
    Feb 27, 2007
    Messages:
    4,502
    Likes:
    67
    Location:
    California USA
    You don't forward bias a class C stage; you bias it so that it, as you say, is off until a signal enters. Perhaps the term bias is confusing. It doesn't just mean turning the transistor on - it means setting an operating point.

    For example, an inductor from base to emitter sets the bias to zero, so that only the AC input voltage greater than the base threshold can turn the stage on. Bias can also be negative.
     
  11. unclejed613

    unclejed613 Well-Known Member

    Joined:
    Apr 3, 2009
    Messages:
    1,828
    Likes:
    99
    that's why i didn't say "bias"..... maybe a more accurate term would have been "drive it into conduction". to add any positive bias would make it a class B amp or a class A amp. adding negative bias would improve the efficiency of the amp, but would require more drive signal. it's best left at zero bias. the first transistor is running class A, and picking a good operating point for it will maximize the gain, and that's why i didn't make an offhand prediction of how much minimum signal is required at the input to drive the output transistor into conduction. i don't like using transistors in self biasing configurations, because normal device-to-device variations in Hfe and Vbe make self biased circuits inconsistent. better to set a stable operating point and minimize the effect of random variations in Hfe and Vbe.
     
  12. Damo666

    Damo666 Member

    Joined:
    Dec 3, 2008
    Messages:
    58
    Likes:
    1
    Location:
    Manchester, England. UK
    @mneary and unclejed, thanks for clearing it up for me. Much appreciated.
     

Share This Page