Class C amp

Discussion in 'Wireless & RF Design' started by powzoom, Jul 27, 2009.

  1. powzoom

    Thread Starter Active Member

    Jan 18, 2009
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    I'm trying to build a class c bjt amplifier to use as a frequency multiplier. I researched it and found this schematic. I'm not sure on how to bias this. I know that its biased into the cutoff region and that only the peaks of the signal get amplified. This keeps the transistor from using too much power. The tank circuit tunes the frequency.

    What's throwing me off is that there is no Rc or Re resistors so I don't know how to analyze the circuit. How do you find Ic?? Ic=Vcc/? and how do I set the value of R1 shown in the circuit. Lets say Vcc=5V and Vin = 1V sine wave.

    I'd rather not use negative supplies.
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Howdy:

    Class C amplifiers are a lot easier to build and operate than to analyze! Needless to say the operation is very non-linear. As a VERY crude approximation, you can figure out the Ic by taking the PEAK current and multiplying it by the duty cycle. Your best bet is to get something like the RCA transistor, thyristor, and diode manual and go through the excellent chapters on transistor design. It's not for the faint of heart, but well worth the journey.

    The GOOD news is, your circuit is almost guaranteed to work....with or without analysis.

    Hope this helps a bit,

    Eric
     
  3. powzoom

    Thread Starter Active Member

    Jan 18, 2009
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    What about R1 in the picture? How do I decide that value? When I model the circuit in multisim, different values don't change much in the output, maybe the time it takes to stabilize. Also the output is in uV. How can I set the gain? Is there a link to a site that goes into detail about class c amps?
     
  4. bertus

    Administrator

    Apr 5, 2008
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  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    As you have discovered, that resistor is rather non critical. It''s function is to simply establish a ground potential for the base; it's value is usually on the order of 1000 ohms.


    You should be getting a LOT more than a microvolt out of the thing, though! How hard are you driving it? A single ended Class C doubler should be around 20% efficient, so you should be getting at least a few hundred millivolts out at the second harmonic. Does the output tuning have much effect? Is it really at the second harmonic?

    eric
     
  6. ELECTRONERD

    Senior Member

    May 26, 2009
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    Is this really a Class C amplifier? It looks like more of a Class A amplifier.
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello ELECTRONERD,

    It is really a class C amplifier.
    There is no biasing of the transistor, the base is even hold down by a resistor.

    Take a look at the attached PDF about amplifier classes.

    Greetings,
    Bertus
     
  8. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    The problem is that your input signal is not large enough..

    As the input is biased at 0V and the transistor base will not conduct until around 0.6 - 0.7V, you need 1.2 - 1.4V P-P before it will start to turn on.

    Your 1V sine wave (1.4V peak) is probably right on the edge for that particular transistor.
    About 50% more drive would give plenty of output, or if you cannot do that put a slight bias on the input, say 0.2 - 0.3V so the 1V signal causes the transistor to conduct.
     
  9. fanie

    Active Member

    Jan 20, 2007
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    R1 should be about 100k and C1 should be ok at 100nF. Q1 could be a 2N3553 or a 2n3866. R1 is only to keep the transistor off when no signal (dc) exist on the input. At 2MHz the 100nF cap will become a short and let the current through.

    The input frequency should be 2MHZ to get the RC to resonate if that's it's resonance freq. Remove the C and it will resonate at the input frequency. Lots of harmonics if you don't add a Hi/Lo pass filter.
     
  10. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    Fanie:
    R1 needs to be a lowish value - 1K or so should be fine.

    The base-emitter junction acts as a diode and C1 will charge on each positive half cycle as the base draws current. R1 is the discharge path for C1 and controls the base drive current.

    Powerzoom states explicitly he's building a frequency Multiplier - the input frequency should be an integer fraction of the circuit's resonant frequency.

    This is the point of using a Class C bias circuit; the transistor only gives a brief current 'kick' to the tuned circuit during each input cycle, allowing the tuned circuit to 'ring' at it's own frequency.

    Think of pushing a swing - you don't have to do it every time, each second / third / forth time etc is OK as long as you give it enough of a push each time to keep it going in between.
     
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