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  1. #1
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    Replacing base resistors in RF amplifier

    Hi, in this circuit http://qrp.gr/cwqrpp/index.htm I would like to replace the two base resistors of the first bfr96s buffer transistor (1k-270R) with higher values, so the oscillator sees a higher impedance and at the same time the buffer bias (voltage) do not change.

    Can you suggest which base resistors should I use?

    Is it enough to just scale them up?
    For example multiply 1k by 10 = 10k and 270R by 10 = 2.7k

    Since it is a voltage divider base bias it should achieve the same base voltage, but what about the impedance that the oscillator sees?
    Last edited by neazoi; 1st July 2015 at 08:30.
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    •   Alt1st July 2015, 08:23

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    Re: Replacing base resistors in RF amplifier

    Have you heard about transistor operation point?

    Resistors in base circuit must be set to have the needed operation point, and other resistors around the transistor to stabilize it over the temperature range.



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    Re: Replacing base resistors in RF amplifier

    The oscillator does not wnat to see a high impedance. You want all the crystal current to pass into the buffer amplifer which has a relatively low iput impedance anyway. Taking it to an extreme if you removed the tuning capacitors and the crystal was connected to an infinite load impedance the oscillator would not work, replacing the load with a very low impedance would get it going again.
    You could increase the bias resitors, but to mantain the sam bias conditions you will need to take the base current into account. I would leave the bias alone unless you need to change it to suit the resistors to hand.


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    •   Alt1st July 2015, 13:22

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    Re: Replacing base resistors in RF amplifier

    Quote Originally Posted by jiripolivka View Post
    Have you heard about transistor operation point?

    Resistors in base circuit must be set to have the needed operation point, and other resistors around the transistor to stabilize it over the temperature range.

    I do not get your point. I am talking only about the base resistors (potential divider)
    Apart from base current limitations, I do not see why I could not scale the resistors up, since the same DC voltage would be applied to the base (bias), even with the scaled-up resistors.
    Am I getting this wrong?
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



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    Re: Replacing base resistors in RF amplifier

    Problem you have is when resistors are changed Ic current must not be changed. Resistor 1k can have so high resistance that 270ohm resistor is not needed. In this case temperature stability is lowest possible. Changing resistors you also change the gain and phase of amplifier which may stop oscillating.
    10mA current of a divider is a bit high for QRP.
    Try and you will see what will happen.



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    Re: Replacing base resistors in RF amplifier

    "Scaling" the base voltage divider by a factor of 10 will change the bias point according to the actual BFR96 dc current gain which is specified with a range of 30 to 200. Doing a few calculations can save a lot of guesses.

    If a higher buffer input impedance is advantageous or unfavourable for oscillator operation is less obvious. I'm not sure if the circuit designer could clearly answer the question.


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    •   Alt1st July 2015, 15:53

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    Re: Replacing base resistors in RF amplifier

    1. You could isolate the resistors from the Rf side by feeding the output from the potential divider to the base via a RF choke.
    2. The 1K provides RF negative feedback from the collector to the base, connecting the 1K directly to the +12V line would remove this feedback, giving you increased gain and input impedance.
    3. Any thing you do might cause the stage to oscillate and its frequency response will get more "peaky".
    Frank


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    Re: Replacing base resistors in RF amplifier

    2. The 1K provides RF negative feedback from the collector to the base, connecting the 1K directly to the +12V line would remove this feedback, giving you increased gain and input impedance.
    That's an important point, I must confess that I didn't think about it yet. Most likely, the low input impedance created by the feedback circuit is essential for oscillator operation.



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