Noob transistor replacement question

Discussion in 'Wireless & RF Design' started by Synaps3, Apr 9, 2015.

  1. Synaps3

    Thread Starter Member

    Jun 5, 2013
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    Let's say I have an amplifier with a certain transistor. If I find another transistor that runs on the same supply voltage and bias voltage with a higher gain, can it simply be replaced or does the circuit have to be somehow designed specifically for that transistor (assuming the circuit is capable of handling the higher current). If not what "calculations" have to be done to find out what changes need to be made?

    Also, I am confused why there is no "bias voltage" in the datasheets of any transistors. Is it the same as "Gate Threshold Voltage"?

    For example, I want to buy this amplifier on ebay: http://www.ebay.com/itm/281595136312?_trksid=p2060353.m1438.l2649&ssPageName=STRK:MEBIDX:IT

    It comes with a 100W MRF186. The seller told me the circuit can handle up to 600W, so could I replace it with something like the MRF151G without any changes to the circuit?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    You have three questions in the one post.

    1) The operational characteristics of a circuit depend on both the specs of the transistor as well as the circuit components. Transistors are not manufactured with a specified gain but will come with a range of gain. The circuit designer takes this into account and will design the circuit to be somewhat independent of the gain of the transistor.

    2) The bias voltage of a transistorized circuit is determined by the components in the circuit and not by a specific "bias voltage" of the device. One way to determine the proper bias voltage is to superimpose a "load line" on top of the I-V characteristic curves of the device.

    [​IMG]

    From this one is able to establish a DC operating point (Q-point) and to analyze the AC performance of the circuit.

    3) You can replace one device with another. The result would depend on the differences in the two devices as well as component values and supply voltages.
     
  3. Synaps3

    Thread Starter Member

    Jun 5, 2013
    67
    2
    None of that really answers my questions.

    I know.

    Yes, I understand that it is determined by the components, not the transistor. Every device has a certain range that is required though, correct? And I am saying I replace it with another with the same range.

    The "differences". WHAT DIFFERENCES? My questions is: If I replace one transistor with another that has the same voltage ranges as the original, but has more gain or power capability, will the circuit need to be modified? In other words, do transistors with the same voltage specs need to be "matched" differently?

    Super clear: Can I replace the MRF186 with the MRF151G??
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    Why would you want to replace the MRF186 with MRF151G?

    Are you expecting to see an increase in power output? The answer is NO, as explained in post #2.
    You would have to alter input signal, component values, power supply voltage or any such combination.
     
  5. Synaps3

    Thread Starter Member

    Jun 5, 2013
    67
    2
    If I did increase the input power and supply voltage, but did no modifications to the circuit, do you think it would work?
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    If the only difference is the max power then you should expect an increase in power output.
    But it begs the question, are the other components suitably rated to handle the higher voltage and current?
     
  7. Synaps3

    Thread Starter Member

    Jun 5, 2013
    67
    2
    I think they probably would handle the higher voltage because the seller said it could handle up to 600W and I don't think they make a 600W transistor that is only 24V. I think it would have to be 48V for that if I'm not mistaken.

    Didn't mean to come off as rude in my other post.
     
  8. Synaps3

    Thread Starter Member

    Jun 5, 2013
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    I'm still wondering this:

     
  9. MrChips

    Moderator

    Oct 2, 2009
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    I already explained this. There is no fixed bias voltage. It depends on what part of the characteristic curve you want to operate. Is the amplifier to function as a class A/AB/B/C/D amplifier?

    http://en.wikipedia.org/wiki/Amplifier
     
  10. Papabravo

    Expert

    Feb 24, 2006
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    No. Bipolar Junction Transistors (BJT) do not have a "gate" terminal, only Field Effect Transistors (FET) have such a terminal. So "Gate threshold Voltage" has no meaning for a BJT.

    There is no "bias voltage" in the datasheet because a BJT can have any bias voltage that a designer wants to have. for a Class A amplifier it might be in the middle of the load line, but for a Class C amplifier it might be close to cutoff, so only the peaks would cause conduction.
     
  11. Synaps3

    Thread Starter Member

    Jun 5, 2013
    67
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    I didn't say it was fixed. The amplifier says the bias voltage range is 3.5 - 4.2V. Does this characteristic curve change depending on the amplifier or the MOSFET? If I put a different MOSFET in the same amplifier, do I need to alter the bias? If so, how do I find out what it should be for the new MOSFET? Sorry, I don't know the class of the amplifier.

    As far as I know the transistors I'm talking about are MOSFETS.
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I don't know what the amp manufacturer means by "the bias voltage range is 3.5 - 4.2V".
    The datasheet for the MRF186 specifies Vgs(th) as 2.5V-4V.
    The datasheet for the MRF151G specifies Vgs(th) as 1.0V-5.0V.
    The typical input capacitance specified for those FETs is worth noting: it's 177pF for the MRF186 but 350pF for the MRF151G.
    Clearly there are differences between the two FETs. Whether or not they are sufficient to justify altering the circuit design (apart from obvious heat-sinking requirements) to accommodate the MRF151G would depend on what the present design actually is. Have you asked the amp manufacturer if FET substitution is ok?
     
  13. Synaps3

    Thread Starter Member

    Jun 5, 2013
    67
    2
    Thanks. That clears things up a bit. I'm still waiting for a response from the manufacturer.
     
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