Blowing transistors when biasing amp

Discussion in 'Amateur Radio Amplifiers' started by KD5W, Jul 22, 2021.

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  1. KD5W

    KD5W Ham Member QRZ Page

    I built a test circuit to measure drain current and bias voltage (following this

    I measured a few irf530n(s)

    bias drain
    2.7v - .79a
    3.2v - .79a
    3.7v - .82a

    So I believe the transistors are good and will start looking for self-oscillation.
  2. KM3F

    KM3F Ham Member QRZ Page

    Good, but you have a 1 volt difference and a 300 ma difference.
    I would deem that a large difference.
    That's why just setting voltage is not good enough for replacement or substitution.
  3. AC9YY

    AC9YY Ham Member QRZ Page

    Looks like 30 ma to me.
  4. KM3F

    KM3F Ham Member QRZ Page

    Your right, to quick on the trigger.
    AC9YY likes this.
  5. LU6PSG

    LU6PSG Ham Member QRZ Page

    The circuit has poor design. Gates should be resistively loaded to ground to avoid a potential self oscillation from a 90 degreees phase shift and also an LC resonance between gates and input transformer (gates are basically capacitors) so the Q of this network must be lowered by resistors, either gate resistors or input resistor. Stability is compromised whenever input and output are floating and not connected to a 50 ohm resistor.
  6. LU6PSG

    LU6PSG Ham Member QRZ Page

    Also please make sure that the negative feedback networks (resistors and capacitors) are ok
  7. HB9PJT

    HB9PJT Ham Member QRZ Page

    The gates are grounded by R3.

    73, Peter - HB9PJT
  8. LU6PSG

    LU6PSG Ham Member QRZ Page

    R3 is only working on the common mode. But doesn't do anything for the differential mode, which, for a floating input, is only loaded by the magnetizing resistance of the input transformer. Not a healthy condition.

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