Amp Idle Current

Discussion in 'Amateur Radio Amplifiers' started by K6BSU, Jan 15, 2020.

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  1. K6BSU

    K6BSU Ham Member QRZ Page

    I'm finishing a home brew linear amplifier. Uses a Motorols NPN, single-ended, at 40 Watts. It uses a separate, adjustable, low Z source for base bias. I can't decide how much idle current the PA should have. The maximum peak current with RF drive is 1.8A @ 30V. I set resting bias to give 130 mA. Is this too much, or too little?
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Does the data sheet give recommended operating parameters for linear amplifier service?

    What's the device type?
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Also what mode of operation are you planning to use with this amp?

    If this is a general purpose linear amp or will support SSB voice then you'll want to bias it for good linearity which typically means in a Class AB region. If it's purely a CW amp that's not so important.

    As it sits your DC input power is 54 watts (1.8A @30V) if you're really seeing 40 watts RF output out of this amp at that input power it's operating at 74% efficiency which implies it's not biased to be very linear and is operating closer to Class C operation.

    For typical Class AB operation with 40 watts RF output you'd typically see approximately 67 watts DC input power or around 2.25 amps at 30V. Either that or with the 54 watts DC input power you're currently seeing you'd expect to get around 30 to 35 watts RF output power with the amp actually operating in a Class AB region.
  4. K6BSU

    K6BSU Ham Member QRZ Page

    More info. The transistor is MRF 314. Collector characteristic curves are not available, so I can't determine linear region. This is a SSB linear running (I hope) Class AB. Maximum collector current with RF drive was measured at 1.7 A. I figured that close to 8-10% idle current would get it away from non-linear at low currents. I don't have a way to measure distortion in operation. Target output is about 30W.
  5. KM3F

    KM3F Ham Member QRZ Page

    Application comes into play.
    What mode are you intending to use the amplifier?
    A bias current rule is about 10%+ of expected peak current.
    Make sure you have sufficient heat sinking/cooling.
    Once you get this attended to, do an input power/output power run,
    Measure the input power vs output power but keep within the max. current rating.
    Then plot the curve on graph paper,
    Look for the part of the transfer curve that is the [straightest] within about a 1% change at the top and above min.. This is the most linear part, your looking for and operate in.
    Don't drive the amplifier on SSB higher than the top of the curve where it begins to stop rising/ begins flatten/ to nose over in(saturation).
    If you do, compression/distortion begins to become significant.
    You can run the amp into the compression area on FM, or CW but not using the other modes because FM/CW doesn't depend on >>varying drive signal amplitude changes<< such as SSB or AM.
    Also there is a difference in bias setting for SSB and CW/FM modes that has an effect on efficiency and heat dissipation as well as linearity.
    Notice that many vhf/uhf SS amps have a switch for SSB-FM and some FM only use.
    This it for two reasons. A. possible BIAS change per mode. B. to adjust the key-up relay (hang time) if the amp has no provision for external keying. {RF sensing keying} hang time before releasing and adjustable.
    The Bias change is changing the class of operation from C for FM/CW to class AB1 area for SSB/AM.
    CW/FM can still be done in class AB1 but heat dissipation is higher.
    Normally the heat sink should not be run much over 150F without forced cooling fan to protect the device.
    A note; to be more accurate, SS does not normally use the term class AB1 that originates from vacuum tube application but no problem if one understands what is being meant.
    Class C is below current cut-off of the device.
    For AM operation, drive requirement must be set so AM peak envelope power does not rise beyond the linear peak of the curve.
    The MRF 314 SPECS come up for me by MACOM. It's a 28 volt device<<<<<.
    Look again.
    Good luck.
    Last edited: Jan 18, 2020
  6. K6BSU

    K6BSU Ham Member QRZ Page

    KM3F. Your observations are all valid, and I'm familiar with all of them. Since my power supply turned out to be 31V, I have since ordered new final. Will use MRF 426. This has higher ratings (35V Vceo) and characterized for Class AB.

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