Power rating of the BLF278 in class AB

The BLF278 is a VHF push-pull power MOS transistor that is rated for 250W in class AB operation.

The data sheet contains a plot of drain current as a function of gate-source voltage, albeit at only one Vds, 10V.

BLF278 drain characteristics

Above is a recreation of the BLF278 characteristics from 9 digitised points from the data sheet graph.

The RF Power Amplifier Tube Performance Computer is a tool for modeling the operation of a RF power amplifier, given the transfer characteristic of the active device under a given load line. The published characteristics for the BLF278 do not include a family of curves to allow extraction of characteristics under an arbitrary load line, but the curve shown above (effectlively for load line Rl=0Ω) can be used as an approximation. The load line for Rl>0Ω with 48V supply is probably very similar, although the saturation point will be at slightly lower drain current, so this model is likely to slightly overestimate the power output that can be obtained.

 Inputs: Output cct efficiency (%) 94% Drain peak RF voltage (V) 44 DC Drain supply voltage (V) 48 Bias voltage (V) 3.4 Drive voltage peak (V) 2.65 Calculated results: Dies 1 Drain current idle (A) 0.251 Drain current peak (A) 17.113 Drain current DC (A) 5.009 Drain current fundamental (A) 8.139 Drain RFpower (W) 179 Drain dissipation (W) 61 Drain efficiency (%) 74.5% Resonant load (ohms) 5.41 Output power (W) 168 Output efficiency (%) 70.0%

The table above shows the key model inputs and calculated values for 330W output from a pair of dies.

Modelled drain current waveform

Above is the modelled drain current as per the table above.

This model allows exploration of power output vs drive level, albeit within the assumption that the drain transfer characteristics are relatively constant as discussed above.

Modelled gain compression

Above is a plot of output power compression relative to 100W from a pair of dies for various drive levels. A commonly used indicator of maximum power output in linear operation is the point where gain is compressed by 1dB, which in this model is around 190W, and gain compression is 2dB at 330W out.

Adjusting the output circuit and drive for such an amplifier to achieve power in excess of about 330W reduces linearity, increases distortion products, reduces efficiency, and requires higher power supply current. Note that if the amplifier output circuit was adjusted for say 450W maximum output, operation at 330W would require higher power supply current, be at reduced efficiency, and degraded linearity since the transistors operate at higher peak current to deliver the 300W into a reduced load impedance.

The datasheet also shows operating conditions for class AB. Class AB is shown with quiescent current of 0.5A per die, and rated output of 250W. The model above uses quiescent current of 0.25A per die and 330W out.

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