Power rating of the BLF248 in class AB

The BLF248 is a VHF push-pull power MOS transistor that is rated for 300W 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.

BLF248 drain characteristics

Above is a recreation of the BLF248 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 BLF248 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 28V 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)	24
DC Drain supply voltage (V)	28
Grid bias voltage (V)	3.05
Drive voltage peak (V)	5.95


Calculated results:
Dies	1
Drain current idle (A)	0.248
Drain current peak (A)	31.084
Drain current DC (A)	8.899
Drain current fundamental (A)	14.686
Drain RFpower (W)	176
Drain dissipation (W)	73
Drain efficiency (%)	70.7%
Resonant load (ohms)	1.63
Output power (W)	166
Output efficiency (%)	66.5%

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 just below 300W.

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 ouput, 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.

A recent demonstration on the ‘net of these transistors producing 500W did not demonstrate linear operation and more importantly, measurement was made with a Bird 43 wattmeter outside the specified frequency range of the slug.