Designers seeking to generate high output
power in broadcast and radar systems are nonetheless hoping to do
so in the smallest device/circuit footprint possible. For HF/VHF
broadcast and UHF radar markets, the new BLF369 LDMOS transistor
from Philips Semiconductors is not only compact, but also delivers
a generous 500 W CW output power from HF through 500 MHz.
Fast pulse rise and fall times result in large differential
current-versus-time (di/dt) values for a device. This is multiplied
by whatever inductance is present between the device and the power
supply. All of this results in a large voltage swing within the
device itself given by V = L di/dt, where V is the voltage swing
within the device and L is the inductance between the device and
the bias supply.
When evaluating a power transistor for processing a particular
type of pulsed waveform, the rise and fall times of the waveform
should be well known in order to accurately evaluate a device for
that application. Once the specification of the pulse waveform is
known, the device should be tested with a waveform that has a rise
and fall time AT LEAST as quick as used in the real application.
The SOT800A-housed BLF369, although an unmatched device, was
optimized for UHF prior to testing. It was evaluated with est gear
from Agilent Technologies (www.agilent.com): an HP8648C signal
generator and HP8990A peak power meter. At 440 MHz, device gain was
typically 12 dB from 100 to 500 MHz, with 500 W output
power across that range. Test conditions included dual drain
voltage supplies of +32 VDC and quiescent current draw of 500 mA
for each supply. Compression is slight even at 500 MHz, indicating
raw power capability.