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 (Fig. 1), 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.
The device current draw and efficiency were measured under the same bias conditions and power ramp as used for the gain measurements (Fig. 2). The current draw is about 2.7 A for output power of 100 W at 440 MHz, rising to less than 5 W current draw for 500 W output power. The drain efficiency is better than 40 percent at 500 W output power. For these tests, no tuning was performed other than the initial setup for the test fixture, but the device can be matched at input and output ports for better power, gain, and efficiency. The LDMOS transistor is rated for as much as 60 percent drain efficiency at VHF with rated intermodulation distortion (IMD) of -28 dBc.
Philips Semiconductors, 2178 Mendon Rd., Ste. 300, Cumberland, RI 02864; (401) 305-5052, Internet: www.philips.com.
