AN1224 STMicroelectronics, AN1224 Datasheet - Page 2

• Manufacturer Part Number: AN1224
• Description: LDMOS RF POWER TRANSISTORS FOR FM BROADBOARD APPLICATION
• Manufacturer: STMicroelectronics

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AN1224 - APPLICATION NOTE
from the low impedance port of the transformers (12.5 Ohms) to the measured impedances of the device
(see table 1). This design uses printed series inductors on a 30 mil Glass Teflon board.
The gain of any power FET is extremely high from DC throughout the low HF frequency band. A feedback
network is necessary to suppress the low frequency gain, as well as give a nominal amount of gain at the
frequency of interest. This feedback also helps to increase the input impedance. Since LDMOS has such
a high gain at low frequencies, a low value, high power, flange mount resistor must be comprised in the
design. The capacitor in the feedback path (C3) provides negative feedback at low frequencies. This
component was designed to be self-resonant.
Far below the FM band, at 100 MHz, the capacitor looks slightly inductive, reducing the amount of
feedback in the band of interest.
Figure 1: Broad Band 4:1 Transformer
Unbalanced transformers offer an efficient matching method from 50
transformers have a zero impedance point over a broad bandwidth, offering an ideal DC feeding point to
the gate and drain circuits. In order to prevent high frequency oscillations, a bypass capacitor is used at
the zero impedance point of the transformer. Capacitor value must be selected so that its self resonant
frequency is above the frequency of interest. Depending on the application, additional low frequency
bypass capacitors isolated with lossy elements (ferrite beads) may be required to prevent power supply
noise affecting gate and drain circuits.
-30dB
-60dB
0dB
80MHz
S11
90MHz
100MHz
110MHz
to low impedance. Besides, auto