Circuit design

AN1224

4/10

1

Circuit design

1.1

Description and consideration

Input and output impedances for the SD57045 are shown in Table 1 below:

With respect to these impedances, two 4:1 transmission line auto transformers were

designed using a 25

Ω, 1/8 wavelength, semi rigid coaxial cable. To achieve this

transformation across the band, a capacitor was added to the low impedance port of each

transformer to cancel the leakage inductance. The frequency response is shown in Figure 1.

Simple L-sections were utilized to make the final transformation from the low impedance

port of the transformers (12.5

Ω) 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.

Broadband 4:1 transformer

Unbalanced transformers offer an efficient matching method from 50 W to low impedance.

Besides, auto 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. The

capacitor value must be selected so that its own resonant frequency is above the frequency

Table 1.

Input and output impedances

Frequency (MHz)

Z input

Z output

88

10.8-j7.60

7.5-j0.15

95

10.6-j8.36

7.8-j0.34

108

10.5-j9.87

8.1-j0.61

S11

0dB

-30dB

-60dB

80MHz

90MHz

100MHz

110MHz