AN1229

Amplifier design

3/7

Figure 2.

Input Impedance of 1:1 balun in cascade with 9:1 balun

SD2932 input matching network was tuned in order to achieve the best compromise in

terms of power gain (Gp) and input return loss (Rtl) over the frequency range 88 - 108 MHz.

Best results were achieved by adding a 10 pF chip capacitor (C1) between RFIN and the

1 nF blocking capacitor (C2).

1.3

Output matching network

The output impedance of each side is a combination of the output capacitance Coss (

195 pF) and the optimum load resistance which can be determined as follows:

Equation 3

The total optimum load, seen by SD2932 (drain to drain), is 2 x 6.02 = 12.04

Ω. Therefore, a

simple two section balun (1:1 balun in cascade with a 4:1 balun transformer) is used to

transform the unbalanced 50

Ω to a balanced 12.5 Ω (2 x 6.25 Ω) which is very near to the

total optimum load resistance.

The first section, a 5" long - 50

Ω flexible coaxial cable, and the second section, two 5" long

- 25

Ω flexible coaxial cables, are connected as described in Figure 6.

To compensate for the output capacitance Coss of SD2932 , a 40 nH inductor (L2) is

connected between the two drains. This LC network (L2 & Coss) is a high pass filter with a

resonance frequency calculated at 10 % below the minimum operating frequency:

Equation 4

Equation 5

Equation 6

0

50

100

150

200

250

300

350

Frequency (MHz)

-35

-30

-25

-20

-15

-10

-5

0

Rp

0.85

Vdd

•

()

2

2

Pout

•

-------------------------------------

0.85

2500

•

2

150W

•

-------------------------------

6.02

Ω

=

=

=

C

OSS

C

OSS

2

---------------

=

180pF

2

-----------------

90pF

=

=

(per side)

Frequency of resonance

0.9

88MHz

80MHz

=

•

=

L2

Coss

2

pi

F

•

•

()

•

•

2

1

L2

44nH

=

→

=