Measurement Results for the Dynamic Supply CMOS RF Power Amplifie - Linear CMOS RF Power Amplifiers for Wireless Applications - page 40

Table 4.1 RO4350
characteristics [ 25 ]

Parameter Value

Thickness (mm) 0.508

ε r 3.48

tan δ 0.0031 @ 2.5 GHz

Copper cladding (µm) 17

Fig. 4.1 Photograph of the dynamic supply die lodged in the evaluation PCB

was used as the substrate. The reason for choosing an RF laminate, instead of a
conventional FR-4, 1 stems from its superior RF performance, mainly the tight tol-
erances specified for its electrical characteristics, such as the relative dielectric con-
stant ( ε r ), and its low dielectric loss (tan δ [ 24 , Chap. 1]). The main characteristics
of the substrate used are given in Table 4.1 .
The 2-layer PCB was fabricated with a special window opened in it where the
die of the dynamic supply RF PA was inserted. The photograph of the assembly
in Fig. 4.1 shows the window used to reduce the length of the bondwires resulting
from the chip-on-board mounting. The equivalent inductances from the pad of the
integrated circuit to the pad of the PCB were therefore reduced. The bottom layer of
the PCB was used as a ground plane and several metalized vias were employed to
connect the ground nodes in the top layer to the ground plane. The back of the die
was also connected to the ground plane using a piece of metal sheet and conductive
glue. With such an arrangement, bondwires with a length of less than 0.5 mm were
possible in critical points like the RF input, RF output and RF ground, resulting
in less than 0.5 nanohenry per bondwire—if we consider a value of 1 nH/mm [ 15 ,

1 FR-4 stands for Flame Retardant type 4, which is a woven fiberglass reinforced epoxy resin.