TDA5250D2 Datasheet, PDF (47/97 Pages) Infineon Technologies AG – ASK/FSK 868MHz Wireless Transceiver

confidential
TDA 5250 D2
preliminary
Applications
“Critical” operation is characterized by the RF peak voltage swing at the
collector of the PA transistor to just reach the supply voltage V_S. The high
efficiency under “critical” operating conditions can be explained by the low
power loss at the transistor.
During the conducting phase of the transistor there is no or only a very small
collector voltage present, thus minimizing the power loss of the transistor
(i_C*u_CE). This is particularly true for low current flow angles of θ<<π. In practice
the RF-saturation voltage of the PA transistor and other parasitics will reduce
the “critical” R_LC.
The output power P_o will be reduced when operating in an “overcritical” mode
at a R_L > R_LC. As shown in Figure 4-7, however, power efficiency E (and
bandwidth) will increase by some degree when operating at higher R_L. The
collector efficiency E is defined as
E
=
P_O
V_S I_C
[4 – 10]
The diagram of Figure 4-7 has been measured directly at the PA-output at
V_S=3V. A power loss in the matching circuit of about 2dB will decrease the
output power. As shown in the diagram, 240 Ohm is the optimum impedance
for operation at 3V. For an approximation of R_OPT and P_OUT at other supply
voltages those 2 formulas can be used:
R_OPT ~ VS
[4 – 11]
and
P_OUT ~ R_OPT
[4 – 12]
Wireless Components
4-7
Specification, July 2002