TDA5250D2_07 Datasheet, PDF (45/94 Pages) Infineon Technologies AG – ASK/FSK 868MHz Wireless Transceiver

TDA5250 D2
Version 1.7
Application
E=
P
O
VI
SC
[3 – 10]
The diagram of Figure 3-7 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
and
P ~R
OUT
OPT
[3 – 11]
[3 – 12]
Power_E_vs_RL.wmf
Figure 3-7 Output power P_o (mW) and collector efficiency E vs. load resistor R_L.
The DC collector current Ic of the power amplifier and the RF output power Po vary with the load
resistor RL. This is typical for overcritical operation of class C amplifiers. The collector current will
show a characteristic dip at the resonance frequency for this type of “overcritical” operation. The
depth of this dip will increase with higher values of RL.
As Figure 3-8 shows, detuning beyond the bandwidth of the matching circuit results in a significant
increase of collector current of the power amplifier and in some loss of output power. This diagram
shows the data for the circuit of the test board at the frequency of 868 MHz. The effective load
resistor of this circuit is RL= 240Ohm, which is the optimum impedance for operation at 3V. This will
lead to a dip of the collector current f approx. 20%.
Data Sheet
45
2007-02-26