TDA5251F1 Datasheet, PDF (43/88 Pages) Infineon Technologies AG – ASK/FSK 315MHz Wireless Transceiver

TDA5251 F1
Version 1.0
Confidential
Application
The output power Po will be reduced when operating in an “overcritical” mode at a R_L > R_LC. As
shown in Figure 3-7, however, power efficiency E (and bandwidth) will increase by some degree
when operating at higher RL. The collector efficiency E is defined as
E=
P₀
V _S IC
[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 3dB will decrease the output power. As shown in the diagram, 250
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_315.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.
Preliminary Specification
43
2003-02-18