TDA7110 Datasheet, PDF (29/39 Pages) Infineon Technologies AG – ASK/FSK Transmitter 868/433 MHz

TDA7110
Applications
The output power P_o is reduced by operating in an “overcritical” mode
characterised by R_L > R_LC.
The power efficiency (and the bandwidth) increase when operating at a slightly
higher R_L, as shown in Figure 3-5.
The collector efficiency E is defined as
E = ^PO
VI
SC
The diagram of Figure 3-5 was measured directly at the PA-output at V_S = 3 V.
Losses in the matching circuitry decrease the output power by about 1.5 dB. As
can be seen from the diagram, 250 Ω is the optimum impedance for operation
at 3 V. For an approximation of R_OPT and P_OUT at other supply voltages those
two formulas can be used:
R_OPT ~ VS
and
P ~R
OUT
OPT
18
16
14
12
10
8
6
4
2
0
0
Pout [mW]
10*Ec
100
200
300
400
500
RL [Ohm]
Figure 3-5
Power_E_vs_RL.pdf
Output power P_o (mW) and collector efficiency E vs. load resistor R_L.
The DC collector current I_c of the power amplifier and the RF output power P_o
vary with the load resistor R_L. 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 R_L.
Wireless Components
3-9
Data Sheet, December 2008