TDA7110F Datasheet, PDF (26/36 Pages) Infineon Technologies AG – 434 MHz ASK/FSK Transmitter in 10-pin Package

TDA7110F
Applications
In practice the RF-saturation voltage of the PA transistor and other parasitics reduce the
“critical” R_LC.
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 14.
The collector efficiency E is defined as
E = ^PO
VI
SC
The diagram of Figure 14 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_OUT ~ R_OPT
18
16
14
12
10
8
6
4
2
0
0
Pout [mW]
10*Ec
100
200
300
400
500
RL [Ohm]
Figure 14 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.
Data Sheet
26
V 1.0, 2009-02-26