TDA7110 Datasheet, PDF (28/39 Pages) Infineon Technologies AG – ASK/FSK Transmitter 868/433 MHz

TDA7110
Applications
3.6 Application Hints on the Power-Amplifier
The power amplifier operates in a high efficient class C mode. This mode is
characterized by a pulsed operation of the power amplifier transistor at a current
flow angle of θ<<π. A frequency selective network at the amplifier output
passes the fundamental frequency component of the pulse spectrum of the
collector current to the load. The load and its resonance transformation to the
collector of the power amplifier can be generalized by the equivalent circuit of
Figure 3-4. The tank circuit L//C//RL in parallel to the output impedance of the
transistor should be in resonance at the operating frequency of the transmitter.
V_S
LC
R_L
Figure 3-4 Equivalent power amplifier tank circuit
Equivalent_power.pdf
The optimum load at the collector of the power amplifier for “critical” operation
under idealized conditions at resonance is:
V²
R =^S
LC 2P
O
A typical value of R_LC for an RF output power of P_o= 10 mW is:
R_LC
=
3² ₌
2 ∗ 0.01
450Ω
“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 degree of efficiency under “critical” operating conditions can be
explained by the low power losses at the transistor. During the conducting
phase of the transistor, its collector voltage is very small. This way the power
loss of the transistor, equal to i_C*u_CE , is minimized. This is particularly true for
small current flow angles of θ<<π.
In practice the RF-saturation voltage of the PA transistor and other parasitics
reduce the “critical” R_LC.
Wireless Components
3-8
Data Sheet, December 2008