TDA7110F Datasheet, PDF (25/36 Pages) Infineon Technologies AG – 434 MHz ASK/FSK Transmitter in 10-pin Package

TDA7110F
Applications
Even harmonics of the signal at CLKOUT can interact with the crystal oscillator input
COSC preventing the start-up of oscillation. Care must be taken in layout by sufficient
separation of the signal lines to ensure sufficiently small coupling.
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 13. 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 13 Equivalent power amplifier tank circuit
The optimum load at the collector of the power amplifier for “critical” operation under
idealized conditions at resonance is:
R _LC
=
V_S²
2 * P_O
The theoretical value of R_LC for an RF output power of P_o= 10dBm (10mW) 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.
Data Sheet
25
V 1.0, 2009-02-26