TDA7100 Datasheet, PDF (30/41 Pages) Infineon Technologies AG – 434 MHz ASK/FSK Transmitter in 10-pin Package

V_S
LC
R_L
TDA7100
Applications
Figure 16 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= 5 dBm (3.16 mW) is:
R _LC
=
3²
2 * 0.00316
= 1423 Ω
“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.
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 17.
The collector efficiency E is defined as
E=
P
O
V_S I_C
The diagram of Figure 17 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
Data Sheet
30
V 1.0, 2007-05-02