tda5251f1 Infineon Technologies Corporation, tda5251f1 Datasheet - Page 42

• Manufacturer Part Number: tda5251f1
• Description: Ask/fsk 315mhz Wireless Transceiver
• Manufacturer: Infineon Technologies Corporation

Confidential
3.1.4
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 q<<p. 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 3-6. 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.
Figure 3-6
The optimum load at the collector of the power amplifier for “critical” operation under idealized
conditions at resonance is:
A typical value of R LC for an RF output power of P o = 13mW is:
Critical” operation is characterized by the RF peak voltage swing at the collector of the PA transistor
to just reach the supply voltage V
explained by the low power loss at the transistor.
During the conducting phase of the transistor there is no or only a very small collector voltage
present, thus minimizing the power loss of the transistor (i
current flow angles of q<<p
parasitics will reduce the “critical” R
Preliminary Specification
R
LC
R
=
LC
V
2
Power-Amplifier
=
S
P
0
2
Equivalent power amplifier tank circuit
2
*
. 0
3
2
013
=
350
.
In practice the RF-saturation voltage of the PA transistor and other
W
S
. The high efficiency under “critical” operating conditions can be
LC
L
.
C
42
R
V
L
S
C
*u
CE
). This is particularly true for low
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TDA5251 F1
Version 1.0
Equivalent_power_wmf.
Application
2003-02-18