100-Watt Transmitter; Figure 3-22. 100-Watt Rf Power Amplifier (Rfpa) Gain Stages (Vhf) - Motorola ASTRO Digital XTL 5000 Detailed Service Manual

3-28

3.6.1.2 100-Watt Transmitter

The following text discusses the 100-W transmitter.
3.6.1.2.1 RF Power Amplifier (RFPA)
The RFPA consists of three gain stages, which are shown in
Tx_inj
2mW

Figure 3-22. 100-Watt RF Power Amplifier (RFPA) Gain Stages (VHF)

First Stage
The RFPA first stage provides gain that is determined by the control voltage, RFPA_CNTRL. This
control voltage is generated in the power control section and is a function of the final-stage output
power, temperature, and current, as well as the control and A+ voltage levels. See
Control (for 50W and 100W Transmitter)"
control section.
The 2 mW TX_INJ signal is routed to the U3500 first-stage device (Pin 16, RFIN) via C3500 to an
integrated, wide-band input match. U3550 is a two-stage LDMOS device with a bandpass interstage
match consisting of C3502 and L3501 routed between VD1 (pin 14) and G2 (pin 11). L3501 and
L3500 provide the K9.1V drain bias voltage for the first and second stages to VD1 (pin 14) and
RFOUT1/2 (pins 6 and 7), respectively. The RFPA_CNTRL gate bias is provided to both stages
internally via VCNTRL (pin 1). Both U3500 stages are operated Class A.
Driver Stage
C3521, L3520, C3520, and a transmission line form a low-pass, interstage match that transfers
power to the Q3520 LDMOS transistor. R3520-5 provide device stability, and R3526 and C3522
supply the VGBIAS2 gate bias. L3521, R3528, C3525, C3524, C3536, L3522 and R3527 form the
A+ drain bias circuit. Q3520 is operated Class AB.
May 25, 2005
Key9.1V
100mW
U3500
Q3520
30C65
A+
RFPA_cntrl
on
Figure 3-22.
A+
Vg_bias
50W
5W
Q3540
10W
Vg_bias
Q3541
50W
5W
Vg_bias
A+
page 3-30 for a detailed explanation of the power
Theory of Operation: Transmitter
RFPA_out
100W
"3.6.1.4. Power
6881096C74-B