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Part 3 : The P.A. Circuit and its Control
To Antenna
mech
Diplexor
switch
The P.A Circuit and its control
The power amplifier control circuit ensures that the RF signal is regulated to the
required limits of operation.
The TX RF signal is input on #1(IC106) or (IC107) and the power amplifier outputs
the signal at #4.
IC105 and IC108 are op-amps, which output a control signal to the P.A.
Power control is carried out by measuring the current drawn by the P.A (see Fig.9:
P.A Circuit Control). The current drawn by the P.A. is directly proportional to the
output power and is measured as a voltage drop across a known resistance path
or "Fixed Control Loop Stripline". This consists of a 47mOhm resistor (R144) for
GSM or two resistors in series, a 47mOhm (R144) and 27mOhm (R136) for PCN.
The voltage drop (V_ERROR) is then integrated by the operational amplifier
(IC108) to give an error voltage, which regulates the TXP signal.
The TXP input signal to the power control circuit is generated by the IC27 #90. TXP
provides a limit for the frequency spectrum caused by burst modulation.
Fig.8: The Power Amp Circuit
PCN = (1710-1785MHz)
GSM = (890-915MHz)
(FL106) (IC106)
(IC105)
GSM_PRE_ON
(IC107)
PCN_PRE_ON
(IC103)
PA
BPF
V_ERROR
TXP_MOD
(FL105)
PA
BPF
(IC104)
(IC108)
V_ERROR
TXP_MOD
From IC101
(FL103)
From IC102
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