Motorola GM950E Service Manual page 64

Power AmpliÞer (PA) 5-25W
In receive mode the PA control line (PWR CNTL) is at ground level and switches off the collector
current of Q3641 which in turn switches off the current source transistor Q3642 and the RF
transistor Q3511.
12.2 PA Stages
The following stage uses an enhancement mode N-Channel MOS FET device (Q3521) and requires
a positive gate bias and a quiescent current ßow for proper operation. The voltage of the line BIAS
VLTG is set in transmit mode by a Digital to Analog (D/A) converter (U0731-4) and fed to the gate of
Q53521 via the resistive network R3613 - R3615. The bias voltage is tuned in the factory. If the
transistor is replaced, the bias voltage must be tuned with the Radio Service Software (RSS). Care
must be taken, not to damage the device by exceeding the maximum allowed bias voltage. The
collector current is drawn from the supply voltage 9V3 SUPP.
The Þnal stage uses the bipolar device Q3531 and operates off the A+ supply voltage. For class C
operation the base is DC grounded by two series inductors (L3521, L3522). A matching network
consisting of C3530-C3534, L3532, L3533 and two striplines transform the impedance to 50 Ohms
and feed the directional coupler.
12.3 Directional Coupler
The directional coupler is a microstrip printed circuit which couples a small amount of the forward
power off the RF power from Q3531. The coupled signal is rectiÞed to an output power proportional
negative DC voltage by the diode D3657 and sent to the power control circuit in the controller
section via the line PWR DETECT for output power control. The power control circuit holds this
voltage constant, thus ensuring the forward power out of the radio to be held to a constant value.
12.4 Antenna Switch
The antenna switch is switched synchronously with the PWR CNTL signal and feeds either the
antenna signal coming through the harmonic Þlter to the receiver or the transmitter signal coming
from the PA to the antenna via the harmonic Þlter.
In transmit mode, this PWR CNTL signal is above 1 V and biases Q3511 through Q3641 and Q3642
to allow a collector current to be drawn. The collector current of Q3511 drawn from A+ ßows via
L3631, L3531, L3532, L3533, directional coupler, D3551, L3651, D3651, L3652, Resistors R3645,
R3646, R3648 and switches the PIN diodes D3551 and D3651 to the low impedance state. D3551
leads the RF signal from the directional coupler to the harmonic Þlter. The low impedance of D3651
is transformed to a high impedance at the input of the harmonic Þlter by the resonant circuit formed
by L3651,C3652 and the input capacitance of the harmonic Þlter.
In receive mode the PWR CNTL signal at ground level turns off the current through the PIN diodes
and switches them to the high impedance state. The antenna signal, coming through the harmonic
Þlter, is channelled to the receiver via L3651, C3651 and line PA RX. The high impedance of D3651
in off state does not inßuence the receiver signal.
4-26 Theory of Operation