Power Controlled Driver Stage; Final Stage; Directional Coupler; Antenna Switch - Motorola CM160 Service Information

UHF Transmitter Power Amplifier (403-440 MHz)
PA.Op-amp U103-3 monitors the drain current of U101 via resistor R122 and adjusts the bias
voltage of U101.
In receive mode, the DC voltage from RX_EN line turns on Q101, which in turn switches off the
biasing voltage to U101.
3.2

Power Controlled Driver Stage

The next stage is an LDMOS device (Q105) which provides a gain of 12dB. This device requires a
positive gate bias and a quiescent current flow for proper operation.The voltage of the
PA_CURRENT is set in transmit mode by ASFIC (U504 pin 5) and fed to the gate of Q105 via
resistive network R186, R187. This bias voltage is tuned in the factory. If the transistor is replaced,
the bias voltage must be tuned using Global Tuner. Care must be taken so that the transistor is not
tuned exceeding the allowed bias voltage. This device directly drains current from B+ via L122.
3.3

Final Stage

The final stage is an LDMOS device (Q100) providing a gain of 12dB. This device also requires a
positive gate bias and a quiescent current flow for proper operation. The voltage of the line PA_BIAS
is set in transmit mode by ASFIC (U504 pin 4) and fed to the gate of Q100 via the resistive network
R134, R131. This bias voltage is tuned in the factory. If the transistor is replaced, the bias voltage
must be tuned using the Global Tuner. Care must be taken not to damage the device by exceeding
the maximum allowed bias voltage. The device's drain current is drawn directly from the radio's DC
supply voltage input, B+, via L117, L115, L124 and L125.
A matching network consisting of C1005, C1017, C1004, C1009, C1008, C1007, C1274, C1279,
C1275, C1276, C1277, C1278, C1021, C1280, C1013, L126, L127 and two striplines, transforms
the impedance to 50 ohms and feeds the directional coupler.
3.4

Directional Coupler

The directional Coupler is a microstrip printed circuit, which couples a small amount of the forward
power of the RF power from Q100. Coupled power is rectified by diode D105 to produce a
proportional DC voltage; and the resulting DC voltage is routed to the power control section to
ensure that the forward power out of the radio is held to a constant value.
3.5

Antenna Switch

The antenna switch utilizes the existing dc feed (B+) to the last stage device (Q100). The basic
operation is to have both PIN diodes (D103, D104) turned on during key-up by forward biasing
them. This is achieved by pulling down the voltage at the cathode end of D104 to around 12.4V
(0.7V drop across each diode). The current through the diodes needs to be set around 100 mA to
fully open the transmit path through resistor R108. Q106 is a current source controlled by Q103
which is turned on in Tx mode by TX_EN. VR102 ensures that the voltage at resistor R107 never
exceeds 5.6V.
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