Pa Stages; Directional Coupler; Antenna Switch - Motorola GM-950 Manual

9.2

PA Stages

The following stage uses an enhancement mode N-Channel MOS FET device (Q2521) and requires
a positive gate bias and a quiescent current flow for proper operation. The voltage of the line BIAS
VLTG is set in transmit mode by a Digital to Analogue (D/A) converter (U0731-4) and fed to the gate
of Q2521 via the resistive network R2613, R2614, R2615. 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 A+ via L2622, L2623, L2624.
The final stage uses the bipolar device Q2531 and operates off the A+ supply voltage. For class C
operation the base is DC grounded by two series inductors (L2521, L2522). A matching network
consisting of C2530-C2534, L2532, L2533 and two striplines transforms the impedance to 50 Ohms
and feeds the directional coupler.
9.3

Directional Coupler

The directional coupler is a microstrip printed circuit which couples a small amount of the forward
power off the RF power from Q2531. The coupled signal is rectified to an output power proportional
negative DC voltage by the diode D2657 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.
9.4

Antenna Switch

The antenna switch is switched synchronously with the K9V1 voltage along with the voltage PWR
CNTL signal and feeds either the antenna signal coming through the harmonic filter to the receiver
or the transmitter signal coming from the PA to the antenna via the harmonic filter.
In transmit mode, this PWR CNTL signal is above 1 V and biases Q2511 through Q2641 and Q2642
to allow a collector current to be drawn. The collector current of Q2511 drawn from A+ flows via
L2631, L2531, L2533, directional coupler, D2551, L2651, D2651, L2652, R2645, R2646, L2641,
and switches the PIN diodes D2551 and D2651 to the low impedance state. D2551 leads the RF
signal from the directional coupler to the harmonic filter. The low impedance of D2651 is transformed
to a high impedance at the input of the harmonic filter by the resonant circuit formed by
L2651,C2652 and the input capacitance of the harmonic filter.
Transistor Q2643, controlled by the microprocessor via Signal K9V1, is used to switch the collector
current of Q2641 on in transmit mode and off in receive mode. In receive mode the low K9V1 and
the low PWR CNTL turn off the collector current of Q2511 through Q2641 and Q2642. With no
current drawn by Q2511 and resistor R2651 pulling the voltage at PIN diode D2651 to A+ both PIN
diodes are switched to the high impedance state. The antenna signal, coming through the harmonic
filter, is channelled to the receiver via L2651, C2651 and line PA RX. The high impedance of D2551/
D2651 in off state does not influence the receiver signal.
Theory of Operation
Transmitter Power Amplifier (PA) 5-25W
5C.3-21