Motorola GM160 Service Information - Page 20

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2-10

5.2

Pre-Driver Stage

The next stage is an LDMOS device (Q3421) providing a gain of 13 dB. This device requires a

positive gate bias and a quiescent current ﬂow for proper operation. The voltage of the line

PCIC_MOSBIAS_1 is set during transmit mode by the PCIC pin 24, and fed to the gate of Q3421

via the resistive network R3410, R3415, and R3416. The bias voltage is tuned in the factory.

5.3

The following stage is an enhancement-mode N-Channel MOSFET device (Q3431) providing a gain

of 10dB. This device also requires a positive gate bias and a quiescent current ﬂow for proper

operation. The voltage of the line MOSBIAS_2 is set in transmit mode by the ASFIC and fed to the

gate of Q3431 via the resistive network R3404, R3406, and R3431-5. This bias voltage is also tuned

in the factory. If the transistor is replaced, the bias voltage must be tuned using the Customer

Programming Software (CPS). 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, PASUPVLTG, via L3431 and L3432.

5.4

The ﬁnal stage uses the bipolar device Q3441. The device's collector current is also drawn from the

radio's DC supply voltage input. To maintain class C operation, the base is DC-grounded by a series

inductor (L3441) and a bead (L3442). A matching network consisting of C3446-52, C3467, L3444-5,

and two striplines, transforms the impedance to approximately 50 ohms and feeds the directional

coupler.

5.5

The directional coupler is a microstrip printed circuit, which couples a small amount of the forward

and reﬂected power delivered by Q3441. The coupled signals are rectiﬁed by D3451-2 and

combined by R3463-4. The resulting DC voltage is proportional to RF output power and feeds the

RFIN port of the PCIC (U3501 pin 1). The PCIC controls the gain of stage U3401 as necessary to

hold this voltage constant, thus ensuring the forward power out of the radio to be held to a constant

value.

An abnormally high reﬂected power level, such as may be caused by a damaged antenna, also

causes the DC voltage applied to the PCIC to increase, and this will cause a reduction in the gain of

U3401, reducing transmitter output power to prevent damage to the ﬁnal device due to an improper

load.

THEORY OF OPERATION