Motorola GM640 Сервісна інформація - Сторінка 14

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2-4

Op-amp U3402-1 monitors the drain current of U3401 via resistor R3444 and adjusts the bias

voltage of U3401 so that the current remains constant. The PCIC (U3501) provides a DC output

voltage at pin 4 (INT) which sets the reference voltage of the current control loop. A raising power

output causes the DC voltage from the PCIC to fall, and U3402-1 adjusts the bias voltage for a lower

drain current to lower the gain of the stage.

In receive mode the DC voltage from PCIC pin 23 (RX) turns on Q3442, which in turn switches off

the biasing voltage to U3401.

Switch S3440 is a pressure pad with a conductive strip which connects two conductive areas on the

board when the radio's cover is properly screwed to the chassis. When the cover is removed, S3440

opens and the resulting high voltage level at the inverting inputs of the current control op-amps

U3402-1 & 2 switches off the biasing of U3401 and Q3421. This prevents transmitter key up while

the devices do not have proper thermal contact to the chassis.

3.2

Power Controlled Driver Stage

The next stage is an LDMOS device (Q3421) providing a gain of 12dB. This device requires a

positive gate bias and a quiescent current ﬂow for proper operation. The bias is set during transmit

mode by the drain current control op-amp U3402-2, and fed to the gate of Q3421 via the resistive

network R3429, R3418, R3415 and R3416.

Op-amp U3402-2 monitors the drain current of U3421 via resistors R3424-27 and adjusts the bias

voltage of Q3421 so that the current remains constant. The PCIC (U3501) provides a DC output

voltage at pin 4 (INT) which sets the reference voltage of the current control loop. A raising power

output causes the DC voltage from the PCIC to fall, and U3402-2 adjusts the bias voltage for a lower

drain current to lower the gain of the stage.

In receive mode the DC voltage from PCIC pin 23 (RX) turns on Q3422, which in turn switches off

the biasing voltage to Q3421.

3.3

The ﬁnal stage is an LDMOS device (Q3441) 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

MOSBIAS_2 is set in transmit mode by the ASFIC and fed to the gate of Q3441 via the resistive

network R3404, R3406, and R3431-5. This bias voltage is 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. In receive

mode U3402-2 pulls the bias voltage to low via D3401. The device's drain current is drawn directly

from the radio's DC supply voltage input, PASUPVLTG, via L3436 and L3437.

A matching network consisting of C3441-49, L3443, and two striplines, transforms the impedance to

50 ohms and feeds the directional coupler.

3.4

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

power delivered by Q3441. The coupled signal is rectiﬁed by D3451. The DC voltage is proportional

to the RF output power and feeds the RFIN port of the PCIC (U3501 pin 1). The PCIC controls the

gain of stage U3401 and Q3421 as necessary to hold this voltage constant, thus ensuring the

forward power out of the radio to be held to a constant value.

THEORY OF OPERATION