Motorola pro7100 Manuel de service télécharger pdf (Page 42)

2-28 Theory of Operation

2.8 Transmitter Power Amplifier (PA) 45 W

The radio’s 45 W PA is a four-stage amplifier used to amplify the output from the VCOBIC to the radio

transmit level. The line-up consists of three stages which utilize LDMOS and VMOS technology,

followed by a final stage using a bipolar device. The gain of the first stage (U3401) is adjustable,

controlled by pin 4 of PCIC (U3501) via Q3501 and Q3502 (VCONT). It is followed by an LDMOS

pre-driver stage (Q3421), a VMOS driver stage (Q3431) and a bipolar final stage (Q3441).

Figure 2-12. VHF Transmitter Block Diagram

Devices U3401 and Q3421 are surface mounted. The remaining devices are directly attached to the

heat sink.

2.8.1 Power Controlled Stage

The first stage (U3401) is a 20 dB gain integrated circuit containing two LDMOS FET amplifier

stages. It amplifies the RF signal from the VCO (TXINJ). The output power of stage U3401 is

controlled by a dc voltage applied to pin 1 from the power control circuit (U3501 pin 4, with transistors

Q3501 and Q3502 providing current gain and level-shifting). The control voltage simultaneously

varies the bias of two FET stages within U3401. This biasing point determines the overall gain of

U3401 and therefore its output drive level to Q3421, which in turn controls the output power of the

PA .

In receive mode the voltage control line is at ground level and turns off Q3501-2, which in turn

switches off the biasing voltage to U3401.

2.8.2 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 factory tuned.

PCIC

Pin Diode

Antenna

Switch

RF Jack

Antenna

Harmonic

Filter

Power

Sense

PA -F i na l

Stage

Driver

From VCO

Controlled

Stage

Vcontrol

Bias 1

Bias 2

To Microprocessor

Temperature

Sense

SPI BUS

ASFIC_CMP

PWR

SET

To Microprocessor

1 2 ... 37 38 39 40 41 42 43 44 45 46 47 ... 108 109

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Motorola pro7100 Manuel de service Page 42

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