RF3140 データシートの表示（PDF） - RF Micro Devices

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RF3140

QUAD-BAND GSM850/GSM900/DCS/PCS POWER AMP MODULE

RF Micro Devices 

RF3140

Theory of Operation

Overview

The RF3140 is a quad-band GSM850, EGSM900,

DCS1800, and PCS1900 power amplifier module that

incorporates an indirect closed loop method of power

control. This simplifies the phone design by eliminating

the need for the complicated control loop design. The

indirect closed loop appears as an open loop to the

user and can be driven directly from the DAC output in

the baseband circuit.

Theory of Operation

The indirect closed loop is essentially a closed loop

method of power control that is invisible to the user.

Most power control systems in GSM sense either for-

ward power or collector/drain current. The RF3140

does not use a power detector. A high-speed control

loop is incorporated to regulate the collector voltage of

the amplifier while the stage are held at a constant

bias. The VRAMP signal is multiplied by a factor of 2.65

and the collector voltage for the second and third

stages are regulated to the multiplied VRAMP voltage.

The basic circuit is shown in the following diagram.

TX ENABLE

VRAMP

VBATT

H(s)

RF IN

TX ENABLE

RF OUT

By regulating the power, the stages are held in satura-

tion across all power levels. As the required output

power is decreased from full power down to 0dBm, the

collector voltage is also decreased. This regulation of

output power is demonstrated in Equation 1 where the

relationship between collector voltage and output

power is shown. Although load impedance affects out-

put power, supply fluctuations are the dominate mode

of power variations. With the RF3140 regulating collec-

tor voltage, the dominant mode of power fluctuations is

eliminated.

PdBm = 10 ⋅ log -(-8-2----⋅⋅---RV----LC---O-C--A--–--D---V-⋅---S1---A0---T-–--)3--2- (Eq. 1)

There are several key factors to consider in the imple-

mentation of a transmitter solution for a mobile phone.

Some of them are:

• Effective efficiency (ηeff)

• Current draw and system efficiency

• Power variation due to Supply Voltage

• Power variation due to frequency

• Power variation due to temperature

• Input impedance variation

• Noise power

• Loop stability

• Loop bandwidth variations across power levels

• Burst timing and transient spectrum trade offs

• Harmonics

Talk time and power management are key concerns in

transmitter design since the power amplifier has the

highest current draw in a mobile terminal. Considering

only the power amplifier’s efficiency does not provide a

true picture for the total system efficiency. It is impor-

tant to consider effective efficiency which is repre-

sented by ηEFF. (ηEFF considers the loss between the

PA and antenna and is a more accurate measurement

to determine how much current will be drawn in the

application). ηEFF is defined by the following relation-

ship (Equation 2):

m

∑ PN – PIN

ηEFF

=

n----=-----1----------------------- ⋅ 100 (Eq. 2)

PDC

Where Pn is the sum of all positive and negative RF

power, PIN the input power and PDC is the delivered

DC power. In dB the formula becomes (Equation 3):

ηEFF = 1----0--V-P------B-P-----A--A-----+--T-1------0P--⋅-----L--I---O--B----S---A--S---T–----⋅-1--1-0--0--P--1----I-0--N-- (Eq. 3)

Rev A6 040113

2-499

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