RF2140 Просмотр технического описания (PDF) - RF Micro Devices

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RF2140

3V DCS POWER AMPLIFIER

RF Micro Devices 

RF2140

Theory of Operation and Application Information

The RF2140 is a three-stage device with 28 dB gain at

full power. Therefore, the drive required to fully satu-

rate the output is +5dBm. Based upon HBT (Hetero-

junction Bipolar Transistor) technology, the part

2

requires only a single positive 3V supply to operate to

full specification. Power control is provided through a

single pin interface, with a separate Power Down con-

trol pin. The final stage ground is achieved through the

large pad in the middle of the backside of the package.

First and second stage grounds are brought out

through separate ground pins for isolation from the out-

put. These grounds should be connected directly with

vias to the PCB ground plane, and not connected with

the output ground to form a so called “local ground

plane” on the top layer of the PCB. The output is

brought out through the wide output pad, and forms the

RF output signal path.

The amplifier operates in near Class C bias mode. The

final stage is "deep AB", meaning the quiescent current

is very low. As the RF drive is increased, the final stage

self-biases, causing the bias point to shift up and, at

full power, draws about 1500mA. The optimum load for

the output stage is approximately 4.5Ω. This is the load

at the output collector, and is created by the series

inductance formed by the output bond wires, vias, and

microstrip, and 2 shunt capacitors external to the part.

The optimum load impedance at the RF Output pad is

4.5-j3.9Ω. With this match, a 50Ω terminal impedance

is achieved. The input is internally matched to 50Ω

with just a blocking capacitor needed. This data sheet

defines the configuration for GSM operation.

The input is DC coupled; thus, a blocking cap must be

inserted in series. Also, the first stage bias may be

adjusted by a resistive divider with high value resistors

on this pin to VPC and ground. For nominal operation,

however, no external adjustment is necessary as inter-

nal resistors set the bias point optimally.

When the device is driven at maximum input power self

biasing would occur. This results in less isolation than

one would expect, and the maximum output power

would be about -15dBm. If the drive power to the PA is

turned on before the GSM ramp-up, higher isolation is

required. In order to meet the GSM system specs

under those conditions, a PIN diode attenuator con-

nected to the input can be turned on. The figure below

shows how the attenuator and its controls are con-

nected.

VCC

RF IN

APC

AT_EN

750 Ω 500 Ω

5 kΩ

2 kΩ

PIN

From Bias

Stages

The current through the PIN diode is controlled by two

signals: AT_EN and APC. The AT_EN signal allows

current through the PIN diode and is an on/off function.

The APC signal controls the amount of current through

the PIN diode. Normally, the AT_EN signal will be

derived from the VCO ENABLE signal available in

most GSM handset designs. If maximum isolation is

needed before the ramp-up, the AT_EN signal needs to

be turned on before the RF power is applied to the

device input. The current into this pin is not critical, and

can be reduced to a few hundred micro amps with an

external series resistor. Without the resistor, the pin will

draw about 700µA.

Because of the inverting stage at the APC input, the

current through the PIN diode is inverted from the APC

voltage. Thus, when VAPC is high for maximum output

power, the attenuator is turned off to obtain maximum

drive level for the first RF stage. When VAPC is low for

maximum isolation, the attenuator is be turned on to

reduce the drive level and to avoid self-biasing.

The PIN diode is dimensioned such that a low VAPC the

impedance of the diode is about 50 Ohm. Since the

input impedance of the first RF stage become very

high when the bias is turned off, this topology will main-

tain a good input impedance over the entire VAPC con-

trol range.

VCC1 and VCC2 provide supply voltage to the first and

second stage, as well as provides some frequency

selectivity to tune to the operating band. Essentially,

2-134

Rev A12 011031

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