RF2138 查看數據表（PDF） - RF Micro Devices

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RF2138

3V GSM POWER AMPLIFIER

RF Micro Devices 

RF2138

Theory of Operation and Application Information

The RF2138 is a three-stage device with 32 dB gain at

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

rate the output is +3dBm. Based upon HBT (Hetero-

junction Bipolar Transistor) technology, the part

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 2000mA. The optimum load for

the output stage is approximately 1.2Ω. 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

1.2-j1.7Ω. 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.

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,

the bias is fed to this pin through a short microstrip. A

bypass capacitor sets the inductance seen by the part,

so placement of the bypass cap can affect the fre-

quency of the gain peak. This supply should be

bypassed individually with 100pF capacitors before

being combined with VCC for the output stage to pre-

vent feedback and oscillations.

The RF OUT pin provides the output power. Bias for

the final stage is fed to this output line, and the feed

must be capable of supporting the approximately 2A of

current required. Care should be taken to keep the

losses low in the bias feed and output components. A

2

narrow microstrip line is recommended because DC

losses in a bias choke will degrade efficiency and

power.

While the part is safe under CW operation, maximum

power and reliability will be achieved under pulsed con-

ditions. The data shown in this data sheet is based on

a 12.5% duty cycle and a 600µs pulse, unless speci-

fied otherwise.

The part will operate over a 3.0V to 5.0V range. Under

nominal conditions, the power at 3.5V will be greater

than +34.5dBm at +90°C. As the voltage is increased,

however, the output power will increase. Thus, in a sys-

tem design, the ALC (Automatic Level Control) Loop

will back down the power to the desired level. This

must occur during operation, or the device may be

damaged from too much power dissipation. At 5.0V,

over +38dBm may be produced; however, this level of

power is not recommended, and can cause damage to

the device.

The HBT breakdown voltage is >20V, so there are no

issue with overvoltage. However, under worst-case

conditions, with the RF drive at full power during trans-

mit, and the output VSWR extremely high, a low load

impedance at the collector of the output transistors can

cause currents much higher than normal. Due to the

bipolar nature of the devices, there is no limitation on

the amount of current de device will sink, and the safe

current densities could be exceeded.

High current conditions are potentially dangerous to

any RF device. High currents lead to high channel tem-

peratures and may force early failures. The RF2138

includes temperature compensation circuits in the bias

network to stabilize the RF transistors, thus limiting the

current through the amplifier and protecting the

devices from damage. The same mechanism works to

compensate the currents due to ambient temperature

variations.

To avoid excessively high currents it is important to

control the VAPC when operating at supply voltages

higher than 4.0V, such that the maximum output power

is not exceeded.

Rev A9 011031

2-123

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