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| 型號(hào)： | CX77314 |
| 英文描述： | CX77314 Product Summary|PA Modules for GSM/GPRS Handsets |
| 中文描述： | CX77314產(chǎn)品概要|功率放大器模塊的GSM / GPRS手機(jī) |
| 文件頁(yè)數(shù)： | 12/14頁(yè) |
| 文件大小： | 517K |
| 代理商： | CX77314 |
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Data Sheet CX77301
PA Module for Dual-band EGSM900 DCS1800 / GPRS
12
SEPTEMBER 19, 2003
Skyworks Solutions, Inc. Proprietary Information
[781] 376-7000 FAX [781] 376-3100 SALES@SKYWORKSINC.COM
WWW.SKYWORKSINC.COM
100956E
Band Select
The Combinational Logic cell also includes a simple gate ar-
rangement that selects the desired operational band by activating
the appropriate current buffer. The voltage threshold level at the
Band Select input (pin 16) will determine the active path of the
bias output to the GaAs die.
Voltage Clamp
The Voltage Clamp circuit will limit the maximum bias voltage
output applied to the bases of the HBT devices on the GaAs die.
This provides protection against electrical overstress (EOS) of the
active devices during high voltage and/or load mismatch condi-
tions.
Figure 8
shows the typical transfer function of the APC
input to buffer output under resistively loaded conditions. Notice
the enable function near 600 mV, and the clamp acting at 2.15 V,
corresponding to a supply voltage of 4.0 V.
0.0
0.5
1.0
1.5
2.0
2.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
APC Input (volts)
B
clamping
occurs
100956_009
Figure 8. Base Bias Voltage vs. APC Input, V
CC
CC
= 4.0 V
Due to output impedance effects, the bias of the GaAs devices
increases as the supply voltage increases. The Voltage Clamp is
designed to gradually decrease in level as the battery voltage
increases. The performance of the clamp circuit is enhanced by
the band gap reference that provides a supply-, process-, and
temperature-independent reference voltage. The transfer function
relative to V
BAT
is shown in
Figure 9
. For battery voltages below
3.4 V, the base bias voltage is limited by the common mode
range of the buffer amplifier. For battery voltages above 3.4 V,
the clamp limits the base bias.
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
2.6
3.00
3.25
3.50
3.75
4.00
4.25
4.50
V
cc
(Volts)
B
clamp
100956_010
Figure
. Base Bias Clamp vs. Supply Voltage
Current Buffer
The output buffer amplifier performs a vital function in the CMOS
device by transferring the APC input voltage ramp to the base of
the GaAs power devices. This allows the APC input to be a high
impedance port, sinking only 10
μ
A, typical, assuring no loading
effects on the PAC circuit. The buffers are designed to source the
high GaAs base currents required, while allowing a settling time
of less than 8
μ
s for a 1.5 V ramp.
POWER RAMPING CONSID
COMPLIANCE
These are the primary variables in the power control loop that the
system designer must control:
ERATIONS FOR 3GPP
software control of the DSP / DAC
software control of the transmitter timing signals
ramp profile attributes - pedestal, number of steps, duration of
steps
layout of circuit / parasitics
RC time constants within the PAC circuit design
All of these variables will directly influence the ability of a GSM
transmitter power control loop to comply with 3GPP
specifications.
Although there is a specific time mask template in which the
transmitter power is allowed to ramp up, the method is very
critical. The 3GPP system specification for switching transients
results in a requirement to limit the edge rate of output power
transitions of the mobile. Switching transients are caused by the
transition from minimum output power to the desired output
power, and vice versa. The spectrum generated by this transition
is due to the ramping waveform amplitude modulation imposed
on the carrier. Sharper transitions tend to produce more spectral
"splatter" than smooth transitions. If the transmit output power is
ramped up too slowly, the radio will violate the time mask
specification. In this condition, the radio may not successfully
initiate or maintain a phone call. If the transmit output power is
ramped up too quickly, this will cause RF "splatter" at certain
frequency offsets from the carrier as dictated by the 3GPP
specification. This splatter, known as Output RF Spectrum (ORFS)
due to Switching Transients, will increase the system noise level,
which may knock out other users on the system. The main
difficulty with TDMA power control is allowing the transmitter to
ramp the output power up and down gradually so switching
transients are not compromised while meeting the time mask
template at all output power levels in all operational bands. The
transmitter has 28
μ
s to ramp up power from an off state to the
desired power level.
The GSM transmitter power control loop generally involves
feedback around the GaAs PA, which limits the bandwidth of
signals that can be applied to the PA bias input. Since the PA is
within the feedback loop, its own small-signal frequency
response must exhibit a bandwidth 5 to 10 times that of the
power control loop. As discussed in the previous section, the PA
bias is held at ground for inputs less than 700 mV. As the APC
input exceeds the enable threshold, the bias will activate. After
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