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ML2751DT Datenblatt(Datasheet) 6 Page - Micro Linear Corporation

Teile-Nr. ML2751DT
Beschreibung  Power Amplifier and LNA
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Hersteller  MICRO-LINEAR [Micro Linear Corporation]
Homepage  http://www.microlinear.com
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 6 page
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ML2751
6
January, 2000
PRELIMINARY
PRELIMINARY DATASHEET
FUNCTIONAL DESCRIPTION
TRANSMIT FUNCTIONALITY
Power Amplifier
The power amplifier (PA) consists of three cascaded gain
stages. The last stage (output stage) is powered directly
from the unregulated supply to increase end-product
battery life. The TPS and TPA pins are used to control its
output power. The TPS and TPA pins are connected to an
internal power control op amp as shown in Figure 1. The
op amp’s output voltage (VP) determines the PA's gain.
The voltage at VP drives a buffer that serves as a collector
supply for the first two stages of the power amplifier.
A ramp generator circuit between the power control op
amp and the buffer facilitates a gradual transition from
the on-state to the off-state of the PA on the rising/falling
edge of the PAON signal.
The DC voltage on the TPS pin determines VP, and may be
varied between 0V and 1V, with a nominal 100mW/V
sensitivity to this pin as shown in Figure 2. To compensate
for the IC manufacturing process variations of the power
amplifier the TPS voltage can be scaled by varying the
resistance between the TPA pin and ground. The value of
Figure 2. Output vs. TPS Curve
Figure 1. PA Power Setting and Ramp Generator
BUFFER
VC
PAON
GAIN SET
TPS
TPA
FEEN
+
RAMP
GENERATOR
VP
100k
100k
0
Ω to
open circuit
PA
RTPA
Power
Control
Op Amp
TRFI
TRFO
120
100
80
60
40
20
0
VOLTAGE (V)
0.1
0.5
0.7
0.9
0.3
0.2
0.6
0.8
1.0
0.4
0
resistor RTPA effectively determines the gain of the op
amp. RTPA can range in value from 0
W to open circuit.
For +20dBm output power the typical full-scale signal on
the TPS will lie between 0.5V and 1V with TPA grounded.
RECEIVE FUNCTIONALITY
Low Noise Amplifier (LNA)
The LNA is a two-stage transistor amplifier designed with
internal inter-stage impedance matching. The output is an
open collector that requires a load to a DC supply and
external matching. All associated bias circuits are
integrated internally. The LNA is controlled by the PAON
signal, as shown in Figure 3.
A nominal gain of 20dB and a noise figure of 2.5dB
complement the ML2721, optimizing the balance
between sensitivity and high signal performance. The
open-collector LNA output provides flexibility in
matching the impedance that an RF filter presents to the
LNA. The input can be matched for low noise using a
small value series capacitor, and the two-stage design
minimizes the interaction between the input and output
match. The RF filter is placed between the ML2751 and
the ML2721.
PIN Diode Driver
A pair of driver circuits is used to drive an external PIN
diode Transmit/Receive switch. The PIN diode drivers are
controlled from the timer’s internal enable signals. The
PIN and PINB output pins act as low impedance voltage
sources that switch between VCC and GND, with the
ability to both source and sink current.
The PIN and PINB outputs are under control of the FEEN
and PAON control input pins (see Figure 3). The switching
between the transmit and receive paths is synchronized
with the PA ramp function, so that the transmit path is
selected at the rising edge of the transmit ramp (when
PAON = 1) and held until the PA has ramped down (about
3µs after PAON = 0).
Simple PIN diode switches can be driven directly, without
the need for current limiting resistors, as each output has a
nominal 100
W series impedance. This reduces the total
component count. For more complex (or lower power)
switches individual current limiting resistors may be used,
forcing the PIN and PINB pins to act as voltage sources.
The total current drawn from each output must not exceed
the current source/sink specification limits.




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