Datasheet

Base-Station PA Gain Control
While the MAX9611 is designed to control high-side
pFETs, the MAX9612 can be similarly used to control
low-side nFETs. For example, the MAX9612 can be used
to control the DC bias point of power amplifier LDMOS
or GaN nFETs in base-station applications. The circuit
shown in Figure 4 also allows the option to apply negative
bias voltages to the PA FET, which is required for certain
types of transistors for proper operation.
In the circuit shown, the nFET is in a linear mode of opera-
tion to allow it to amplify high-frequency RF signals, while
the MAX9612 sets the DC operating point. The gain of
the FET can be varied by changing its drain current. This
operating point can be varied by an external DAC voltage
that feeds the SET pin.
V
NEG
and V
CLAMP
together with R1, R2, and R3 set the
DC bias point limits for the PA transistor. V
CLAMP
is a suit-
able positive voltage and V
NEG
is a suitable negative volt-
age. When V
OUT
= 0V, the gate voltage of the PA FET is:
( )
NEG
OUT
V R2
V
R1 R2
×
=
+
When the OUT open-dran transistor is off, the gate volt-
age of the PA FET is:
( )
NEG
CLAMP
GATE
V R2 R3
V R1
V
R1 R2 R3 R1 R2 R3
+
= +
++ ++
R
COMP
and C
COMP
connected to the OUT pin compen-
sate the internal amplifier. Choose a corner frequency of
100kHz.
Choose suitable R
SENSE
as required for the application.
The inductor isolates the DC measuring point of current
from the high-frequency AC signals through the PA FET,
as well as helping with the high-frequency gain.
Power-Supply Bypassing and Grounding
The MAX9611/MAX9612 share a common ground pin for
both the analog and digital on-chip circuitry. It is therefore
very important to properly bypass the V
CC
to GND, and to
have a solid low-noise ground plane on the circuit board so
as to minimize ground bounce. Bypass V
CC
to GND with
low ESR 0.1µF in parallel with a 4.7µF ceramic capacitor
to GND placed as close as possible to the device.
Figure 4. Base-Station PA Gain Control
MAX9612
C
IN
V
CC
RS- RS+
A0
A1
SET
OUT
2.7V TO 5.5V
V
NEG
V
CLAMP
(OUTPUT SET TO OP-AMP MODE)
BASE-STATION PA GAIN CONTROL
RF
IN
GND
I
2
C CLOCK
INPUT
I
2
C DATA
INPUT/OUTPUT
SCL
SDA
10-BIT
DAC
R
COMP
R2
R1
R3
N
C
COMP
RF
OUT
MAX9611/MAX9612 High-Side, Current-Sense Ampliers with
12-Bit ADC and Op Amp/Comparator
www.maximintegrated.com
Maxim Integrated │ 18