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Automatic RF MESFET Amplifier
Drain-Current Controllers
V RCS _ + ? V RCS _ ? = FILT FILT
Once the control loop has been set, the MAX11014
automatically maintains the drain-current value. Figure
5 details the amplifiers that bias the channel 1 and
channel 2 control loops.
The dual current-sense amplifiers amplify the voltage
between RCS_+ and RCS_- by four and add an offset
voltage (+12mV nominally). These current-sense ampli-
fiers amplify sense voltages between 0 and 625mV
when V REFDAC = +2.5V. See the Current-Sense
Amplifiers section.
The current-sense amplifier output injects a scaled-down
replica of the MESFET drain current at the summing
node to complete the internal analog feedback loop. The
summing node drives the gate-drive amplifier through a
100k Ω series resistor. The gate-drive amplifier is config-
ured as an integrator by the external capacitor connect-
ed between GATE1/GATE2 and FILT2/FILT4. The
gate-drive amplifier includes automatic offset cancella-
tion between 0 and 24mV to null the 12mV offset from the
current-sense amplifier. See the Register Descriptions
and PGACAL (Write) sections.
The MAX11014 ’s analog control loop setpoint is
described by the following equation:
V ( CODE = 000 h ) ? V
4
where:
V FILT (CODE = 000h) = V FILT1 (channel 1) and V FILT3
(channel 2) when the THRUDAC1/THRUDAC2 register
code is set to 000h.
V FILT = V FILT1 (channel 1) and V FILT3 (channel 2).
V RCS_+ - V RCS_- = the voltage drop across the current-
sense resistor.
Connect a capacitor from FILT2 to GATE1 to form an
integrator (setting the control-loop dominant pole) with
the channel 1 internal 100k Ω resistor. Connect a
capacitor from FILT4 to GATE2 to form an integrator
(setting the control-loop dominant pole) with the chan-
nel 2 internal 100k Ω resistor. The gate-drive amplifier’s
output drives the MESFET gates. See the Gate-Drive
Amplifiers section.
The channel 1 DAC voltage is output to FILT1 through a
series 580k Ω resistor. The channel 2 DAC voltage is
output to FILT3 through a series 580k Ω resistor.
Connect a capacitor from FILT1 to AGND and FILT3 to
AGND to set the filter’s time constant for the respective
channel.
MAX11015 Class AB Control
The MAX11015 is designed to be used with a Class AB
amplifier configuration to independently measure the
drain current and set the GATE_ output voltages through
the serial interface. After sensing the drain current with
no RF signal applied, set the DAC code to obtain the
desired GATE_ voltage. Figure 6 details the amplifiers
that bias the channel 1 and channel 2 control.
The MAX11015 internal 12-bit DAC voltage is applied to
the gate-drive amplifier, which has a preset gain of
-2. See the Gate-Drive Amplifiers section. Setting the
DAC code between FFFh and 000h typically produces
a GATE_ voltage between 0 and (-2 x V REFDAC ). See
the HCFG (Read/Write) section for details on adjusting
the GATE_ maximum voltage.
The channel 1 DAC voltage is output to FILT1 through a
series 580k Ω resistor. The channel 2 DAC voltage is
output to FILT3 through a series 580k Ω resistor.
Connect a capacitor from FILT1 to AGND and FILT3 to
AGND to set the filter’s time constant for the respective
channel. Connect FILT2 and FILT4 to AGND
(MAX11015 only).
The dual current-sense amplifiers amplify the voltage
between RCS_+ and RCS_- by four and add an offset
voltage (+12mV nominally). The current-sense ampli-
fiers amplify sense voltages between 0 and 625mV
when V REFDAC = +2.5V. See the Current-Sense
Amplifiers section.
Current-Sense Amplifiers
The dual current-sense amplifiers amplify the voltage
between RCS_+ and RCS_- and add an offset voltage.
Connect a resistor between RCS_+ and RCS_- to sense
the MESFET drain current. The current-sense amplifiers
scale the sense voltage by four. These amplifiers also
reject the drain supply voltage that appears as a DC
common-mode level on the current signal.
The gate-drive amplifier includes automatic offset can-
cellation between 0 and 24mV to null the 12mV offset
from the current-sense amplifier. See the PGACAL
(Write) section.
Gate-Drive Amplifiers
The gate-drive amplifiers control the MESFET gate bias
settings. The MAX11014’s channel 1 and channel 2
DAC voltages are routed through a summing node and
into the gate-drive amplifiers. The MAX11015’s channel
1 and channel 2 DAC voltages are routed directly to the
gate-drive amplifiers, which have a preset gain of -2.
See the 12-Bit DAC section for details on setting the
DAC codes.
18
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