FET BIAS CONTROLLER AND
POLARITY SWITCH
ISSUE 1 - FEBRUARY 1998
DEVICE DESCRIPTION
The ZNBG series of devices are designed to
meet the bias requirements of GaAs and
HEMT FETs commonly used in satellite
receiver LNBs, PMR cellular telephones etc.
with a minimum of external components.
With the addition of two capacitors and a
resistor, the devices provide drain voltage
and current control for three external
grounded source FETs, generating the
regulated negative rail required for FET gate
biasing whilst operating from a single supply.
This negative bias, at -3 volts, can also be
used to supply other external circuits.
The ZNBG3010/11 includes bias circuits to
drive up to three external FETs. A control
input to the device selects either one of two
FETs as operational, the third FET is
p e r m a n e n t l y a ct i v e . T hi s f e a t u r e i s
particularly used as an LNB polarisation
switch.
Drain current setting of the ZNBG3010/11 is
user selectable over the range 0 to 15mA,
this is achieved with addition of a single
resistor. The series also offers the choice of
drain voltage to be set for the FETs, the
ZNBG3010 gives 2.2 volts drain whilst the
ZNBG3011 gives 2 volts.
ZNBG3010
ZNBG3011
These devices are unconditionally stable
over the full working temperature with the
FETs in place, subject to the inclusion of the
recommended gate and drain capacitors.
These ensure RF stability and minimal
injected noise.
It is possible to use less than the devices full
complement of FET bias controls, unused
drain and gate connections can be left open
circuit without affecting operation of the
remaining bias circuits.
In order to protect the external FETs the
circuits have been designed to ensure that,
under any conditions including power
up/down transients, the gate drive from the
bias circuits cannot exceed the range -3.5V
to 1V. Furthermore if the negative rail
experiences a fault condition, such as
overload or short circuit, the drain supply to
the FETs will shut down avoiding excessive
current flow.
The ZNBG3010/11 are available in QSOP16
for the minimum in device size. Device
operating temperature is -40 to 70°C to suit
a wide range of environmental conditions.
FEATURES
APPLICATIONS
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•
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Provides bias for GaAs and HEMT FETs
Drives up to three FETs
Dynamic FET protection
Drain current set by external resistor
Regulated negative rail generator
requires only 2 external capacitors
Choice in drain voltage
Wide supply voltage range
Polarisation switch for LNBs
QSOP surface mount package
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Satellite receiver LNBs
Private mobile radio (PMR)
Cellular telephones
4-114
ZNBG3010
ZNBG3011
FUNCTIONAL DIAGRAM
FUNCTIONAL DESCRIPTION
The ZNBG devices provide all the bias requirements for external FETs, including the generation
of the negative supply required for gate biasing, from the single supply voltage.The diagram
above shows a single stage from the ZNBG series. The ZNBG3010/11 contains 3 such stages. The
negative rail generator is common to both devices.
The drain voltage of the external FET Q
N
is set by the ZNBG device to its normal operating voltage.
This is determined by the on board V
D
Set reference, for the ZNBG3010 this is nominally 2.2 volts
whilst the ZNBG3011 provides nominally 2 volts.
The drain current taken by the FET is monitored by the low value resistor I
D
Sense. The amplifier
driving the gate of the FET adjusts the gate voltage of Q
N
so that the drain current taken matches
the current called for by an external resistor R
CAL
.
Since the FET is a depletion mode transistor, it is often necessary to drive its gate negative with
respect to ground to obtain the required drain current. To provide this capability powered from
a single positive supply, the device includes a low current negative supply generator. This
generator uses an internal oscillator and two external capacitors, C
NB
and C
SUB
.
4-118
PCB Design
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