FET BIAS CONTROLLER WITH POLARISATION
SWITCH AND TONE DETECTION
ISSUE 2 - APRIL 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 ZNBG3110/11 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. Also specific to LNB applications is
the 22KHz tone detection and logic output
feature which is used to enable high and low
band frequency switching.
Drain current setting of the ZNBG3110/11 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 3110 gives
2.2 volts drain whilst the 3111 gives 2 volts.
ZNBG3110
ZNBG3111
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 ZNBG3110/11 are available in QSOP20
for the minimum in device size. Device
operating temperature is -40 to 70擄C to suit
a wide range of environmental conditions.
FEATURES
APPLICATIONS
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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
22KHz tone detection for band
switching
Compliant with ASTRA control
specifications
QSOP surface mount package
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Satellite receiver LNBs
Private mobile radio (PMR)
Cellular telephones
4-123
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FET BIAS CONTROLLER
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