FET BIAS CONTROLLER WITH POLARISATION
SWITCH AND TONE DETECTION
ISSUE 1 - OCTOBER 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 ZNBG3113/14 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.
The facility to control the tone switching
delay is provided. This allows the rejection
of other lower frequency tones tat may be
present in multiple LNB applications.
ZNBG3113
ZNBG3114
Drain current setting of the ZNBG3113/14 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 3113 gives
2.2 volts drain whilst the 3114 gives 2 volts.
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.
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 ZNBG3113/14 are available in QSOP20
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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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
Programmable tone delay
Compliant with ASTRA control
specifications
QSOP surface mount package
4-123
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Satellite receiver LNBs
Private mobile radio (PMR)
Cellular telephones
ZETEX [ ZETEX SEMICONDUCTORS ]
