FET BIAS CONTROLLER
ISSUE 1- AUGUST 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 3 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 ZNBG3000/1 contains three bias stages.
A single resistor allows FET drain current to
be set to the desired level. The series also
offers the choice of drain voltage to be set
for the FETs, the ZNBG3000 gives 2.2 volts
drain whilst the ZNBG3001 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.
ZNBG3000
ZNBG3001
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 0.7V. 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 ZNBG3000/1 are available in QSOP16
packages for the minimum in devices size.
Device operating temperature is -40 to 70°C
to suit a wide range of environmental
conditions.
FEATURES
APPLICATIONS
•
•
•
•
•
•
•
•
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
QSOP surface mount package
•
•
•
•
Satellite receiver LNBs
Private mobile radio (PMR)
Cellular telephones
Single in single out C Band LNB
4-137
ZNBG3000
ZNBG3001
SYMBOL PARAMETER
CONDITIONS
Min
LIMITS
Typ
Max
2000
µA
UNITS
GATE CHARACTERISTICS
I
GO
V
OL
Output Current Range
Output Voltage
Output Low
I
D1
to I
D3
=12mA
IG1
to I
G3
=0
I
D1
to I
D3
=12mA
I
G1
to I
G3
= -10µA
V
OH
Notes:
1. The negative bias voltages specified are generated on-chip using an internal oscillator. Two external capacitors, C
NB
and C
SUB
, of
47nF are required for this purpose.
2. The characteristics are measured using an external reference resistors R
CAL1
of value 33kΩ wired from pin R
CAL1
to ground.
-30
-3.5
-3.5
0.4
-2
-2
1
V
V
V
Output High
I
D1
to I
D3
= 8mA
I
G1
to I
G3
= 0
3. Noise voltage is not measured in production.
4. Noise voltage measurement is made with FETs and gate and drain capacitors in place on all
outputs. C
G
, 4.7nF, are connected between gate outputs and ground, C
D
, 10nF, are connected
between drain outputs and ground.
4-139
FUNCTIONAL DIAGRAM
ZNBG3000
ZNBG3001
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 ZNBG3000/1 contains 3 such
stages.
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 ZNBG3000 this is nominally 2.2 volts
whilst the ZNBG3001 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
. Both ZNBG devices have the facility to program
different drain currents into selected FETs.
Since the FET is a depletion mode transistor, it is usually 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
.
4-141
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