FET BIAS CONTROLLER
ISSUE 2 - JUNE 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
resistors the devices provide drain voltage
and current control for a number of 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 ZNBG4000/1 and ZNBG6000/1 contain
four and six bias stages respectively. In
setting drain current the ZNBG4000/1 two
resistors allows individual FET pair control
to different levels, the ZNBG6000/1 two
resistors split control between two and four
FETs. This allows the operating current of
input FETs to be adjusted to minimise noise,
whilst the following FET stages can
separately be adjusted for maximum gain.
The series also offers the choice of drain
v ol t a ge t o b e s e t f or t he F E T s , the
ZNBG4000/6000 gives 2.2 volts drain whilst
the ZNBG4001/6001 gives 2 volts.
ZNBG4000 ZNBG4001
ZNBG6000 ZNBG6001
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 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 ZNBG4000/1 and ZNBG6000/1 are
available in QSOP16 and 20 pin packages
respectively 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 four or six 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
4-137
ZNBG4000 ZNBG4001
ZNBG6000 ZNBG6001
SYMBOL PARAMETER
CONDITIONS
Min
LIMITS
Typ
Max
2000
µA
UNITS
GATE CHARACTERISTICS
I
GO
Output Current Range
Output Voltage
ZNBG4000/1
V
OL
Output Low
I
D1
to I
D4
=12mA
I
G1
to I
G4
=0
I
D1
to I
D4
=12mA
I
G1
to I
G4
= -10µA
V
OH
Output High
Output Voltage
ZNBG6000/1
V
OL
Output Low
I
D1
to I
D6
=12mA
I
G1
to I
G6
= 0
I
D1
to I
D6
=12mA
I
G1
to I
G6
= -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 two external reference resistors R
CAL1
and R
CAL2
of value 33kΩ wired from pins R
CAL1/2
to
ground. For the ZNBG4000, resistor R
CAL1
sets the drain current of FETs 1 and 2, resistor R
CAL2
sets the drain current of FETs 3 and 4.
For the ZNBG6000, resistor R
CAL1
sets the drain current of FETs 1 and 4, resistor R
CAL2
sets the drain current of FETs 2, 3, 5 and 6.
-30
-3.5
-3.5
0
-2
-2
1
V
V
V
I
D1
to I
D4
= 8mA
I
G1
to I
G4
= 0
-3.5
-3.5
0
-2
-2
1
V
V
V
Output High
I
D1
to I
D6
= 8mA
I
G1
to I
G6
= 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
ZNBG4000 ZNBG4001
ZNBG6000 ZNBG6001
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 ZNBG4000/1 contains 4 such
stages, the ZNBG6000/1 contains 6. The negative rail generator is common to all 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 ZNBG4000/6000 this is nominally
2.2 volts whilst the ZNBG4001/6001 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. Two R
CAL
inputs are provided. For the ZNBG4000,
resistor R
CAL1
sets the drain current of FETs 1 and 2, resistor R
CAL2
sets the drain current of FETs
3 and 4. For the ZNBG6000, resistor R
CAL1
sets the drain current of FETs 1 and 4, resistor R
CAL2
sets the drain current of FETs 2, 3, 5 and 6.
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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