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 o l t a g e t o b e s e t f o r t h e F E T s , t h e
ZNBG4000/6000 gives 2.2 volts drain whilst
the ZNBG4001/6001 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.

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

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

APPLICATIONS

Satellite receiver LNBs

Private mobile radio (PMR)

Cellular telephones

ZNBG4000

ZNBG4000 ZNBG4001
ZNBG6000 ZNBG6001

4-137

ABSOLUTE MAXIMUM RATINGS

Supply Voltage

-0.6V to 15V

Supply Current

100mA

Drain Current (per FET)

0 to 15mA

(set by R

CAL1

and R

CAL2

)

Output Current

100mA

Operating Temperature

-40 to 70°C

Storage Temperature

-50 to 85°C

Power Dissipation (T

amb

25°C)

QSOP16

500mW

QSOP20

650mW

ELECTRICAL CHARACTERISTICS TEST CONDITIONS (Unless otherwise stated):
T

amb

25°C,V

=5V,I

=10mA (R

CAL1

=33k

CAL2

=33k

)

SYMBOL PARAMETER

CONDITIONS

LIMITS

UNITS

Min

Typ

Max

Supply Voltage

Supply Current
ZNBG4000/1

to I

=10mA

10
50

mA
mA

Supply Current
ZNBG6000/1

ID1 to I

to I

=10mA

15
75

mA
mA

SUB

Substrate Voltage
(Internally generated)

SUB

= 0

SUB

= -200

-3.5

-3

-2
-2

V
V

Output Noise
Drain

Voltage

Gate

Voltage

=4.7nF, C

=10nF

=4.7nF, C

=10nF

0.02
0.005

Vpkpk
Vpkpk

Oscillator Freq.

200

350

800

kHz

DRAIN CHARACTERISTICS

Current

Current Change

with V

=5 to 12V

0.02

%/V

with T

=-40 to +70°C

0.05

%/°C

Voltage
ZNBG4000, ZNGB6000
ZNBG4001, ZNBG6001

2
1.8

2.2
2

2.4
2.2

V
V

Voltage Change

with V

= 5 to 12V

0.5

%/V

with T

= -40 to +70°C

ppm

ZNBG4000

ZNBG4000 ZNBG4001
ZNBG6000 ZNBG6001

4-138

SYMBOL PARAMETER

CONDITIONS

LIMITS

UNITS

Min

Typ

Max

GATE CHARACTERISTICS

Output Current Range

-30

2000

Output Voltage
ZNBG4000/1

Output

Low

to I

=12mA

to I

-3.5

-2

to I

=12mA

to I

= -10

-3.5

-2

Output High

to I

= 8mA

to I

= 0

Output Voltage
ZNBG6000/1

Output

Low

to I

=12mA

to I

= 0

-3.5

-2

to I

=12mA

to I

= -10

-3.5

-2

Output High

to I

= 8mA

to I

= 0

Notes:

1. The negative bias voltages specified are generated on-chip using an internal oscillator. Two external capacitors, C

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 33k

wired from pins R

CAL1/2

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.

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

, 4.7nF, are connected between gate outputs and ground, C

, 10nF, are connected

between drain outputs and ground.

ZNBG4000

ZNBG4000 ZNBG4001
ZNBG6000 ZNBG6001

4-139

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

is set by the ZNBG device to its normal operating voltage.

This is determined by the on board V

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

Sense. The amplifier

driving the gate of the FET adjusts the gate voltage of Q

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

and C

SUB

ZNBG4000

ZNBG4000 ZNBG4001
ZNBG6000 ZNBG6001

4-141

ZNBG4000

ZNBG4000 ZNBG4001
ZNBG6000 ZNBG6001

TYPICAL APPLICATION CIRCUIT

APPLICATIONS INFORMATION

The above is a partial application circuit for the ZNBG series showing all external components
required for appropriate biasing. The bias circuits are unconditionally stable over the full
temperature range with the associated FETs and gate and drain capacitors in circuit.

Capacitors C

and C

ensure that residual power supply and substrate generator noise is not

allowed to affect other external circuits which may be sensitive to RF interference. They also
serve to suppress any potential RF feedthrough between stages via the ZNBG device. These
capacitors are required for all stages used. Values of 10nF and 4.7nF respectively are
recommended however this is design dependent and any value between 1nF and 100nF could
be used.

The capacitors C

and C

SUB

are an integral part of the ZNBGs negative supply generator. The

negative bias voltage is generated on-chip using an internal oscillator. The required value of
capacitors C

and C

SUB

is 47nF. This generator produces a low current supply of approximately

-3 volts. Although this generator is intended purely to bias the external FETs, it can be used to
power other external circuits via the C

SUB

pin.

Resistors R

CAL1/2

sets the drain current at which all external FETs are operated. 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. If the same drain current

is required for all FETs on either device then pins R

CAL1

and R

CAL2

can be wired together and

shunted to ground by a single calibration resistor of half normal value.

If any bias control circuit is not required, its related drain and gate connections may be left open
circuit without affecting the operation of the remaining bias circuits. If all FETs associated with
a current setting resistor are omitted, the particular R

CAL

should still be included. The supply

current can be reduced, if required, by using a high value R

CAL

resistor (e.g. 470k).

4-142

ZNBG4000

ZNBG4000 ZNBG4001
ZNBG6000 ZNBG6001

APPLICATIONS INFORMATION (Continued)

The ZNBG devices have been designed to protect the external FETs from adverse operating
conditions. With a JFET connected to any bias circuit, the gate output voltage of the bias circuit
can not exceed the range -3.5V to 0.7V, under any conditions including powerup and powerdown
transients. Should the negative bias generator be shorted or overloaded so that the drain current
of the external FETs can no longer be controlled, the drain supply to FETs is shut down to avoid
damage to the FETs by excessive drain current.

The following diagrams show the ZNBG4000/1 and ZNBG6000/1 in typical LNB applications.
Within each FET gain stage the numbering system indicates how the bias stages relate to the
application circuits. This is important when RCAL values are used to set differing drain currents.

Dual standard or enhanced LNB block diagram

4-143

PACKAGE DIMENSIONS

PIN No.1

IDENTIFICATION
RECESS
FOR PIN 1

QSOP16

Millimetres

Inches

MIN

MAX

MIN

MAX

4.80

4.90

0.033

0.039

0.635

0.025 NOM

0.177

0.267

0.007

0.011

0.20

0.30

0.008

0.012

3.81

3.99

0.15

0.157

1.35

1.75

0.053

0.069

0.10

0.25

0.004

0.01

5.79

6.20

0.228

0.244

0°

8°

0°

8°

Zetex plc.
Fields New Road, Chadderton, Oldham, OL9-8NP, United Kingdom.
Telephone: (44)161 622 4422 (Sales), (44)161 622 4444 (General Enquiries)
Fax: (44)161 622 4420

Zetex GmbH

Zetex Inc.

Zetex (Asia) Ltd.

These are supported by

Streitfeldstraße 19

47 Mall Drive, Unit 4

3510 Metroplaza, Tower 2

agents and distributors in

D-81673 München

Commack NY 11725

Hing Fong Road,

major countries world-wide

Germany

USA

Kwai Fong, Hong Kong

Zetex plc 1998

Telefon: (49) 89 45 49 49 0

Telephone: (516) 543-7100

Telephone:(852) 26100 611

Fax: (49) 89 45 49 49 49

Fax: (516) 864-7630

Fax: (852) 24250 494

Internet:http://www.zetex.com

This publication is issued to provide outline information only which (unless agreed by the Company in writing) may not be used, applied
or reproduced for any purpose or form part of any order or contract or be regarded as a representation relating to the products or
services concerned. The Company reserves the right to alter without notice the specification, design, price or conditions of supply of
any product or service.

QSOP20

Millimetres

Inches

MIN

MAX

MIN

MAX

8.55

8.74

0.337

0.344

0.635

0.025 NOM

1.42

1.52

0.056

0.06

0.20

0.30

0.008

0.012

3.81

3.99

0.15

0.157

1.35

1.75

0.053

0.069

0.10

0.25

0.004

0.01

5.79

6.20

0.228

0.244

0°

8°

0°

8°

ZNBG4000

ZNBG4000 ZNBG4001
ZNBG6000 ZNBG6001

Page Number

Ð­Ð»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: ZNGB6000

ÐÐ»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: ZNGB6000