znbg4000 Zetex Semiconductors plc., znbg4000 Datasheet - Page 5

• Manufacturer Part Number: znbg4000
• Description: Fet Bias Controller
• Manufacturer: Zetex Semiconductors plc.

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
This is determined by the on board V
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
driving the gate of the FET adjusts the gate voltage of Q
the current called for by an external resistor R
different drain currents into selected FETs. Two R
resistor R
3 and 4. For the ZNBG6000, resistor R
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
CAL1
sets the drain current of FETs 1 and 2, resistor R
N
CAL1
D
is set by the ZNBG device to its normal operating voltage.
Set reference, for the ZNBG4000/6000 this is nominally
sets the drain current of FETs 1 and 4, resistor R
CAL
4-141
. Both ZNBG devices have the facility to program
CAL
inputs are provided. For the ZNBG4000,
N
so that the drain current taken matches
CAL2
sets the drain current of FETs
NB
ZNBG4000
ZNBG4000 ZNBG4001
ZNBG6000 ZNBG6001
and C
D
Sense. The amplifier
SUB
.
CAL2