znbg3000 Zetex Semiconductors plc., znbg3000 Datasheet - Page 5

• Manufacturer Part Number: znbg3000
• 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 ZNBG3000/1 contains 3 such
stages.
The drain voltage of the external FET Q
This is determined by the on board V
whilst the ZNBG3001 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.
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
D
N
Set reference, for the ZNBG3000 this is nominally 2.2 volts
is set by the ZNBG device to its normal operating voltage.
CAL
4-141
. Both ZNBG devices have the facility to program
N
so that the drain current taken matches
NB
and C
ZNBG3000
ZNBG3001
D
Sense. The amplifier
SUB
.