znbg3110 Zetex Semiconductors plc., znbg3110 Datasheet - Page 8

• Manufacturer Part Number: znbg3110
• Description: Fet Bias Controller With Polarisation Switch And Tone Detection
• 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 ZNBG3110/11 contains 3 such stages. The
negative rail generator is common to both devices.
The drain voltage of the external FET Q
This is determined by the on board V
whilst the ZNBG3111 provides nominally 2 volts.
The drain current taken by the FET is monitored by the low value resistor ID Sense. The amplifier
driving the gate of the FET adjusts the gate voltage of Q
the current called for by an external resistor R
Since the FET is a depletion mode transistor, it is often 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
ZNBG3110
ZNBG3111
D
N
Set reference, for the ZNBG3110 this is nominally 2.2 volts
is set by the ZNBG device to its normal operating voltage.
4-130
CAL
.
N
so that the drain current taken matches
NB
and C
SUB
.