ZNBG3113 Datasheet, PDF (8/17 Pages) Zetex Semiconductors – FET BIAS CONTROLLER WITH POLARISATION SWITCH AND TONE DETECTION

ZNBG3113
ZNBG3114
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 ZNBG3113/14 contains 3 such stages. The
negative rail generator is common to both 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 ZNBG3113 this is nominally 2.2 volts
whilst the ZNBG3114 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_N so that the drain current taken matches
the current called for by an external resistor R_CAL.
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_NB and C_SUB.
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