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Transmit amplifier chain

A transmit amplifier chain consists of a driver amplifier followed by an open collector output stage. The

open collector TXOUT output (pin 5) requires an external pull-up inductor for bias. This inductor may be

used as part of an L-C matching circuit for transforming the amplifier output impedance to that of an exter-

nal loop antenna. To achieve optimum power-added efficiency, the peak-to-peak RF voltage swing at the

TXOUT pin should be twice the supply voltage. Also, the output amplifier has its own ground pin (VSS) in

order to improve isolation.

The transmitter chain is designed for amplitude-shift-keying/on-off keying (ASK/OOK), with the modula-

tion data provided at the MODIN input (pin 8). When V

when V

Operation modes

The table below summarizes the TX4915’s three operating modes — power-down, PLL-only and transmit,

which are controlled by the TXEN and MODIN inputs.

TXEN

MODIN

Operation Mode

L

L

Power-down

HL

PLL-only

H> 0.7 VTransmit

Rev.2.02 8/06/2003

SSTX4915