Which device supplies the radio signal into a DAS (distributive antenna system)?

An amplifier increases signal strength along the distribution path but does not originate or supply the RF signal; it depends on an upstream source.

A receiver only captures and processes incoming RF signals and has no ability to inject a downlink signal into the DAS.

A transmitter only sends signals in the downlink direction and cannot handle the uplink receive path required for a complete two-way DAS.

In a DAS, the transceiver functions as the head-end unit that originates the downlink RF signal distributed through the antenna network and also processes the uplink signals received from the coverage area. Because two-way radio coverage requires simultaneous transmit and receive capability, a transceiver - not a one-directional device - is required at the signal source. This bidirectional nature is what distinguishes the transceiver as the correct signal-supplying component in a DAS architecture.