The GPA-9 is an RF power amplifier designed for use in the 896-940MHz frequency range at up to 40
Watts. The A version is intended to serve as a 100% duty cycle amplifier for MDS SD9 and other MDS
radios operating in point-to-multipoint repeater or base applications.
The GPA-9 power amplifier consists of an RF amplifier and PCB mounted to a heat sink, with a DC Power
interface, PTT and input/output RF connections on the sidewalls of the chassis. DC power is supplied to
the amplifier from a regulated and filtered DC source capable of supplying 10-16 Vdc at a maximum
current of 10 Amperes. The DC power source should be current limited or have a protective fuse or circuit
The GPA-9 amplifier uses a feedback circuit within the amplifier to control the output power. The set
point is set by potentiometer R138, accessed near the RF input connector.
Further, the PA performs TX/RX switching to provide a low loss receive path when not transmitting. The
TX/RX control is performed by RF detection within the PA. There is also a PTT (Push to Transmit) input on
the terminal strip.
1.4 Product Description
1.5 Power Control Loop
Figure 1 – R138 Location