Feed forward amplifier employing bias circuit topologies for minimization of RF amplifier memory effects
View Patent ↗An RF power amplifier having reduced memory effects is disclosed. This is achieved by a novel design of the DC supply feed network to achieve low impedance across video frequencies, whilst maintaining the correct RF output matching. One or more transmission zeros are provided in the bias circuit transfer function, which are positioned in the video bandwidth so as to provide low and relatively constant impedance across the video bandwidth. Also, a parallel DC feed line may be employed to reduce impedance across the video bandwidth. The reduction in memory effects allows improved performance of predistortion linearization techniques and an implementation in a feed forward amplifier employing predistortion linearization is also disclosed.
1. A feed forward amplifier, comprising:
an RF input for receiving an RF signal having an RE carrier and a modulation bandwidth of at least 10 MHz;
a predistortion circuit receiving and predistorting said RF input signal;
a main amplifier receiving and amplifying said predistorted RF signal, said main amplifier comprising an active device and a bias circuit coupled to a DC power supply, said bias circuit having a transfer function having a plurality of transmission zeros spaced across the modulation bandwidth so as to substantially eliminate amplifier memory effects;
a main amplifier output sampling coupler;
a first delay coupled to the RF input and providing a delayed RF signal;
a carrier cancellation combiner coupling the delayed RF signal to the sampled output from the main amplifier;
an error amplifier receiving and amplifying the output of the carrier cancellation combiner;
a second delay coupled to the output of the main amplifier;
an error injection coupler combining the output from the error amplifier and the delayed main amplifier output from the second delay so as to cancel distortion introduced by the main amplifier; and
an RF output coupled to the error injection coupler output and providing an amplified RF output.