Cancellation of third order intermodulation currents in a two-stage amplifier topology

This paper gives a detailed description of a linearization technique using a two-stage amplifier. The first stage is used to generate third order intermodulation products (IMD3), which will be added to the input signal of the second stage In order to cancel the IMD3 at its output. The technique effectively improves the linearity of the system. To investigate the effect of IMD3 injection, analytical calculations of the drain currents for the fundamental frequencies f₁ and f₂ and for the third order intermodulation currents 2f₁-f₂ and 2f₂-f₁ are established without and with injection of intermodulation components at the input of the power stage. To evaluate the improvement provided by the proposed technique, a two-tone test at carrier frequency of 900 MHz is performed with a MESFET. The test procedure is described and measurements show a great improvement of IMD3. A carrier to third intermodulation ratio C/I3 of 55 dBc was achieved for a 19 dBm output power, compared with 25 dBc achieved without the first stage.