TABLE OF CONTENTS FETs are widely used in MMIC mixers, as many monolithic foundry processes are based on the MESFET structure. Consequently, FET mixers can be fabricated on the same chip with other FET-based circuits, such as amplifiers, oscillators and switches, to form integrated subsystems. In addition, active FET mixers offer the potential to achieve conversion gain. Generally, they also require less LO power, have higher dynamic range and give comparable noise performance to diode mixers.
A study by Fairburn et al. [14] identified several mixing actions using the active mode in a single-gate FET. They can be attributed to the various non-linear elements in the active FET. Figure 7.29 shows the large-signal equivalent circuit of an active FET. The dominant non-linear elements are the gate-source capacitance, transconductance and drain conductance. Depending on the biasing conditions, each of these elements can be employed for frequency mixing. According to which FET terminal the LO is applied to, these active FET mixers can be classified as gate, drain and source mixers. Figure 7.30 shows the biasing points and the I V trajectories of the pumped device, for each mixer type.
The LO voltage is applied to the gate and the time-varying transconductance is responsible for frequency conversion. The gate is biased near pinch-off and the drain biased at V dd to provide...
