Radio Frequency Integrated Circuit Design - Webs

Mixers
Figure 7.34 Mixer with folded switching stage and resistive input matching.
the quad nonlinearity for the folded cascode configuration is slightly more
complex than the standard configuration because the current applied to it is
no longer exactly equal to the current from the RF stage. We can start by
assuming that each nonlinearity contributes equally to the linearity of the circuit
(assuming that the circuit now has enough headroom that the output does not
clip or saturate the quad). Then the input should be designed for 11-dBm IIP3
rather than 8-dBm.
The quad will be more linear as more current is passed through it because
of the reduction in the emitter resistance re of the four transistors and the
resulting reduction of the voltage swing at the emitters. Thus, as a first cut we
will split the available current equally between both stages, allotting 6 mA to
the driver and 6 mA to the quad. We can now start to size the degeneration
resistor R E . An IIP3 of 11 dBm at 100� corresponds to a signal swing of 1.12
Vrms at the input of the mixer or, equivalently, 0.561 Vrms per side for the
differential circuit. Using (6.85) from the previous chapter, R E can be determined
to be
R E = 2�r e� v IP3
2vT� 2/3
561 mV
− re� = 2�8.3��2 � 25 mV� 2/3
− 8.3�� = 66.6�
235