Radio Frequency Integrated Circuit Design - Webs

High-Frequency Filter Circuits
9.4 Achieving Filters with Higher Q
So far, we have considered a few simple second-order filters that can be placed
around amplifiers to provide frequency selectivity. Since on-chip inductors do
not have a high Q, the bandwidth of the passband filter is not very high.
Likewise, with the bandstop filters, the amount of attenuation at the notch
frequency will be limited by the Q of the inductors used in building the filter.
To increase the Q, it is necessary to build some active circuitry around the LC
resonators. This can be done by creating negative resistance to offset the losses
in the inductors and other circuits. In Chapter 8, a few very common circuits
for doing this were studied. There are many possible ways to generate negative
resistance, and in the next few sections, a few of them will be considered.
9.4.1 Differential Bandpass LNA with Q-Tuned Load Resonator
The first example of a circuit that will be considered is the differential version
of the previously shown bandpass circuit with the addition of a −Gm cell, as
shown in Figure 9.7. Note that to complete this design, some form of emitter
degeneration would usually be used in Q 1 and Q 2 and input matching would
be needed.
The −Gm cell generates a negative resistance of −2/g m (or −4vT /Isharp),
which is placed in parallel with R, the equivalent total losses in the tank. If this
parallel combination is used instead of R in (9.3), the gain becomes
Figure 9.7 An LNA circuit with Q-enhanced resonator.
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