Design and Analysis of a Novel Noise Cancelling Topology for Common Gate UWB LNAs - VLSI Design and Test - page 174

Fig. 6. UWB LNA with proposed noise cancellation technique

Fig. 6 shows schematic of the UWB LNA with proposed noise cancelling topolo-
gy. In addition to the conceptual design shown in Fig. 4, extra gain stages i.e. resistive
feed forward cascode CS stage marked by 'A' are added to improve the overall signal
gain and bandwidth of amplifier. Noise cancellation is achieved by combining the
outputs of these gain stages.

4 Results and Discussion

The proposed noise cancelling topology is designed for UWB LNA operating from
frequency range 2.4 GHz to 9 GHz. Small size of M1 transistor demonstrates wide-
band input matching characteristic. Transconductance (g m1 ) of M1 transistor is fixed
at 20 mS for 50 Ω input matching with bias current of 1.8 mA and L2 is 4.44 nH,
which keeps input reflection coefficient well below -10 dB over the entire range of
2.4 GHz to 9 GHz. The g m of M2 (43.53 mS), M3 (43.56 mS) and M4 (47.55 mS) are
decided by considering the radio frequency circuit design tradeoffs and cancellation
of the noise of M1 at output. Simulated noise figures, which are optimized at 6 GHz
for UWB LNA with and without noise cancellation technique, are shown in Fig. 7 and
report approximately 1 dB reduction. Minimum noise figures of 3.16 dB and 4.09 dB
are noted with and without noise cancellation respectively.
Fig. 8 shows the circuit simulated power gain of 24.24 dB (average) in the fre-
quency range of 2.4 GHz to 9 GHz and 17.7 dB (average) in the frequency range of
3.1 GHz to 10.6 GHz for UWB LNA with and without noise cancellation technique
respectively. Fig. 9 compares simulated and theoretical noise figures, which shows an
excellent noise performance at 3 GHz and lower corner frequency. Simulated noise
figure value increases towards upper corner frequency due to the noise contribution of
components that were not considered during NF analysis and are not included in (6).
However, in this paper an UWB LNA is designed which exploits thermal noise
cancellation of its first stage CG transistor (M1). Equation (6) is used to get the plot
for theoretical noise figure as shown in Fig. 9, which in turn is compared with the
simulated noise figure plot and they are found to be in close agreement near a fre-
quency of 3 GHz. Table I compares the performance of UWB LNA with and without
noise cancelling technique.