Analog Design
Kevin Aylward
B.Sc.
Low Noise (RF) Amplifier
LNA Design
Overview
A reference LNA
(low noise RF amplifier) topology, due to Professor Behzad
Razavi, UCLA, is compared to an alternative topology via SuperSpice simulation.
The design problem of a R.F LNA is usually, to obtain a
low noise amplifier, with high gain, with large output swing, with a constant
resistive input impedance, typically 50 ohms or 75 ohms, and with a high
operating frequency and bandwidth, typically in the several GHz range. The
design described here targets a 10 GHz 3dB BW.
Schematics
A nominal schematic topology, from Razavi’s “RF Microelectronics” is
shown as follows:
Fig. 1 – Razavi Schematic
To perform the simulations, device
models and sizes were selected that would be representative of a real design. The mosfet
models
chosen for design
were nonpropriety device models from http://ptm.asu.edu/, representative of a 45nm process.
The principle design procedure being to chose a
value for the feedback resister that achieves a flatband
6dB attenuation at the amplifier input with zero
phase shift, directly after the source input resistor. What ever gain is
obtained is what it is! In the above schematic, LBOND
and CSTRAY are nominal values chosen to represent ASIC parasitics. These may be significantly different in a
real design. Simulations with different values of Cin
were also performed.
By inspection, the following blemishes are
immediately obvious with the Razavi topology.
1
Reduced output headroom due to the direct coupling
of the gain stage to its pmos source follower buffer.
2
Use of a slower pmos device rather than an nmos
as the buffer.
3
The bias current source injects
noise into the amplifier, requiring mitigation by a noise filtering capacitor
4
The feedback does not include Cin, causing additional source mismatch at lower
frequencies.
Fig.
2 – Aylward Schematic
The
following points are made with respect to the Fig.2
topology:
1
The output buffer is AC coupled,
with its bias point being such that its gate input may go above the supply,
allowing for a larger final output swing. This is achieved without the device
voltage ratings being exceeded.
2
The output buffer is a faster,
smaller nmos device.
3
Reduction of the dual push pull
output and current determining devices to a single push pull device. This is achieved
by AC coupling the input to the bias source making it serve both functions.
4
Inclusion of the input blocking
capacitor within the feedback path.
Results Table
Topology 
Aylward 
Razavi 
Units 




Gain 
14.3 
10.6 
dB 
Bandwidth 
10 
8.7 
GHz 
Gain Bandwidth
Product 
51.9 
29.4 
GHz 
Output Swing 
0.67 
0.49 
V 
Equivalent
Input Noise 
1.14 
1.25 
nV/rtHz 
N.F. 
2.15 
2.95 
dB 
Zin MAG Match BW (+/ 1dB) 
0.1 – 6.5 
0.5 – 6.5 
GHz 
Zin Phase Match
BW (+/ 10 Degs.) 
0.2  11 
1  11 
GHz 




Graphs
Fig. 3 – Output Swing
Orange – Razavi topology, output
Red – Aylward topology, output
Green and violet,
inputs.
Fig. 4 – Bandwidth – Cin=10p
Orange – Razavi topology, output
Red – Aylward topology, output
White – Razavi topology, input magnitude
Green – Aylward topology, input
magnitude
Light Blue – Razavi topology, input phase
Pink – Aylward topology, input
phase
Fig. 5 – Bandwidth – Cin=100p
Orange – Razavi topology, output
Red – Aylward topology, output
White – Razavi topology, input magnitude
Green – Aylward topology, input
magnitude
Blue – Razavi topology, input phase
Pink – Aylward topology, input
phase
Fig. 6 – Input Noise – Cfilt=2p
White  Razavi topology
Red – Aylward topology
Notes: Increase of noise at low
frequencies for the Razavi topology due to its current source
Fig. 7 – Input Noise – Cfilt=10p
White  Razavi topology
Red – Aylward topology
Summary
A LNA topology
has been presented that has superior performance in all key parameters, than a reference
topology of Professor Behzad Razavi, UCLA, described in his instructional text
book “RF Microelectronics”
© Kevin
Aylward 2014
All
rights reserved
The
information on the page may be reproduced
providing that this source is
acknowledged.
Website
last modified 10^{th} March 2014