A Low-Power Tunable SiGe HBT LNA for Wireless LAN Applications

A Low-Power Tunable SiGe HBT LNA for Wireless LAN Applications
Corrado Carta, Jörg Carls and Werner Bächtold
Electromagnetic Fields and Microwave Electronics Laboratory (IFH),
Swiss Federal Institute of Technology (ETH), 8092 Zurich, Switzerland
email: carta@ifh.ee.ethz.ch
Abstract
This paper presents the design, implementation and measurements
of a BiCMOS integrated tunable Low Noise Amplifier
for 5 GHz WLAN applications. It consists of a cascode
gain cell, noise and power matched to a 50 Ω input
source, and power matched to a 50 Ω load. In order
to broaden the available bandwidth, the output matching
network is modified by means of a shunt varactor, thus allowing
the tuning of output matching and peak gain over
a band exceeding 1 GHz. The proposed topology modification
proofs to have neglectable effects on other LNA features,
such as noise figure, linearity and input matching.
Realized in a 120 GHz-ft commercial BiCMOS technology,
the circuit exhibits 13.2 dB of flat power gain, 2.3 dB of NF,
-9 dBm of iIP3, -20.5 dBm of P1 dBm, input and output return
losses better than -10 dB in the 5-6 GHz band. Power
consumption is 6 mW.
1. Introduction
The 5-6 GHz wireless LAN standards make use of
three unlicensed U-NII bands, resulting in a total input
band from 5.15 GHz to 5.875 GHz [5]. This sets the
operating bandwidth of the first receiver stages, including
the LNA. In designing this building block, cascode amplifiers
are often preferred as gain cell because of their good
isolation and voltage gain; one drawback this choice implies
is that achieving sufficient preamplifier bandwidth
becomes difficult, even with low-quality silicon technologies:
the problem is typically addressed designing the receiver
for the two lower [5–7] or the upper [2, 4] U-NII
bands, or lowering the resonator quality factors, thus trading
gain for bandwidth [1].
Assuming that no more than one 20 MHz channel would
be received at a time, this paper presents an alternative
approach: with a simple modification of typical
LNA topologies, output matching and power gain can be
tuned and optimized over a band sufficient to cover com-
Vtune R∞
RFin
L2
Ctune
Vbias
C1
C∞
Vcc
T2
T1
L3
L1
C2
RFout
Figure 1. Schematic of the fabricated amplifier.
Bias network is not shown.
pletely the 5 GHz standards. The proposed topology has
been validated by a design and fabrication in an advanced
BiCMOS technology of an LNA. For ease of characterization,
the presented amplifier is designed to drive a
50 Ω load, but the same idea can be easily adapted to the
higher impedance levels typical for on-chip system designs.
2. Circuit
2.1. Design
Fig. 1 shows the schematic of the amplifier. The LNA
is a single-ended HBT cascode, power matched to 50 Ω at
both input and output; the input matching network was designed
to achieve simultaneous noise and power matching
[8] to the source.
The cascode gain cell is often chosen for WLAN re-

ceivers: this is mainly due to its good isolation and its high
output impedance, which improves the gain for given current
consumption and output-matching quality factor.
Both of the transistor dimensions are set for peak-ft operation
at the given DC collector current. This sets the input
capacity of T1: for this reason, capacitor C1 is used to increase
the input capacity to a value convenient to achieve,
together with inductors L1 and L2, noise and power input
matching.
A conventional narrow-band output matching is implemented
by means of L3, Ctune and C2. The value of
L3 and the nominal value of Ctune, i.e. the value for
Vtune = Vcc, are chosen to provide the reactance necessary
for power matching at 5.5 GHz, the center frequency
of the target band. The capacity variation tunes the matching
center frequency and the power-gain peak.
Apart from offering the tuning capability, a positive
side-effect of adding Ctune in parallel to L3 is that the inductance
of L3 needed for power matching is lower: this
results in a smaller size of the spiral inductor and reduces
the overall circuit active area.
Targeting the main 5-6 GHz WLAN standards, the circuit
was designed to achieve 14dB of flat gain over the
whole 5-6 GHz band, a NF of 2 dB [1] with a current consumption
of 3mA from a 2V power supply.
2.2. Fabrication
The circuit was fabricated with the commercial IBM
BiCMOS 7HP process [3]. At optimum bias current, the
HBTs yield transit frequencies ft up to 120 GHz and minimum
noise figures of around 0.5 dB at 5-6 GHz. The process
is targeted for RF, analog and mixed signal applications:
a 4 µm thick Analog Metal and deep-trench insulations
allow the integration of inductors with quality factors
up to 20 at 5 GHz.
The circuit has been implemented on a 0.3 mm 2 active area,
as shown in the photograph in Fig 2. Total area shown measures
1.18 mm×0.92 mm; area efficiency is mainly limited
by bond pads and pattern density requirements.
Transistors T1 and T2 are dual-base-contact HBTs; L1−3
are single layer octagonal inductors over deep-trench lattice;
Ctune is a nFET varactor, while C1−3,∞ are MIM high
density capacitors.
3. Measurements
The circuit has been characterized on-wafer using
RF probes. While biased with a 3 mA collector current,
it showed similar performances for Vcc ranging
from 1.5 V to 2.4 V. Results presented here are those observed
for Vcc =2 V.
Figure 2. Chip microphotograph. Area shown
measures 1.18 mm×0.92 mm
Table 1. Summary of experimental results
Technology 120 GHz-ft BiCMOS
|S21|max
13.2 dB
|S21|max± 0.1 dB 5.1 – 6.2 GHz
|S11|

|S 21 |, |S 22 | [dB]
15
10
5
0
−5
−10
−15
−20
−25
3 4 5 6 7 8
Frequency [GHz]
Figure 3. Effects of the output-matching tuning
on S21 and S22: Vcc − Vtune sweeps from
-0.4 V to 0.3 V
Fig. 5 presents an example of linearity tests preformed on
the LNA at 5.5 GHz: in this case, P1 dB is -20.5 dBm and
iIP3 is -9 dBm at the corresponding optimum Vtune value.
Extrapolations of iIP3 at other frequencies have confirmed
that it is independent of Ctune variations, within measurement
uncertainty.
4. Conclusions
The design, implementation and characterization of a
tunable LNA have been presented. A simple modification
of the topology for the output matching network allows
to double the -10 dB bandwidth of S22 and achieves
constant power gain over the 5.1-6.2 GHz band. The measured
performances are sufficient to meet the specifications
for an RF front-end compliant with the main WLAN
standard in the 5 GHz band [1]. In particular, the flat frequency
response of the gain can significantly improve
the noise performance of the whole receiver, or simplify
the use of noise-critical active mixers relaxing, e.g.,
the LO power requirements.
5. Acknowledgment
This work was funded by the ETH/IBM Center for Advanced
Silicon Electronics (CASE).
|S 21 |, NF [dB]
RF output power [dBm]
15
12
9
6
3
NF
|S 21 |
|S 11 |
|S 22 |
−10
−15
−20
0
3 4 5 6 7 8
Frequency (RF) [GHz]
−25
Figure 4. Optimal S-parameters and noise figure:
for each frequency Vtune is set for best
|S22| value.
10
−10
−30
−50
−70
−90
−45 −40 −35 −30 −25 −20 −15 −10 −5
RF input power [dBm]
11
1st
3rd
Gain
11.5
0
−5
13.5
13
12.5
Figure 5. Linearity tests preformed on the
LNA at 5.5 GHz. Vtune is set for best |S22|
value.
The authors wish to thank H.-R. Benedickter and M. Lanz
(both with IFH-ETH) for their valuable help in characterizing
the circuit and preparing the test boards, M. Schmatz
(IBM) and R. Vogt (IFH) for managing the project.
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