The Design of SiGe HBT Direct-coupled LNA
Yang Weiming*, Chen Jianxin, Xie Wanbo, Shi Chen
Beijing Optoelectronics Technology Laboratory, Beijing University of Technology , Beijing 100022, P.R. China
∗Email: ywmwy@emails.bjut.edu.cn
Abstract-A direct-coupled low noise amplifier was designed
and fabricated on a Teflon substrate, using packaged SiGe
HBTs BFP420 and chip type passive components. This SiGe
LNA has the advantages such as the wide bandwidth
(2.5GHz), the low noise figure (NF ≤ 2.23dB), the high
power gain (S21 ≥ 26.7dB), the input and output VSWR are
all less than 2. The design principle and technology features
of the broadband amplifier was described. The computer
simulated results were coincident with that of the test.
I. INTRODUCTION
Low noise direct-coupled microwave amplifiers are
critical circuit components for high data-rate optical
communications, instrumentation and commercial
industrial-scientific-medical
wireless
applications,
cellular telephone, as well as satellite receiver
applications. The low noise amplifier (LNA) must
balance the specifications of noise figure, bandwidth,
dynamic range and impedance matching. SiGe HBTs can
meet the above demand since they have excellent
characteristics of higher operation frequency: low noise
figure, high gain and high linearity at low current
consumption compared to Si BJTs [1], Given its high
performance and low-cost, SiGe HBTs are emerging as
replacements for expensive HEMT, GaAs MESFETs,
and GaAsHBTs. Up to now, various researchers have
demonstrated high gain, low noise and wideband
amplifier circuits using FET and HBT technologies [2].
Most of them were fabricated by MMIC process [3]. But
MMIC technology is very complicate and difficult to
implement for some research department. In this work,
we have completed a direct-coupled LNA and obtained
the expected microwave performances by using the
common micro-strip hybrid integrated circuit technology.
II. SIGE HBT DEVICE
In the course of the design , we selected the popular SiGe
HBT BFP420 (Siemens Infineon Technologies). This
device has a package of SOT343 with fT and fmax of 25
and 10 GHz, respectively. The noise figure is 0.9dB,
1.05dB, and 1.25dB at 0.9GHz, 1.8GHz, and 2.4GHz for
the bias of 2V/5mA, respectively. The device breakdown
voltage (BVceo) is 4.5V. The typical DC features and
microwave performances are shown in Fig.1 to Fig.4.
For HBT devices, lower current means lower minimum
0-7803-9433-X/05/$20.00 ©2005 IEEE.
noise. However, for low noise amplifier design the
source impedance required to achieve minimum noise
(gamma opt) needs to be considered over bias. This point
is especially important for broadband direct coupled
amplifiers which extend into the microwave frequency
range. Fig.5 shows a plot of the optimum noise source
impedance (gamma opt) from 200MHz to 2GHz as a
function of collector current, where the bias of Vce is 2V,
the collector currents vary from 1.2mA to 7.2mA at a step
of 1.2mA. From the right to the left, each curve
represents a sort of bias of collector currents and the
corresponding collector currents increase in-order. The
impedance plot shows that the optimum impedance is far
from 50 ohms on the above half-circle when the collector
current is very small. But for the high collector current,
gamma opt is mostly real and decreases below 50 ohms.
When the collector current close to 4.8mA, the
impedance is close to 50 ohms over the frequency range.
At this bias point, the LNA would obtain a lower noise
figure, then it would be easier to design for both good
input return-loss and low noise figure when using a
resistive feedback topology. In order to compromise the
noise figure, gain, VSWR, bandwidth and gain roll, we
select 2V/3.1mA as the bias point of the first stage in
terms of the circuit structure.
III. DESIGN OF THE DIRECT-COUPLED LOW NOISE
HBT AMPLIFIER
A.. Design of the circuit configuration
A similar LNA topology using MMIC technology has
been demonstrated [4], but in this work the packaged
devices are used, we cannot design the right device
structure and size. We can only choose the bias
conditions. In this case, the gain and bandwidth product
of the LNA would be affected. So we use a different
implementation in the bias points of Q1 and Q3. A
modified schematic of HBT low noise direct-coupled
amplifier is shown in Fig.6. The HBT direct-coupled
amplifier consists of two gain stages. The first stage is a
common-emitter amplifier comprised of transistor Q1,
the second is a Darlington feedback amplifier comprised
of Darlington connected transistors Q2 and Q3, shunt
feedback resistor Rf1 and inductor L1, load resistor Rload,
output resistor Rout.
APMC2005 Proceedings
Fig.1. The DC characteristics of BFP420
Fig.4. The S12 of BFP420
Fig.2. The minimum noise figure of BFP420
Fig.5. Gamma opt. versus device collector current
Fig.3. The S21 of BFP420
Where the transistor Q4 act as a bias resistor with the
base and collector connected together. The first stage acts
as a low noise common-emitter amplifier which
determines the overall amplifier noise figure. The second
stage Darlington feedback amplifier provides wideband
gain and output drive capability. The gain-bandwidth
characteristics of the Darlington feedback stage can be
optimized by Rf1 without degrading the noise figure of
the overall amplifier. Rf1 also provides a current source
for biasing transistor Q1 of the first stage. The shunt
inductor L1 and the serial inductor L2 of emitter are used
to compensate the high frequency response of the
amplifier so as to improve the flatness within the
effective bandwidth. The bias resistor Rb2 act as an
assistant of Rf1, the load resistor Rload and output resistor
Rout were employed to adjust the output VSWR.
By using the bias transistor Q4 to replace the traditional
bias resistor, a higher gain and better bias stability under
large signal condition can be obtained [5]. The shunt
feedback resistor, Rf2, connected between the emitter of
Q2 and the base of Q1, can be adjusted to change the
effective impedance looking out of the base of Q1 toward
the source, and therefore, optimized for minimum noise
match. In addition, Rf2 provides RF shunt feedback,
which impacts the gain-bandwidth response and
determines the input impedance match of the amplifier to
50Ω. Thus Rf1, L1, L2, Rf2, Rload and Rout can be
adjusted in order to obtain an optimal combination of
gain, noise figure, input VSWR, and output VSWR.
B. Computer simulation
The schematic of the SiGe HBT LNA was designed by
ADS2003C software platform. The passive component
parameters, the micro-strip line structure, and the ground
design were optimized by the layout simulation tools.
When collector emitter voltage (Vce) was 2V, and
collector emitter current (Ice) was 3.1mA for Q1, Vce is
2.3V and 3.3v, Ice is 4.2mA and 4.4mA for Q2 and Q3,
respectively. The simulation results are shown in Fig.7 to
Fig.12. The simulated performances of LNA are as
follows:
S21=27.5dB,
BW=2.5GHz,
input
VSWR=1∼1.77, output VSWR=1∼1.97, NF≤2.2dB,
P1dB=3dBm. Pass-band ripple < ± 0.5dB.
Fig.9. The simulated S12 of LNA.
Fig.6. The topology structure of the LNA
Fig.10. The simulated input VSWR of the LNA.
Fig.7. The simulated noise figure of LNA.
Fig.11. The simulated output VSWR of the LNA
Fig.8. The simulated S21 of LNA.
Fig.12. The simulated P1dB of LNA
IV. IMPLEMENTATION AND MEASUREMENT RESULTS
The designed LNA was fabricated and mounted on a
Teflon substrate (εr = 2.52, conductor thickness =
0.018mm, substrate height is 0.54mm), using four
packaged SiGe HBTs BFP420 (Infineon technology) and
chip type passive components, some small passive
components were implemented by micro-strip lines. The
corresponding components were all connected by
micro-strip lines. The back of the substrate was covered
with copper with some small vias every certain space
away, which were connected to the ground, and then
fixed in a shield box. The supply voltage was 4.5V with a
filter circuit to decrease the noise of the power. This LNA
circuit need not other passive match components, only
two large DC-block capacitors are needed at the input
port and the output port respectively. The noise figure
was tested by HP8970B, HP8971C, HP8350. The small
signal features of the fabricated LNA was measured by
network analyzer (HP8753). The typical gain of the SiGe
HBT LNA was 27dB, the bandwidth is 2.5GHz, and the
noise figure is 2.2dB or so. The main measured results
are shown in Table I. we can see that the measured results
are close to that of the simulation.
TABLE I
THE MEASURED RESULTS OF LOW NOISE AMPLIFIER
100MHz
2.5GHz
S21
S12
Input VSWR
Output VSWR
NF
27.05dB
26.62dB
-39.20dB
1.05
1.46
2.19
-37.84dB
1.80
1.96
2.23
V. CONCLUSION
A SiGe HBT direct coupled LNA was designed using the
microwave simulation tool ADS2003C. The LNA was
implemented on a Teflon substrate using packaged
SiGe HBTs BFP420 and chip type passive components in
hybrid microwave circuit technology. Compared to the
LNA using MMIC technology, this LNA topology has
some modification. The feedback inductor has been used
to compensate the high frequency response and improve
the bandwidth. The replacement of the bias resistor by
the bias transistor can obtain a larger gain. The bias
resistor Rb2 act as an assistant of Rf1 to adjust the input
VSWR and bandwidth. This SiGe LNA resulted in a
noise figure of 2.2dB, and 27dB gain over 0 to 2.5 GHz
range. The input and output VSWR are all less than 2.
The amplifier uses a direct-coupled topology to achieve
wideband frequency performances from base-band to
L-band. This sort of LNA is useful for instrumentation
and commercial industrial-scientific-medical wireless
applications, cellular telephone and electro-optical
communication applications.
ACKNOWLEDGEMENT
This work has been supported by Beijing Natural Science
Fund of China, grant No. is 4032005.
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