Ultra Wide band Low Noise amplifier for Low frequency
applications in Radio Astronomy
Raghunathan A1 , Raj Kumar P2 , Udaya Shankar N1 , and Vinoy K J2
1
Radio Astronomy Lab, Raman Research Institute, Bangalore-560080, India
E-mail: {raghu, uday}@rri.res.in
2
Dept. of Electrical Communication Engg., IISc, Bangalore-560012, India
E-mail: rajkumarpalwai@yahoo.co.in, vinoy@ece.iisc.ernet.in
1
Introduction
Now it is well recognized that although Jansky discovered radio astronomy at decametric
wavelengths, the quest for higher angular resolution and the difficulties in handling ionospheric effects have left the low frequency (below 150 MHz) spectrum poorly explored. This
is despite many unique astrophysical questions that can be better addressed at low frequencies, and problems requiring accurate source spectra at low frequency end as well. However
the recent advancement in technology and better understanding of imaging algorithm, interest has been rekindled in low frequency astronomy which requires a high dynamic range
imaging in the presence of man made interferences. With this the usage of short dipoles
(whose length is much smaller than wavelength) as feeds for parabolic dishes and as stand
alone antenna elements for arrays has taken the centre stage. Using short dipoles a compact
array could be constructed with less mutual coupling effects. If the dipole is electrically
small in size, then its radiation pattern is shown to be almost independent of frequency
allowing for broad band operation. One of the main disadvantages of a short dipole is that
it offers a very high reactive impedance to the sky radiation as a result of which the latter
gets poorly coupled to the receiver. The poor coupling can result in a system in which
instead of the sky noise the receiver noise may dominate the system temperature. To ensure
a sky noise dominated system even with poor coupling, one requires low noise amplifier even
at low frequencies. The amplifier should also have a high dynamic range since the radio
frequency interference in the low frequency regime is very high.
This paper discusses the design and development of a broad band low noise amplifier successfully built in the laboratory based on the thermal noise cancellation technique (TNC) [1]
to operate in the frequency range of 30 MHz to 300 MHz. The circuit has been optimized
to have the required characteristics like minimum noise temperature (≤ 50K), good input
return loss (≤ −10dB) for a 50 ohm system and a sufficient gain (≥ 20dB) over most of the
operating frequency band. The dynamic range of the amplifier has also been enhanced by
about 10 dB by incorporating a post distortion linearizing circuit at the output of the amplifier. Microwave Integrated Circuit technique has been adopted in our approach to design
the amplifier instead of MMIC since the latter technique is not suitable for implementing in
the laboratory. We present in this paper various simulations made using ADS CAD software
along with the measurement results obtained in the laboratory.
2
Principle of Thermal Noise cancellation technique
In any transistor thermal noise generated in the gate and source terminals, thermal noise
generated in the source-drain channel and the same induced in the gate contribute to the
total noise temperature of the device. When any one of them is added on to itself in
phase opposition, then the net noise contribution due to that component tends to cancel
reducing the effective noise temperature of the device.This is made use of in the thermal
noise cancellation method to cancel dominantly the channel generate noise. The schematic
shown in Fig.1(a), depicts the basic principle involved. In the figure Vnoise and Vs represent
the channel generated noise voltage at the output of transistor Q1 and the input signal
470 Ohm
Vnoise
875 Ohm
A
RF
A
Q2
c
Amplified
Signal
B
50 Ohm
B
Q1
Q1
RS
33 Ohm
10 UH
Vs
Vs
Residual
Noise
Q3
−A
(a)
(b)
Figure 1: Schematics showing (a) the working principle of Thermal Noise Cancelling technique and (b) Optimized LNA circuit .
to be amplified respectively. A small portion of Vnoise appears at the gate terminal of
the transistor as shown in the figure. It is amplified to the same level of Vnoise before
combining it with the latter in phase opposition in the power combiner. If the cancellation
does not occur completely the output will tend to have a residual noise as shown in the
figure. The input signal on the other hand gets equally amplified by both the transistors Q1
and Q2 before getting combined in the power combiner. The input stage of the amplifier
can independently be designed to provide the required input impedance for achieving good
input return loss. Thus both minimum noise figure and good input VSWR could be achieved
by adopting TNC technique.
3
LNA design
The amplifier was designed to have i) a noise temperature of ≃ 50 K, ii) an overall power gain
of 20 dB and iii) an input return loss of -10 dB for a 50 ohm system over the frequency range
of 30-300 MHz. The noise temperature of 50 K was chosen in order to ensure domination
of sky noise in the system temperature when the sky is observed using an antenna having a
coupling efficiency of 1% at 50 MHz. The sky brightness temperature of 2500K is assumed
at this frequency with a spectral index of -2.5.
A PHEMT transistor ATF-54143 from Agilent Technologies was chosen to build the amplifier
since it has low noise and high linearity performance over broad frequency range. The
amplifier consists of i) an input matching section and ii) an output noise cancellation section.
The input matching section which is basically an inverting amplifier is designed to have a
50 ohm input impedance (Ri ) for testing in the lab and a voltage gain (Av ) of ∼10 dB
to minimize the noise contribution from the subsequent stages. The feed back resistor RF
and the required transconductance gm were determined using the relationships given below
derived from the small signal equivalent circuit [2] of the transistor used. We assume the
output impedance of the transistor ro to be much greater than the load resistor RL .
Ri =
ro kRL + RF
;
1 + gm (ro kRL )
Av =
VO
(gm RF − 1)
=−
F
Vb
1 + r RkR
o
(1)
L
The output noise cancellation section consists of two transistors (Q2 and Q3) operating in
the push pull mode. The gains of these transistors are determined in such a way that the
27
24
23
22
Noise Temperature (K)
Input Return Loss (dB)
Gain (dB)
−8
25
21
70
−6
Measured
Simulated
26
Simulated
Measured
−10
−12
−14
−16
−18
−20
50
100
150
200
Frequency (MHz)
250
300
(a)
−22
Measured
60
Simulated
50
40
30
20
50
100
150
200
Frequency (MHz)
250
300
(b)
50
100
150
200
Frequency (MHz)
250
300
(c)
Figure 2: Comparison of simulated and measured (a) gain, (b) input return loss, and (c)
Noise figure of the LNA as a function of frequency.
noise voltages at their gate terminals appear with equal amplitude for effective cancellation
at the output. The gain of Q2 (A2) is maintained at unity where as the gain of Q3 (A3) is
determined in terms of the transconductances (gm2 , gm3 ) and drain source currents Ids3 , Ids2
of Q2 and Q3 as
gm3
A3 ≈
≈
gm2
r
Ids3
= 11
Ids2
Ids3
≈ 121
Ids2
(2)
(3)
Since the current range supported by the transistor ATF 54143 is approximately 80 mA,
the required ratio of currents in both the transistors could not be maintained for complete
noise cancellation. With this constraint the circuit was optimized (Ref. Fig. 1(b)) using
the ADS CAD package to meet the required specifications.
4
Measurement Results
The amplifier was realised on an Ultralam 2000 PCB which is a woven glass reinforced
microwave laminate generally used for high reliability applications. The amplifier was characterized for its gain, input match using scalar network analyser (HP 8757D) and noise
temperature using noise figure meter HP 8970B. The measurement results are shown in the
Fig.2(a-c) along with the simulation results. The simulated noise figure performance appears
better than the measured value since i) the noise contribution due to the input connector
was not taken into account and ii) all the passive elements were treated as noiseless during
the simulation. The photograph of the low noise amplifier built in the lab is shown in the
Fig. 3(a).
5
Enhancement of the dynamic range using Post Distortion
circuit
The third order intermodulation product of the amplifier was measured to be about -40
dBm for an input power of -15 dBm at 180 MHz. In an attempt to improve it, a transistor
based non-linear element was introduced at the output of the amplifier to correct for the
gain and phase responses of the amplifier for higher input power level [3]. Simulation was
carried out and the optimized circuit which could produce an improvement in the third
order intermodulation product by about 10 dB is shown in the Fig.3(b). The relative
improvement in the 3rd order intermodulation product when compared to an LNA having
no post distortion circuit at its output is also shown in the Fig.3(c). We present only the
Q2
RF
B
Q1
c1
c2
ATF−54143
RS
Vs
Ld
Ls
Vds
Rg
Q3
(a)
Cg
Vgs
(b)
Relative 3rd order intermodulation components (dBc)
A
−10
−20
LNA with Post−distorter
LNA without Post−distorter
−30
−40
−50
−60
improvement is seen in this region
−70
−80
−50
−45
−40
−35
−30
−25
−20
Input Power (dBm)
−15
−10
−5
0
(c)
Figure 3: (a) Photograph of the Low Noise Amplifier, (b) Amplifier circuit with post distorter, and (c) Comparison of III order intermodulation products for amplifier with and
without post distorter at its output.
simulation results showing the enhancement in the dynamic range of the amplifier since it
could not be realized prior to the submission of this paper.
6
Conclusion
We have successfully designed and developed a wide band low noise amplifier based on
Thermal Noise Cancellation technique for low frequency applications. Its frequency range is
30-300 MHz. It has a good input return loss of ≤10 dB over most of the band. The amplifier
has a moderate gain of 24 dB and a noise temperature of ≃50 K. The measurements made
matched very closely with the simulated results.Also the simulated result indicates a possible
improvement in the dynamic range of the amplifier when a post distortion circuit is included
at the output of the amplifier. A differential low noise amplifier based on TNC with an input
matching network is being developed so that it can be connected directly to the two halves
of the short dipole instead of using a balun and a single ended amplifier.
7
Acknowledgment
We thank Sandhya P., Raghavendra K.B and Ateequlla and his group (Narayana Swamy
and Venu P.) in the mechanical workshop for having extended their support while building
and testing of this amplifier.
References
[1] Federico Bruccoleri, Eric A. M. Klumperink, and Bram Nauta. Wide-Band CMOS
Low-Noise Amplifier Exploiting Thermal Noise Canceling. IEEE Journal of Solid-State
Circuits, 39(2):275–282, February 2004.
[2] Inder Bahl and Prakash Bhartia.
Interscience Publications, 1988.
Microwave Solid State Circuit Design.
Wiley-
[3] Thomas H. Lee. Planar Microwave Engineering. Cambridge University Press, 1998.