Q-Band Monolithic GaAs PHEMT Low Noise Amplifiers:
Comparative Study of Depletion and Enhancement Mode
Transistors
B. Aja, L. de la Fuente, J.P. Pascual, M. Cryan*, E. Artal
Dpto. Ing. de Comunicaciones. ETSII y de Telecomunicación. University of Cantabria
Avda. Los Castros s/n 39005 Santander. Spain, beatriz@dicom.unican.es
*
Department of Electronic and Electrical Engineering. Queen's Building.University of
Bristol. United Kingdom, m.cryan@bristol.ac.uk
Two Q-band monolithic low noise amplifiers have been designed and characterized. A study about
depletion and enhancement mode HEMTs with the same technology has been performed in order to apply
these results to the design of the low noise amplifiers. These circuits have been developed for being used
in the Back End module of the radiometers in the European Scientific mission Planck, because there are
not commercial circuits available in this frequency band. The main goals for these amplifiers are low
noise with a small DC consumption. A minimum noise figure of 2.8 dB with an associated gain of 23.1
dB at 40.4 GHz has been measured for the E-HEMT MMIC LNA and its DC current consumption was
15.4 mA. The D-HEMT MMIC LNA has a minimum noise figure of 3 dB with an associated gain of 23.2
dB at 42 GHz and 30 mA of DC current consumption.
INTRODUCTION
Planck is a mission of the European Space Agency
(ESA) Science Program (1), to perform astronomical
investigations in the submillimeter and millimeter
wave range. The mission will produce calibrated maps
of the whole sky with high sensitivity. The Planck Low
Frequency Instrument (LFI) receiver is a form of
differential radiometer. It must have enough sensitivity
to measure Cosmic Microwave Background
anisotropies in the frequency range 30 – 100 GHz and
it will be split into 4 channels centered at 30 GHz, 44
GHz, 70 GHz and 100 GHz with a 20 per cent
bandwidth each one. The 44 GHz radiometer is based
on a Front End Module (FEM) with InP HEMT
amplifiers cooled at 20K and a Back End Module
(BEM) with GaAs HEMT amplifiers at room
temperature (300 K). MMIC low noise amplifiers have
been developed due to lack of commercial products in
this frequency band (39.6 GHz-48.4 GHz). The most
critical requirements for these amplifiers are the power
dissipation and the noise figure. Both of them should
be as low as possible because this circuit will be
shipped in a satellite and the instrument will be looking
at background noise so its inherent noise must be very
low.
D-HEMT AND E-HEMT CHARACTERISTICS
A study about enhancement and depletion transistors
has been performed in order to establish the behaviour
of both modes. The design and characterization of two
low noise amplifiers have been used to verify the
conclusions drawn from the study. The technology
chosen has been OMMIC ED02AH process, which
employs a 0.2 µm Pseudomorphic-High Electron
Mobility Transistor (P-HEMT).
The enhancement mode is similar in geometry to the
depletion mode, but in electrical operation it is
normally off and it does not conduct with zero gate
voltage. Transistor width has been chosen taking into
account the minimum noise factor (Fmin) with a fair
drain-source current (Ids). The output noise drain
current increases as W and the signal current increases
as W; big transistors improve the signal-to-noise ratio
and it means low noise figure. On the other hand big
unit gate width means big gate resistance and therefore
the noise figure increases.
Figure 1 depicts the variation of the minimum noise
figure (Nfmin) versus the total gate width (W) of a
transistor with the same unit gate width but different
number of fingers, showing that a larger number of
fingers implies a decrease in the Nfmin. Transistors
with a gate width of 6x15µm have been used for both
designs as a low noise and low DC current fulfil.
They have similar minimum noise figures but the
enhancement mode has a smaller drain source current,
which makes it suitable for on board circuits.
amplifiers of the radiometer must be sensitive to work
with weak signals.
D-HEMT (6x15um)
7
70
6
60
Gm (min(Nfmin))
41.0 mS
Gm (mS)
50
1,6
ON
1,58
5
NFmin
40
4
30
3
1,54
20
2
1,52
10
1,5
1,48
0
2x15um
OFF
0,6
0,5
0,4
0,3
0,2
0,1
0,0
-0,1
10x15um
-0,2
0
-0,3
6x15um
-0,6
2x15um
1,46
1
Gm
-0,4
10x15um
6x15um
-0,5
1,56
NFmin (dB)
Gm (max)
61.2 mS
NFmin (dB)
Figure 2 shows a comparison between the minimum
noise figure versus drain source current of both
transistor modes. Solid line refers to depletion mode
and dot line refers to enhancement mode, which has a
minimum slightly lower and it occurs with small Ids
current.
Vgs (V)
1,44
1,42
Figure 3. Gm and Nfmin versus Vgs of D-HEMT 6x15µm
@ 44 GHz
30
90
150
180
W (um)
E-HEMT (6x15um)
Figure 1. NFmin versus gate width (W) of a D-HEMT and
E-HEMT @ 44 GHz
100
6
90
5,5
80
5
Gm (mS)
70
3
D-HEMT
E-HEMT
2,6
Gm (max)
84.7 mS
Gm(min(Nfmin))
55.2 mS
60
4
3,5
50
3
40
30
Gm
2,5
NFmin
20
2
1,5
10
2,2
1
0,80
0,75
0,70
0,65
0,60
0,55
0,50
0,45
0,40
0,35
0,30
min
1,8
0,25
0
0,20
NFmin (dB)
4,5
NFmin (dB)
1,4
Vgs (V)
1,4
min
18,00
16,00
13,00
11,00
8,84
6,54
4,56
2,69
1,34
0,54
0,19
1
Figure 4. Gm and Nfmin versus Vgs of E-HEMT 6x15µm
@ 44 GHz
Ids (mA)
LOW NOISE AMPLIFIERS DESIGN
Figure 2. NFmin versus Ids of D-HEMT and E-HEMT
6x15µm @ 44 GHz
At low drain currents in order to provide low noise
figures it is required high values of transconductance,
since the increase in Nfmin at low drain currents is
mainly determined by the Gm-Vgs relation. At very
low drain-source currents gm and cut-off frequency
both approach zero, yielding a sharp increase in Nfmin
near the threshold, Soares (2). But this point of
minimum noise figure does not correspond with the
point of maximum transconductance but a slightly
smaller, which means that minimum noise figure is
achieved with a trade off between low drain-source
current and gm-vgs relation.
Transconductance (gm) and the minimum noise figure
(Nfmin) versus Vgs for the transistors under study are
depicted in Figure 3 and Figure 4. For a similar NFmin
the transconductance is a bit higher for the
enhancement mode transistor; the same gain will be
achievable with lower drain-current. Intermodulation
performance for the E-prototype is expected to be
worse than for the D-prototype, however the low noise
The two amplifiers have been designed using the same
design method; one of them with depletion mode
transistors and the other with enhancement mode
transistors. A schematic of the Q-band four-stage
MMIC low noise amplifiers is shown in Figure 5. The
two first stages use inductive source feedback to
achieve a low noise performance, Engberg (3), with
reasonable gain and return loss.
Figure 5. Schematic diagram of the Q-band four-stage low
noise MMIC amplifier
Parallel feedback, Niclas (4) Niclas et al (5), has been
used in the last two stages to obtain flat gain over the
operating bandwidth. In the first stage the load
20
18
30
16
20
14
10
12
0
10
50,0
49,0
48,0
47,0
46,0
45,0
44,0
43,0
42,0
41,0
40,0
0
39,0
2
-50
38,0
4
-40
37,0
6
-30
36,0
8
-20
35,0
-10
NF (dB)
Noise Figure
40
34,0
dB(S21)
Gain
50
33,0
impedance to achieve low noise figure and minimum
reflection coefficient is the same by using source
inductor, but this improvement is at the cost of a loss of
gain. So at the second stage a source inductor has been
included to obtain a bit higher gain with low noise.
Source inductors with different values have been
employed for each stage in order to get a low noise
figure with low input return loss and a reasonable gain,
since in this way the last stages have a little impact on
the total noise figure as a multistage amplifier. Parallel
feedback has the advantage of increasing the stability
factor, improving input and output return losses and
flattering gains over widebands.
Frequency (GHz)
Figure 8. Gain and Noise Figure of the D-HEMT MMIC LNA
Figure 6 and Figure 7 show a photograph of the low
noise amplifiers. The chip size is 3 mm2 each one.
Gain
Noise Figure
15
50
40
12
30
9
10
0
6
-10
NF (dB)
dB(S21)
20
-20
-30
3
-40
50,0
49,0
48,0
47,0
46,0
45,0
44,0
43,0
42,0
41,0
40,0
39,0
38,0
37,0
36,0
35,0
34,0
0
33,0
-50
Frequency (GHz)
Figure 6. Photograph of the MMIC LNA with D-HEMT
Figure 9. Gain and Noise Figure of the E-HEMT MMIC LNA
Figure 7. Photograph of the MMIC LNA with E-HEMT
Figure 10 and Figure 11 show that measured and
simulated noise figure of the low noise MMIC
amplifiers agree quite well, which means that the noise
model used is reasonably good. The solid line is the
simulation and the dot line corresponds to the
measurement.
Simulated
LOW NOISE AMPLIFIERS PERFORMANCE
Measured
10
9
8
7
NF (dB)
The chips have been measured on wafer and the noise
figure and associated gain are depicted in Figure 8 and
Figure 9 .
6
5
4
3
2
1
50,0
49,0
48,0
47,0
46,0
45,0
44,0
43,0
42,0
41,0
40,0
39,0
38,0
37,0
36,0
0
35,0
The minimum noise figure is 2.8 dB with an associated
gain of 23.1 dB at 40.4 GHz for the E-HEMT MMIC
LNA. In the case of the D-HEMT MMIC LNA the
minimum noise figure is 3 dB with associated gain of
23.2 dB at 42 GHz. The total DC consumption for the
D-protoype is 30 mA and 15.4 mA for the E-prototype,
which agrees with the expected results.
Frequency (GHz)
Figure 10. Comparison measurement and simulation of the
Noise Figure D-HEMT MMIC LNA
Simulated
have been measured with a power saving close to the
fifty per cent for the enhancement prototype.
Measured
10
9
8
ACKNOWLEDGEMENT
NF (dB)
7
6
5
This work has been co-financed by the Spanish
“Ministerio de Ciencia y Tecnología” by the grant
reference ESP2001-4543-PE.
4
3
2
1
50,0
49,0
48,0
47,0
46,0
45,0
44,0
43,0
42,0
41,0
40,0
39,0
38,0
37,0
36,0
35,0
0
Frequency (GHz)
REFERENCES
(1)
ESA Astrophysics Home page of Science
team of Planck: http://astro.estec.esa.nl/Planck/
(2)
R. Soares, GaAs MESFET Circuit Design,
Artech House 1988
(3)
J. Engberg, Simultaneous input power match
and noise optimization using feedback, in Proc. 4th
European Microwave Conf. 1974, pp.385-389.
(4)
K.B. Niclas, GaAs MESFET feedback
amplifiers: design considerations and characteristics,
Microwave Journal, pp. 39-48 & 85, Mar. 1980.
(5)
K.B. Niclas, W. T. Wilser, R. B. Gold & W.
R. Hitchens, The matched feedback amplifier:
Ultrawide-band microwave amplification with GaAs
MESFETs, IEEE Trans. Microwave Theory Tech, vol.
MTT-28, pp.285-294, Apr. 1980.
Figure 11. Comparison measurement and simulation of the
Noise Figure E-HEMT MMIC LNA
CONCLUSIONS
A study about depletion and enhancement mode
HEMT has been performed, whose results has been
used in the design of two Q-band low noise MMIC
amplifiers. Both of them have shown similar minimum
noise figures and gains, but the E-HEMT with lower
drain-source current. A small variation of the gatesource voltage is more critical for the noise figure of
the enhancement mode transistors. The low noise
amplifier with D-HEMT has a noise figure of 3 dB and
a gain of 23.2 dB at 42 GHz. For the LNA with EHEMT a noise figure of 2.8 dB with a gain of 23.2 dB
at 40.4 GHz has been measured. Similar noise figures