A monolithic integrated wide-band amplifier is presented, which consists of two 0.3 &#956;m HEMT-stages in GaAs/GaAlAs-Technology. The main signal path consists of a coplanar waveguide, which interfaces conveniently to the HEMT-cells. A feedback circuit with R, L and C connects gate and drain. To close the feedback loop small scale microstrip lines were combined with CPW /1, 2, 4/. Matching is done with a LC circuit at the gate and a combination of transmission line and capacitor at the drain. In addition there are some other passive elements for biasing, which reduce external components to a minimum. All element models were checked with measured data and placed into a CAD-library for easy use in the design process

Berroth, Manfred

Bischof, Werner

Ehrlinger, Wolfgang

Reinert, Werner

Crossref

A Monolithic HEMT-Amplifier with Feedback in Coplanar Waveguide Technology

Online Publikationen der Universität Stuttgart

A Monolithic HEMT -Amplifier with Feedback in Coplanar Waveguide 
Technology 
*w. Bischof, *w. Ehrlinger, * * * * . M. Berreth, W. Remert 
ABSTRACT 
A monolithic integrated wide-band amplifier is presented, which consists of two 0.3 pm 
HEMT-stages in GaAs/GaAIAs-Technology. The main signal path consists of a coplanar 
waveguide, which interfaces conveniently to the HEMT-cells. A feedback circuit with R, 
Land C connects gate and drain. To close the feedback loop small scale microstrip lines 
were combined with CPW /1, 2, 4/. Matching is done with a LC circuit at the gate and a 
combination of transmission line and capacitor at the drain. In addition there are some 
other passive elements for biasing, which reduce external components to a minimum. All 
element models were checked with measured data and placed into a CAD-library for 
easy use in the design process. 
INTRODUCTION 
Monolithic microwave integrated circuits (MMIC) in coplanar waveguide technology offer 
some advantages over conventional microstrip designs. Active and passive components 
lie on top of the GaAs-substrate, eliminating the need for via holes. Coplanar lines and 
HEMT-cells are connected with very low source inductance. The process allows the 
integration of a broad range of passive lumped elements li ke NiCr-thin film resistors, and 
MIM-capacitors. Airbridges are used for spiral inductors and for interconnections 
between different sections of the ground plane on top of the GaAs substrate in order to 
have one common ground plane. 
A new type of small scale microstrip line was created. It consist of a chain of airbridges 
which rest on metal posts. The characteristic impedance was adjusted to 50 Ohms. The 
lines were very useful to realize T -junctions with coplanar lines and space-saving 
microwave connections in compact layouts. 
The two stage amplifier was designed as gain block with wide band flat gain. The topo-
logy includes feedback stages with additional reactive matching. All biasing circuits are 
included on chip. The amplifier is absolutely stable and therefore easy to handle and 
cascadable. 
PASSIVE CIRCUITS 
Most passive circuits used in the amplifier design were characterized with data of a pre-
vious wafer run. Theoretical models and measured data from DC to 40 GHz were 
compared to create library models. Straight sections of CPW-transmission lines showed 
nearly constant phase velocity in the whole frequency range /3/ . Spiral inductors with 1, 
2 and 3 turns were realized and measured accurately /6/. Figure 1 shows the model with 
its parasitic elements divided into N equal sections. The series resistance Rs is frequency 
dependent. Measured and computed impedance of a spiral with 2 turns is given in figure 
2. MIM-capacitors embedded in coplanar lines and used as coupling capacitors, exhibited 
the same electrical length like line sections without capacitor . The small scale microstrip 
line was formed as a chain with inductive (air bridges) and capacitive parts (metal posts, 
Fig. 3). The line is called "telegraph-line" due to the construction with wires and posts . 
The geometric dimensions are choosen to form a 50 Ohm characteristic impedance 
(Fig. 4). Losses are slightly higher as with a comparable CPW-Iine due to the small 
dimensions (width = 10 pm, loss = 2.2 dB/mm at 40 GHz). 
833 
HEMTS 
The active layers were deposited with MBE. The gates with a length of 0.3 pm were 
written with e-beam . The HEMTs were modelled to fit the equivalent circuit up to 40 
GHz. The elements were extracted with a direct solution of the analytical formulas of the 
Y-parameters of the intrinsic FET /5/ and cold-FET-modelfing for the parasitic parts. This 
procedure is very fast, because no iteration is neccessary. 
AMPLIFIER 
A two stage amplifier consisting of two identical stages was constructed. The design 
started with ideal values for the feedback and matching circuits (Fig. 5). 
The combination of R and L in feedback reduces the high gain at low frequencies while 
gain is not changed at high frequencies. This results in a flat response across the fre-
quency range. The input impedance becomes lower and is easier to match to the source 
with a two element reactive transformer (LC). Gate bias is fed via a high impedance re-
sistor, which is connected to a low impedance point. 
The drain output impedance is also lower with feedback and the matching network requi-
res only a low transformation ratio to 50 Ohm. This gives broadband performance. Drain 
bias runs through a spiral inductor, which is blocked to ground with a capacitor at one 
end. The two stage amplifier is made up of two identical one stage amplifiers, which are 
connected with a short section of line. 
The initial model was refined during the design procedure with the parasitic elements of 
the passive circuits /3/. The geometric shape of the components in the layout required 
further adjustment of the electrical model. 
The feedback loop added two transmission lines to close the path around the transistor. 
A blocking capacitor seperates De-voltages between gate and drain. 
The amplifier can be supplied with one single supply voltage if required. Without external 
connections the DC-terminals are kept to ground via high impedance resistors thus pro-
tecting the HEMTs against static discharge. 
RESULTS 
The circuits were processed on 2" GaAs-Wafers. The amplifier chip is shown in Fig. 6. 
All chips were measured on wafer w ith coplanar probes. The small-signal S-parameter 
analysis agreed well with the computed values (Fig. 8). The yield was around 60 %. The 
gain curve of good amplifiers kept the same shape from chip to chip with an absolut de-
viation of less than 0.5 dB. This shows a main advantage of the feedback concept: the 
circuit is less sensitive to transistor parameter variations. Due to the feedback loop the 
amplifier is absolutely stable. The two stages deliver more than 10 dB gain between 8 
and 25 GHz. 
CONCLUSION 
A MMIC amplifier was realized, which exhibits wide-band stable gain. The coplanar wa-
veguide approach on the IC integrates passive and active elements easily. The chip can 
be used in a variety of applications either as single gain block in a chip and wire as-
sembly or as element within a CAD-library for more complex circuits . 
834 
ACKNOWLEDGEMENT 
This work was supported by the Ministry of Research and Technology of the Federal Re-
publik of Germany (NT 2776 D9). 
* 
** 
ANT Nachrichtentechnik GmbH, 
Gerberstraf3e 33, D-71 50 Backnang/FRG 
Fraunhofer Institute for Applied Solid State Physics, 
Tullastraf3e 72, D-7800 Freiburg/FRG 
REFERENCES 
/1 I 
/2/ 
/3/ 
/4/ 
/5/ 
/6/ 
M. Muraguchi; T. Hirota; A. Minakawa; Y . lmai: 
Uniplanar MMICs and their Applications 
Review of the Electrical Communications laboratories 
Vol. 36, No. 6 (1988) 
I. Wolff; K. Heime: 
Entwicklung von monolithisch integrierten rauscharmen 
Breitbandverstarkern im Frequenzbereich von 1 2 GHz 
bis 40 GHz, 
Arbeits- und Ergebnisbericht, SFB 254 (1989), 
University of Duisburg 
W. Bischof, W. Ehrlinger, M. Berreth; W. Reinert: 
Modelling and Realization of a Monolithic 27 GHz HEMT 
Amplifier in Coplanar Waveguide Technology 
20th European Microwave Conference, 
Budape~ 1990, pp. 943 
R. K. Hoffmann: 
lntegrierte Mikrowellenschaltungen 
Springer Verlag ( 1983) 
M. Berreth; R. Bosch: 
Broad Band Determination of the FET Small-Signal 
Equivalent Circuit, 
IEEE-MTT, Vol. 38, July 1990, pp. 891 
G. Kempa: 
Small- and large-Signal Characterization Methods, 
Modelling and Verification 
IEEE-MTT, Workshop on Measurement Techniques for 
Microwave Device Characterization and Modelling, 
Stuttgart, April 1 990 
835 
Ls / N Ls/ N Ls/ N 
Rs/N Rs/N Rs/N 
Cp Cp Cp Cp C.p (p 
Rs = Rs(t) 
Fig. 1: spiral inductor with parasitic elements 
·. : . 25 
\····":"··· ./ f/GHz .. ·· 
·. ..·· ··.. : ... · ··.. . ... ··· 
·~::·< ... ::'\. '1 ... < ·.... ...:=:.::· 
~ .... ~· .: ...... : ........... ; ...... ; .......... . 
• • 0 • • 
. . . . 
. .. . .. . 
,.':(.... . ~: .. ~-. <. ······.-:: 
. : : ·.. .· 
·. ,: 
.··· : ;· .. : .. · 
····:····· 
F ig. 2: measured and computed impedance of spiral inductor 
Fig. 3: small scale microstripline 
836 
.. 
" • 
d 
• 
-to.-
10.00 
0 
10.-
0.00 to. 
00 
110.00 110.00 ~.oo 110 .00 110,00 7D.OO 
Fig. 4: insertion and return loss of small scale microstripline 
u 
G 
LFEEO CKF 
- -1 
RFE£0 
CPOI 
ROI 
l.ORAJN 
u 
D 
TRAN 
Fig.-5: schematic circuit of two-stage feedback a1nplifier 
with ideal elements 
Fig. 6: photograph of a1nplifier, chipsize 2.5nun * l.Onun 
837 
CPOI 
flO I 
1.01\AIN 
u 
0 
eu X 
II 
• 
• 
.. 
• 
~.o 
00 
so.-
0.-
-tO.-
... 
o.- o.oo 
X measured 
6 computed 
~.00 
to.oo 
Fig. 7: comparison between n1easured and 
computed gain 
\ 
80.00 
measured 
computed f= 0 .. .40 GHz 
Fig. 8: comparison between 1neasured and . 
computed input and output 111atching 
838 
40. 00 
0 


A monolithic HEMT-amplifier with feedback in coplanar waveguide technology

http://elib.uni-stuttgart.de/bitstream/11682/8223/1/ber23.pdf

A monolithic HEMT-amplifier with feedback in coplanar waveguide technology

Abstract

Similar works

Full text

Available Versions

Crossref

Online Publikationen der Universität Stuttgart