World Applied Programming, Vol (4), Issue (1), January 2014. 8-17
ISSN: 2222-2510
©2013 WAP journal. www.tijournals.com
Increase the Flatness of the Gain over the Bandwidth
in a Low Noise Amplifier using Inductive Gate
and Active Inductor
Meisam Tahmasbi *
Alireza Kashaniniya
Masoud Moasefi
Master of science student of
electrical and electronic engineering
of Islamic Azad university of
central Tehran branch, Iran.
Tahmasbi_meisam@yahoo.com
Assistant professor of electrical and electronic
engineering of Islamic Azad university of
central Tehran branch, Iran.
Master of science student of
electrical and power engineering of
Islamic Azad university of
Shabestar branch, Iran .
Md_moasefi@yahoo.com
Ali.kashaniniya@iauctb.ac.ir
Abstract: This paper presents: a 0.18 m coms low noise amplifier. The main goals: reducing the amount of
noise by the noise cancelling method, and more smooth gain, is improved by using inductive gate. In addition
providing, the gain of the circuit is improved by modifying the value of the gate inductance. The simulated gain
and noise figure of low noise amplifier are 20db and 2.2db , respectively.
Keywords: Gain flatness, low noise amplifier, noise canceling, active inductor
I.
INTRODUCTION
Television frequencies are allocated to a range of less than 900Mhz. In the other hand Digital television technology for
better video and audio quality will soon replace analog technology. Growing trend in digital technology will cause the
improvement in the receiver bandwidth. Low-noise amplifiers, are one of the most important parts of broadband
receivers. Features of low noise amplifier is closely related to receiver sensitivity and dynamic range [1]. In wireless
communication systems, we generally place a low-noise amplifier on the front end of the receiver to improve the gain
and reduce the noise figure. Now people’s requirements of the wireless communication tools have become more
sophisticated. Small power of radiation and the role of distance have become the common pursuit of wireless
communication equipment operators and manufacturers, so the requirement of receiver sensitivity is very high [2].
One of the common methods in the development of broadband amplifier, is achieved by using parallel feedback and
common gate circuit [1,3]. Among the efforts that have been made in the development of broadband amplifiers,
increasing the flatness of the gain has been caused less attention yet. Using noise canceling technique reduces noise in
amplifier performance in the TV band. But can’t improve gain flatness within a broadband amplifier. In this paper,
inductive gate method is presented to improve the gain flatness is presented. The purpose of using this method is to
separate the poles of the amplifier and transmission complement poles to higher frequencies to achieve more flatting
gain.
II.
NOISE CANCELLING TECHNIQUE
The main goal is reducing the noise figure of the amplifier in low range GHz frequencies. Noise Cancelling is an
acceptable method to design low-noise and broadband amplifiers. Thermal noise, which is one of the dominant noise in
micron cmos components, can be modeled as the source of the noise in a parallel feedback amplifier. Usually the (FRII)
formula is used to calculate the noise figure for multi- stage amplifier [3].This formula, usually calculates all the noise
factor. The (FRII) is:
Ftotal  F1 
F2  1 F3  1
F 1
Fn  1

 4
 ...
G1
G1G 2 G1G 2G 3
G1G 2 ...G n
(1)
where Fn, Gn and n, are noise figure, power gain and position of each amplifier respectively.
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Meisam Tahmasbi, et al. World Applied Programming, Vol (4), No (1), January 2014.
I
Figure (1) illustrates a parallel feedback amplifier. noise current source is n [4]. A fraction of the noise current source
 I n flows through the feedback resistor R f to the gate of amplifier, where 0<  <1 [2]. Voltage noise from the noise in
the x and y points are induced, are so dependent on each other as [5]:
V x ,n   I n R s
(2)
V y , n   I n (R s  R f )
(3)
Figure 1. noise canceling technique
In from the other hand, the voltages; at node x, y are in different polarity. as a common source amplifier has negative
boost[6].so , the following equation is obtained:
V out , n   I n ( R s  R f )  Av ( I n R s )
(4)
with the elimination of output noise, The Av value is:
Av  1 
Rf
Rs
(5)
Since the signal at node x and Y are of opposite polarity, And assuming the impedance matching between the amplifier
input and the source impedance, voltage gain will be achieved as:
Gv  2
Rf
RS
(6)
III.
INCREASING THE GAIN IN FLATNESS
The broadband LNA is suitable for TV bands. Using noise canceling method, just cause reducing the noise in the
amplifiers performance. But doesn’t improve the gain flatness of the amplifier.
Figure(2); shows noise canceling technique architecture , which is actually a combination of a matching amplifier with
noise canceling technique [5].parallel feedback makes gain flat and; Increase the bandwidth of the amplifier as well. But,
unfortunately, there is no way to achieve noise cancelation, matching and gain flatness simultaneously.
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Meisam Tahmasbi, et al. World Applied Programming, Vol (4), No (1), January 2014.
Figure 2. A scheme of broadband amplifier architecture with the flat gain
3.1 Inductive Gate
Inductive gate method is proposed to increase the gain of a cascode amplifier and to improve flatness of the gain too.
Lg inductor is connected to the gate of a common-gate amplifier, as shown in Figure (3)[5]. The dominant poles are
separate and complementary poles are shifted to higher frequencies. The movement of complementary poles, causes the
gain flatness increasing. Small signal equivalent circuit of cascode amplifier in and Norton equivalent circuit are shown
in Fig. (4) (a) and (b) respectively [5,6].
Figure 3. Cascode amplifier with inductive gate
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Meisam Tahmasbi, et al. World Applied Programming, Vol (4), No (1), January 2014.
Figure 4. (a) small signal equivalent circuit of cascode amplifier (b) Norton equivalent circuit.
The short circuit current source and the equivalent resistance of the following circuit; formulas are obtained as:
g m 1V in ro 1 (1  g m 2 ro 2 )
i sc  
ro 1  ro 2  ro 1ro 2 (sC gs 2  g m 2 )
R eq 
(7)
ro 1  ro 2  ro 1ro 2 (sC gs 2  g m 2 )
sC gs 2 ro 1  1
(8)
The voltage gain of the cascode amplifier , regardless of the inductor, is obtained in (9). Its transfer function has a pole
approximated as,
g
s   m2
C gs 2
.
Av 
i sc  ( R eq
RD
RL )
V in

g m 1ro 1 (1  g m 2 ro 2 ) R L
2 ro 1  ro 2  ro 1ro 2 (sC gs 2  g m 2 )
(9)
After inserting the inductor at the gate, the short circuit current source, the equivalent resistance and transfer function
will be as followed:
ro 1 (s 2 
   g m 1V in
i sc
( ro 1  ro 2 )(s 2  s
(ro 1  ro 2 ) s 2  s
 
Req
g R
Av   m 1 L 
2
s2 s
g m 2 ro 2
)
L g C gs 2
ro1 ro 2 g m 2 ro1 ro 2

)
Lg
L g C gs 2
(10)
ro1 ro 2 g m 2ro1 ro 2

Lg
L g C gs 2
(11)
ro 1
1

L g L g C gs 2
g r
ro1 (s 2  m 2 o 2 )
L g C gs 2
r
r
g r
r
(ro1  ro 2 )(s 2  s o1 o 2  m 2 o1 o 2 )
Lg
L g C gs 2

11
g m 1R l
ro1


2
ro 1  ro 2
s 2   2n
s2 
0
 0 2
Q0
(12)
Meisam Tahmasbi, et al. World Applied Programming, Vol (4), No (1), January 2014.
n 
0 
Q0 
g m 2 ro 2
L g C gs 2
(13)
g m 2  ro1 ro 2
LgC gs 2
(14)
Lg g m 2
C gs 2  ro1 ro 2
(15)
IV.
ACTIVE INDUCTOR
An on-board (chip) spiral inductor shows large size and low Quality Factor. Although there are many ways to increase
Q in cmos technology, however, these methods have Q less than 10 [7] [8]. Other methods such as the use of active
elements are introduced to produce magnetic induction with much higher Q while having small size. As shown in
fig.(5).a, an inductor, based on gyrator structure can be employed which its equivalent is shown in Fig.(5).b[5].Gm and R
are the transconductance and output resistance of the amplifiers, respectively. Co1 and,Co2 capacitors are associated
capacitances ; at nodes.
Figure 5. (a) Inductor (b) equivalent circuit
Circuit elements can be calculated as follows :
R eq 
1
G m 1G m 2 R o1
G p  G m1
C p  Co 2
(16)
(17)
(18)
An active inductor using gain boosting technique and negative feedback is designed to increase the coefficient of
magnetic induction and quality factor. The active inductor with resistive feedback is shown in Fig.6 [4,5].
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Meisam Tahmasbi, et al. World Applied Programming, Vol (4), No (1), January 2014.
Figure 6. Active inductor with a resistive feedback
Lact and Ract of the active inductor is calculated as follows:
Lact 
R act 
2
g m 1g m 3C gs 1   2C gs
1C gs 3 ( R Lf g ds 3  1)
(19)
2
g m2 1 g m 2 g m 3   2 g m 2 g m 3C gs
1
2
g m 1g ds 2 g ds 3   2 g m 3C gs
1  g m 1C gs 1C gs 3 ( R Lf g ds 3  1)
2
g m2 1g m 2 g m 3   2 g m 2 g m 3C gs
1
(20)
V.
CIRCUIT DESIGN
The proposed scheme of broadband low noise amplifier with flat gain is shown in figure (7)[4,5] .The main amplifier
stage contains M 1n and M 1p .and parallel feedback resistor, causes a flattening of the amplifier gain[7,9]. The noise
canceling amplifier stage , contains the cascode transistors , M 2a and M 2b , and an active inductor is connected to the
gate M 2b . The inductor connected to the gate of the transistor, causes increasing the stability of the amplifier, since the
real part of the impedance Z in at frequencies above the resonant frequency,( r  1
L g C gs 2
) is negative.
Impedance Z in is:
Z in 
1
gm2
(1  s 2 L g C gs 2 )
(21)
As well, the stability can be investigated using detailed design of the active inductor.
VI.
IMPLEMENTATION RESULTS
The main circuit of low noise amplifier is shown in Figure (7).
Figure 7. Main circuit of LNA
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Meisam Tahmasbi, et al. World Applied Programming, Vol (4), No (1), January 2014.
Values for the low-noise amplifier elements is presented in table (1) [3,5]. Employing these values in the primary circuit
of low noise amplifier (Fig 7),the results for gain and noise are achieve as shown in ,figure (8) and (9) .
Table 1. Component value of LNA
Component
Value
Rf
665
M 1n
W
M 1p
W
M 2a
W
M 2b
W
W
M3
L
L
L
L
L
 150 m
0.18 m
 150 m
0.18 m
 480  m
0.18 m
 350  m
 80 m
0.18 m
C1
112fF
Lg
114nH
VII.
0.18  m
OPTIMIZATION
In order to optimize the performance of the amplifier (mainly , increasing the gain value and flatness , as well as
reducing the noise level), we persuade changing some elements of the circuit by tuning value of the elements. The
reduction of active inductor value showed the improvement of the gain flatness. The result of the optimization of the
amount of the noise and gain of amplifier , are shown in figure (10) and (11). It can be seen that within
200 MHz  f  1400 MHz frequency, the gain is equal 18db  S 21  20db and noise is 2.2 db.
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Meisam Tahmasbi, et al. World Applied Programming, Vol (4), No (1), January 2014.
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dB(S(2,1))
10
8
6
4
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.2
1.4
freq, GHz
Figure 8. Gain of LNA
4.10
4.05
NFmin
4.00
3.95
3.90
3.85
3.80
0.2
0.4
0.6
0.8
freq, GHz
Figure 9. Noise of LNA
15
1.0
Meisam Tahmasbi, et al. World Applied Programming, Vol (4), No (1), January 2014.
20.0
dB(S(2,1))
19.5
19.0
18.5
18.0
17.5
0.2
0.4
0.6
0.8
1.0
1.2
1.4
freq, GHz
Figure 10. Simulated Gain of LNA
3.0
NFmin
2.5
2.0
1.5
1.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
freq, GHz
Figure 11. Simulated Noise of LNA
VIII. CONCLUSION
In this paper a broadband low noise amplifier, has been designed by using inductive gate , in 0.18 m cmos technology.
This amplifier works with 1.8v supply voltage.
The amplifier gain, in frequency range of 200 MHz  f  1400 MHz is approximately equal to 19db , and the gain
flatness is acceptable ,while the amplifier noise is equal 2.2db .
Active gate inductor caused the increase in gain flatness and parallel feedback configuration was used to reduce
amplifier noise level.
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Meisam Tahmasbi, et al. World Applied Programming, Vol (4), No (1), January 2014.
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