Download A Low-voltage Wide-band Current-mode Automatic Gain Control (AGC) Kriangkrai Sooksood and Montree Siripruchyanun

A Low-voltage Wide-band Current-mode
Automatic Gain Control (AGC)
Kriangkrai Sooksood1 and Montree Siripruchyanun2
1, 2
Department of Teacher Training in Electrical Engineering, Faculty of Technical Education,
King Mongkut’s Institute of Technology North Bangkok, Bangkok, 10800, THAILAND
Email: [email protected], [email protected]
ABSTRACT
In this paper, a current-mode automatic gain control
(AGC) is presented. Due to operation in current-mode,
the proposed circuit provides a wide frequency response,
a low supply voltage, low power consumption and
electronic controllability. Thus, it is very suitable for use
in portable and battery-powered equipment such as a
hearing aid instrument and a wireless radio device. The
proposed AGC consists of current controlled exponential
amplifier, current-mode precision rectifier, current-mode
low pass filter and current-mode integrator, which can be
fabricated into Integrated Circuit (IC) form. The
performances of the proposed circuit are explored
through HSPICE simulation program using BSIM3v3
model from MOSIS, they demonstrate good agreement to
the theoretical anticipation. The proposed circuit works at
±1.5 V supply voltage, power consumption is merely 712
µW.
Keywords: AGC, CMOS, Low-voltage circuit
1. INTRODUCTION
As well known, AGCs play a very important role in
modern hearing aid devices and communication systems
[1-3]. An AGC is a closed-loop system that automatically
adjusts the voltage gain such that the output voltage stays
within a desired range. Many published literatures have
proposed different methods to design the AGC circuits
[4-6]. However, as investigated, all above AGCs work in
voltage-mode. As a result, there are some restrictions in
such: high supply voltage and power consumption,
narrow frequency response, narrow dynamic range,
complicated circuit details and absence of electronic
control.
In the last decade, there has been much effort to
reduce the supply voltage of analogue CMOS systems.
This is due to the command for portable and batterypowered equipment. Since a low-voltage operating circuit
becomes necessary, the current–mode technique is ideally
suited for this proposes. Actually, a circuit using the
current-mode technique has many other advantages,
which can be found in many literatures [7-8].
The purpose of this article is to present a realization
of an AGC circuit functioning in current-mode. The
proposed circuit comprises exponential current
controllable amplifier, precision rectifier, low-pass filter
and integrator. All circuits can be realized using CMOS
technology. The simulation results through HSPICE
using 0.5 μm CMOS technology are achieved to confirm
that the realized circuit can provide wide dynamic range,
low supply voltage, and wide bandwidth including low
power consumption. In addition, the gain of AGC can be
controlled by the reference current. Consequently, the
proposed circuit is very appropriate for further
fabricating into Integrated Circuit (IC) form to employ in
portable electronic equipment such as hearing aid
instrument and wireless communication device.
2. PRINCIPLE AND CIRCUIT DESCRIPTION
2.1 The Principle of the Proposed AGC
The proposed AGC scheme can be found in Fig. 1. It
consists of the exponential current amplifier whose gain
can be adjusted by an input bias current. The currentmode precision rectifier is employed to convert the output
sinusoidal current to a positive full wave ( I A ).
Subsequently, this signal will be applied to current-mode
first-order low pass filter to change it to a DC current
level ( I F ). After that, the DC current is subtracted to a
reference current, the remaining current is employed as
input current of integrator ( I E ). The output current of
integrator is used as exponentially control current ( I B ) of
the gain of the current amplifier.
I in
I out
Current Controlled
Exponential
Amplifier
IB
Current-mode
Integrator
−
I REF
IE
∑
+I F
Current-mode
Low Pass
Filter
IA
Current-mode
Precision
Rectifier
Fig.1: The principle of proposed AGC
2.2 Current-mode Exponential Control Current
Amplifier
The current amplifier using in the AGC system must
be exponentially controlled to maintain AGC loop
settling time which is independent of signal levels of
M 6M 7
M1
VDD
M 16
M 10
M 18
IB
IX
M3
M2
I out +
I X + I in
M4
M 5M 8
M 9 M 11
I out −
M 13
I X − I in
M 14M 15
M 12
M 17
VSS
Fig.2: Current-mode exponential control current amplifier
input current [9]. Fig. 2 shows the current-mode
exponential control current amplifier where the output
current ( I out ) can be exponentially controlled using I B ,
modified from [10]. From Fig. 2, the output current; I X
generates exponential function of I B , thus the output
current equation can be shown as
full-wave signal consists of DC fundamentals, a currentmode low-pass filter is required. Fig. 4 shows the firstorder current-mode low-pass filter, which is modified
from using bipolar version [12]. When I 4 = I 5 , the
transconductance gains of M26 and M27 ( g m ) are
equaled and matched. The transfer function of the filter is
given by
IB
I out = I out + − I out − = I in e KG .
2
IF ( s)
(1)
IA (s)
Where K is a transconductance parameter,
considered as all matched PMOS and NMOS. G equals
VDD − VSS − VTP − VTN . From Eqn. (1), we can found that
2
C1
I A = I out .
(2)
VDD
M 23
M 24
M 25
IA
I1
M 22
M 19
I out
I3
C1
s +1
gm
I4 I5
IA
IB
Since the output current of the current amplifier must
be changed to full-wave signal, consequently, a currentmode precision full-wave rectifier is required. Fig. 3
illustrates the circuit, improved from [11]. The current
source I 3 , which equals to I 2 , is used to eliminate a DC
current offset of the output current I A . Hence, I A can be
found as
1
(3)
.
VDD
the controlled gain is e KG .
2.3 Current-mode Precision Rectifier
=
M 26
IF
M 27
VSS
Fig.4: Current-mode low-pass filter
2.5 Current-mode Integrator
It was stated that it is desirable to keep the DC loop
gain as large as possible in order to maintain a constant
output level [1]. One method is to employ an integrator,
since an ideal integrator has infinite gain at DC, the
steady-state output amplitude will not change in response
to slowly varying changes in the input amplitude. Fig. 5
shows the current-mode integrator, developed from the
current-mode differential integrator [13]. When the
current source I 6 , which is equal to I 7 , equals to 2 I and
I 8 is set to K i I . Consequently, the output current ( I E )
can be shown as
IE ( s) =
Ki g m
IE ( s)
C2 s
(4)
I2
M 20
M 21
VSS
Fig.3: Current-mode precision rectifier
2.4 Current-mode Low-pass Filter
The resulting output current of the precision rectifier
must be filtered to achieve a DC current level. Because a
where K i is a positive real number and all matched
transistors are considered.
From the block diagram of the proposed AGC in Fig.
1, the remaining part is a current differential circuit. It is
not required, because the current sources can be directly
connected. This is an advantage of current-mode circuit
over voltage-mode version.
VDD
I6
I7
I8
IE
M 29
M 28
M 30
C2
M 31
M 33
M 32
IB
M 34
VSS
Fig.5: Current-mode integrator
Table 1: Transistor Aspect Ratios
Transistor
M1-M18
M19, M20, M21, M23, M24
M22
M25
M26- M29
M30- M33
M34
W/L (µm/µm)
30/3
4/1
1/4
16/16
10/3
10/5
10/5 (M=5)
Fig.7: Output current when input current is 25 µAp-p
and decreased to 15 µAp-p with delay 200 ms.
3. SIMULATION RESULTS AND DISCUSSIONS
24
22
Iout/μΑ (dB)
The proposed AGC system was simulated by
HSPICE simulation program through BSIM3v3 model of
0.5 µm CMOS technology obtained from MOSIS. It was
operated by ±1.5 V supply voltages and the bias currents
are following: I1 = 3 µA, I 2 = I 3 = 5 µA, I 4 = I 5 = 20 µA,
I 6 = I 7 = 40 µA and I8 = 100 µA. The aspect ratios of all
transistors are listed in Table 1, where capacitor C1 and
C2 are 1 μF and 100 nF, respectively. The feedback
current I B is also amplified with 5 times gain and K i = 5
before feeding to the current amplifier. The responses of
AGC, for 500 Hz sinusoidal input current with 15 μAp-p
and changed up to 25 μAp-p with delay 200 ms and 25
μAp-p and changed down to 15 μAp-p with delay 200
ms, are shown in Fig. 6 and 7, respectively, with I REF = 5
μA. It is readily seen that the AGC system provides a
constant output current when input current is changed,
however some settling times are required.
20
18
16
0
20
40
60
80
100
120
140
160
Iin (μAp-p)
Fig.8: The current transfer characteristic
60
50
Iout (μAp-p)
40
30
20
10
0
5
10
15
20
IREF (μA)
Fig.9: Output current against current reference
variations
Fig.6: Output current when input current is 15 µAp-p
and increased to 25 µAp-p with delay 200 ms.
The current transfer characteristic of the AGC system
is illustrated in Fig. 8, which was achieved with 500 Hz
sinusoidal input current and I REF = 5 μA. It is clearly seen
that AGC provides the output current almost constant for
input current varied between 12 to 150 μAp-p. This
means that the AGC input dynamic range when no more
than 1 dB normalized output current acceptable is 22 dB.
In addition, the behaviour of the AGC system when
I REF changed was also investigated in Fig. 9. It illustrates
output current when I REF was varied from 5 to 20 μA
with I in = 60 μAp-p of 500 Hz sinusoidal signal. The
result shows that AGC output current is proportional to
the current reference ( I REF ).
Fig. 10 shows the frequency response of the AGC
system for 20 μAp-p input current and I REF = 5 μA. It is
obviously seen that the proposed AGC can operate over a
wide range of frequencies, the -3 dB bandwidth of the
proposed AGC system is about 110 MHz.
[2]
[3]
[4]
[5]
[6]
[7]
[8]
Fig.10: Frequency response of the proposed AGC
[9]
4. CONCLUSION
A wide-band current-mode AGC system has been
proposed in this paper. The proposed AGC system
composes of exponential control current amplifier,
precision rectifier, low-pass filter and integrator. The
proposed AGC circuit provides a wide frequency
response, a low supply voltage and low power
consumption because all circuits operate in current-mode.
In addition, the closed-loop gain can be controlled by the
reference current. The simulation results confirm the
performances of the proposed AGC. They show the
dynamic range of 22 dB, controllability of gain, -3 dB
bandwidth of approximately 110 MHz, and power
consumption of 712 µW. With the above features, the
proposed AGC is very suitable for further fabricating into
IC form, where the large-value capacitor ( C1 ) can be an
external element, for use in portable electronic
equipments. Our future work is to develop the circuit to
obtain lower settling time and lower power consumption.
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