Hardware Reference
In-Depth Information
V
1
if V
1
>
V
2
V
0
¼
ð
8
:
28
Þ
V
2
if V
2
V
1
whereas the transfer characteristic of the circuit of Fig.
8.18b
can be expresses as:
V
2
if V
1
V
2
V
0
¼
ð
8
:
29
Þ
V
1
if V
2
>
V
1
The workability of these prepositions has been demonstrated by implementing CCII
+ using a CMOS implementation and choosing the aspect ratio of the MOSFETs as
W/L
¼
2
μ
m/2
μ
m for the MOSFETs, M
1
and M
2
; W/L
¼
3
μ
m/2
μ
m for the diode-
connected MOSFETs D
1,
D
2
,D
3
; W/L
¼
2
μ
m/8
μ
m for the MOSFET M
3
. The
CMOS CCII was biased from
5 V DC power supply and the reference current
source was taken to be I
R
¼
A. SPICE simulation confirmed the high speed
and accurate performance of these circuits.
34.4
μ
8.6 Realization of Analog Switches
It is well known that in many analog applications such as filters, D/A converters etc.
analog switches are needed. MOS technology has been particularly useful in
realizing switching methods.
Premont et al. in [
11
] demonstrated a new approach of designing of analog
switches using two current conveyor cell as switching element for continuous time
application which was claimed to offer high linearity, low noise and high frequency
characteristics. In this context, it may be noted that a CCII can be easily used to
realize either a voltage follower (between ports Y and X; with terminal-Z grounded)
or a current follower (between ports X and Z; with terminal-Y grounded). The
switching element proposed by Premont et al. [
11
] is based upon two current
conveyor cell as shown in Fig.
8.19
.
This switching element contains two ways signal path. By tuning the biasing of
two current conveyors, the circuit simulates two ways analog switch. This behavior
is depicted in Fig.
8.20
. The input resistance R
y
is more than 10 M
ʩ
for OFF state of
the switch. On the other hand, R
x
equals around 350
(for I
bias
of 20 mA) below
5 MHz for the ON state of the switch. For the OFF state, R
x
is considered to be
greater than 10 M
ʩ
below 5 MHz. This circuit when implemented with a CMOS
current conveyor, exhibits characteristics which is comparable to the traditional
commercial circuit of the analog switch such as CD4066.
Figure
8.21
shows an analog switch employing a CCII
ʩ
[
15
]. Using CCII
characterization, it is clear that V
A
¼
V
B
also i
B
¼
i
RL
¼
(V
B
-V
C
)/R
L
¼
(V
A
V
C
)/
R
L
¼
i
A
. These equations mean that the proposed configuration can operate as an
analog switch in both cases when V
A
>
V
C
and V
A
<
V
C
by supplying DC bias
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