Hardware Reference
In-Depth Information
Fig. 2.6 An entirely
op-amp version of Senani ' s
op-amp-OTA CCII,
proposed by Huertas [ 4 ]
Z
r 3
r 4
r 1
r 2
r
Y
X
and finally, (iv) in applications where several CCs are to be connected in parallel at
port-Z, this implementation allows a practical sub-summing of many op-amps as
shown in [ 4 ].
2.2.5 Pookaiyaudom and Samootrut Implementation
Using OTAs
In [ 10 ] Pookaiyaudom and Samootrut demonstrated how a commercial dual oper-
ational transconductance amplifier (OTA) IC CA3280 together with a current
source IC LM334 can be employed to realize both CCII+ and CCII
. These circuits
are shown in Figs. 2.7 and 2.8 respectively.
The OTA CA3280 has the provision that the common junction of the emitters of
the two transistors forming the differential pair is available as an external terminal
outside the chip. In the CCII+ implementation, the basic idea is to keep the
inverting input terminal of the OTA open circuited thereby forcing Q 1 to be off
so that transistor Q 2 conducts the entire current (I B-ix ). Since Q 1 is off, the first
current mirror (CM 1 ) is off and consequently, CM 3 is also off. The collector current
of Q 2 is mirrored by CM 2 and reaches the output node Z where a constant current
source LM334 is connected between terminal-Z and negative supply rail thereby
ensuring i z ¼
i x as shown on the diagram.
It may be noted that the main departures from ideal CCII properties in this circuit
are (i) the voltage drop of one V BE between terminal Y and X and (ii) the nonzero
input resistance at port-X. Since in most applications only AC signals are used
therefore, the one V BE voltage drop does not introduce serious application diffi-
culty. Also, if one takes the maximum bias current of the order of 10 mA, the input
resistance looking into port-X turns out to be the order of 2.5
ʩ
which is quite
adequate for most practical applications.
 
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