Basic Analog Circuit Building Blocks Using CCs and Application of CCs in Impedance Synthesis - Current Conveyors: Variants, Applications and Hardware Implementations - page 84

In this context, it is worth pointing out that a translinear CCII, based upon the
input stage composed of a four-transistor mixed translinear cell is characterized by
i z ¼ 2I B Sinh [(V y -V x )/V T ] and therefore , exhibits a very large (theoretically

unlimited) current for large differential inputs. This leads to theoretically unlimited
charging current in the z-port parasitic capacitance thereby resulting in a very high
value of the slew rate. By contrast, the CCII architectures which are based upon
differential-pair-based input stages, would have an i-v equation of type i z ¼

i x ¼

i x ¼ I B

tanh [(V y -V x )/2V T ] thereby leading to the maximum charging current as I B only

(due to the saturation of the differential pair) for large differential inputs (V y V x )
and hence, such CCII structures are likely to have comparatively much lower slew
rates.
Interestingly, the property of gain bandwidth decoupling is available even if the
inverting and non-inverting amplifiers are realized from CCII using the configura-
tions of Fig. 5.3 . The expressions for the non-ideal gains for these circuits have been
found to be:

1
C p R 2

V o
V in ¼

R 2
R 1

ð 5 : 8 Þ

1
C p

1 1

R p

s þ

R 2 þ

from where the maximum gain and the 3-dB frequency are found to be:

;

R 2
R 1 =

R 2
R p

1
C p

1
R 2 þ

1
R p

þ ð : Þ

Maximum gain ¼ 1 3 dB bandwidth ¼ 5 9

From the above, it can be seen that the bandwidth can be held constant by fixing R 2
while the gain can be continuously adjusted by varying the other resistor R 1 .

5.2.2 Constant-Bandwidth Instrumentation Amplifiers

Wilson [ 67 ] demonstrated that an excellent instrumentation amplifier can be made
using only two CCII+ and no more than two resistors which is a very significant
improvement over the conventional VOA-based topology which typically employs
as many as three VOAs along with as many as seven resistors along with the
requirement of precise matching of at least four of them.
A simplified form of the circuit proposed by Wilson is shown in Fig. 5.4 which
has the voltage gain of V 0 =

R 1 . By taking the Z-port parasitic
impedances R p /(1/sC p ) into account, it can be easily verified that this circuit
exhibits a constant bandwidth set by R 2 and a variable gain adjustable with R 1 .
Figure 5.5 shows an improved instrumentation amplifier proposed by Khan
et al. [ 142 ]. This modified circuit features high CMRR without requiring matched

R 2

ð V 1 V 2 Þ ¼