Environmental Engineering Reference
In-Depth Information
[G93], [B93], [SKW94], and represent an evolution in the architecture of the
voltage-mode op-amps (VOAs), which have otherwise remained much the
same over the years. Implementations of high-performance CFOAs have
become possible thanks to the availability of high quality complementary
bipolar transistors provided by advanced BJT processes. However, the low
transconductance of MOS transistors, does not make this component suitable
for implementing CFOAs. The internal architecture of a CFOA is
exemplified in Fig. 9.10. It is made up of a CCII-based input stage -that
performs a voltage following action between terminal Y to X and a current
following action between X and Z- and an output stage with a voltage
buffering function which properly drives the load and isolates it from the
internal high-resistance, at node Z. The amplifier is hence characterised
by an inverting low-impedance terminal and by a transresistance gain
equal to (this is the reason for the name “transimpedance amplifier”also
used when referring to this opamp).
The CFOA circuit symbol is not different from that of a conventional
single-ended VOA, as can be deduced from Fig. 9.11. The same figure also
explicitly shows in a dotted line, the input voltage follower and the output
current-controlled voltage source. Observe that the voltage follower will
operate outside the feedback loop (which involves the output and the
inverting input terminal, but not the noninverting input). This fact can result
in a source of harmonic distortion especially in noninverting configurations
in which the CFOA is operated under large common mode signal swings.
Nevertheless, it must be observed that although the internal structure of the
CFOA differs greatly from that of a traditional VOA, the external feedback
circuitry and its applications are similar to those of a VOA [S96], [S911]
[SKW94], [TL94], Hence, a variety of configurations with their respective
performance parameters have been directly derived from traditional ones,
without requiring any further study.
