Environmental Engineering Reference
In-Depth Information
From the AC schematic diagram we derive the small-signal circuit of the
cascode current mirror shown in Fig. 9.9.
The output resistance of the cascode current mirror is that of the common
Y transistor T4, which is almost equal to
By superficial analysis of the circuit in Fig. 9.9 we could erroneously
assume to be about equal to the parallel between and (where the
series contribution to given by has been neglected).
However, if resistance is finite, there is a feedback loop that reduces this
value. Indeed, by choosing transconductance
as a critical parameter, we
find that term
is much lower than one and can be neglected.
Otherwise,
is almost equal to one. Hence, resistance
is equal to
its value assuming transconductance
to be equal to zero divided by two
9.4 THE CURRENT FEEDBACK OPERATIONAL AMPLIFIER
AND ITS HIGH-LEVEL CHARACTERISTICS
Recently, we have witnessed the affirmation of a novel op-amp
architecture now available from several of the specialist analogue
semiconductor manufacturers. These op-amps are generally referred to as
Current-Feedback
Operational Amplifiers
(CFOAs)
[S911],
[TLH90],
