Environmental Engineering Reference
In-Depth Information
7.3.3 Single-stage Amplifiers
The last case we shall study is that of the single-stage amplifiers. These
architectures are frequently employed in IC applications (for instance in
switched-capacitor circuits) for their high-frequency performance. Indeed, a
single-stage amplifier exhibits only an (output) high-resistance node.
Moreover, this output node often exploits cascoding, allowing a voltage gain
similar to that of two-stage amplifiers to be achieved. Of course, these
amplifiers are used in closed-loop configurations and, due to the internal
structure, output dominant-pole compensation is invariably utilised.
The small-signal model of a (open-loop) single-stage amplifier is
illustrated in Fig. 7.13, in which
C
is the output compensation capacitor.
In general, there are two sources of harmonic distortion in such
amplifiers. The first is due to the nonlinear V-I conversion accomplished by
the input transconductance stage. The second is due to the nonlinear I-V
characteristic exhibited by the output devices.
Let us first analyse the effect on linearity of the nonlinear output
resistance. Observe that this case does not fall into the category of any of
those already studied because both pole and distortion are generated at the
same circuit node (i.e., the output) by the same nonlinear element. Hence a
specific analysis must be performed.
For easy calculation express the input signal as Moreover, it
is better to characterise the nonlinear resistance in terms of (nonlinear)
conductance
where and are nonlinear coefficients normalised to the linear part of
the output conductance 1/
R.
These cause harmonic distortion components to
appear in the output voltage, according to
