Digital Signal Processing Reference
In-Depth Information
nVdd
Vref
n
outp
n
outn
n
inn
n
inp
Offset compensation
I
bias
circuits are located
AC-coupling not suited
outside the signal path.
for near-DC operation.
Figure 7.1.
For open-loop amplifiers operating at near-dc frequencies, capac-
itive interstage coupling requires very large (off-chip) coupling
capacitors. Differential compensation brings the offset problem
outside the signal path, with only minimal consequences for the
high-speed performance of the amplifier.
problem caused by one special case of differential 'noise', which is the dc-
offset between the outputs of the differential amplifier. Unfortunately, in a dc-
coupled chain of amplifiers, the subsequent stage will handle this dc-offset as
any other valid input signal. The dc-offset will be amplified, passed to the next
stage, amplified again, and so on.
Chances are pretty high that at a certain point further on in the chain, amplifiers
will start to clip due to unrestrained offset voltage levels. But even if clipping
does not occur for some reason, large offsets exist between the operating point
voltages in the two branches of the differential amplifiers. The difference in
biasing conditions will also induce an increase of the common-mode to differ-
ential conversion gain (more information in Section A.2). In the latter case, the
immunity of the amplifier against power supply noise or in-coupling common-
mode noise is thus also affected. At this point, it becomes clear that it is essen-
tial to introduce common-mode and offset regulating circuits into a dc-coupled
multistage amplifier. While the speed of common-mode compensation should
be as fast as possible in order to avoid inter-domain signal conversion, the max-
imum speed of the offset compensation circuit will be determined by the low-
est frequency component in the signal of interest. Obviously, the part of the
spectrum that falls within the operating speed of the offset compensation cir-
cuit will be suppressed. This is especially important for baseband amplifiers,
because they require a low cut-off frequency at the lower end of the spectrum.
The reader may correctly argue that the problem of the large interstage cou-
pling capacitances has now become a problem of generating a low-frequency
pole in the offset compensation circuit. There is one major difference between
the two, however. In the former case, the coupling capacitances lie directly
