IEEE Solid-State Circuits Magazine - Winter 2016 - 7

Vo1, the dc at Vo1 (which is equal to
AVOS) will be completely cancelled by
the dc voltage across the capacitor,
and hence Vout = AVin .
What is interesting about this
technique is that it allows us to cancel the offset voltage without really
knowing its value, and hence the
term auto-zeroing! This technique
does remove the offset voltage from
the output node (Vout) but not from
the intermediate node (Vo1) . In the
example of the amplifier with a gain
of 200 and an offset voltage of 3 mV,
the capacitor will be charged to 0.6 V.
But the output of the amplifier may
not be able to go above 1 V, especially when its power supply limits
it to 1 V. On the other hand, if the
amplifier is allowed to use a higher
supply voltage, then the output node
of the amplifier may not saturate,
maintaining the linear relationship
between the input and output.
Figure 3 shows an alternative autozero technique in which a capacitor (C I ) is placed in series with the
negative input terminal of the amplifier. During Phase I of the operation,
the left terminal of the capacitor is
grounded while the output terminal
of the op-amp is shorted to its negative input terminal as shown in Figure 3(a). One can easily verify that in
this case, the capacitor stores a voltage that is very close to VOS . Under
normal operation (Phase II), we break

PreSIdenT'S COrn er

the feedback and apply the input as
shown in Figure 3(b). During this
phase, a 0-V input produces an output offset voltage that is equal to the
input offset voltage (VOS) . Equivalently, the input offset voltage of the
augmented amplifier is VOS /A! Unlike
the first approach, this approach cancels (or almost cancels) the offset at
the input when it is small.
The auto-zero technique is not without pitfalls. The capacitors do block
the offset voltage, but they also block
any dc component of the input signal.
This may be okay in some applications
such as voice where there is no content
at dc, but it may not be acceptable
when the input signal has a meaningful
dc component. In addition to blocking
the dc component of the input signal,
this technique also attenuates the lowfrequency content of the input signal.
This is because, in both cases, we have
created a high-pass filter (either at the
output or at the input), and this implies attenuating low-frequency components. It is possible to lower the corner
frequencies of the high-pass filters by
using larger capacitor sizes, but again
we should be careful about the cost in
terms of silicon area.
One offset cancellation technique
that does not block the dc and the lowfrequency component of the input signal is known as the chopper stabilization technique. This technique will be
covered in a future article.

+
CI

- V +
os

-+ +
A
-

1 + (1/A))
(a) Auto-Zero Phase
+
Vin
-

CI
-

Vos

-+ +
A
-

Vout

1 + (1/A))
(b) Operation Phase

Figure 3: The auto-zero technique to
cancel offset with the aid of a capacitor at
the input of the amplifier. In (a) the auto-zero
phase, the capacitor is charged close to VOS,
and in (b) the operation phase, the voltage
across the capacitor cancels the effect of
input offset voltage.

In summary, the auto-zero technique measures and stores either
the output offset voltage or the
input offset voltage, respectively,
on a capacitor in series with the
output or the input terminal of the
amplifier. Once stored, the capacitor
is left in place for normal operation.

References

[1] A. S. Sedra and K. C. Smith, Microelectronic Circuits, 7th ed. London, U.K.: Oxford
Univ. Press, 2014.
[2] B. Razavi, Fundamentals of Microelectronics. New York: Wiley, 2008.

(Continued from p. 4)

tours. It is and has been my privilege
to work with all these committed
individuals.
The Executive office has also some
new faces. Abira Sengupta joined us
in July of 2015 and will be working
with Lauren Caruso under the leadership of Executive Director Mike Kelly.
Katherine Olstein retired in March of

2015 after nine years of dedicated
service to the Society. I would like to
thank her for the excellent work she
has done for the Society, through
her involvement with the Chapters,
awards, and the magazine.
I'd like to thank you for being a
member of the Society. I look forward to working with you, as a

member or as a volunteer, in ensuring that the Society will be as or
even more relevant to its members
in 2020. Please feel free to contact
me with your ideas and comments,
Jan Van der Spiegel, president-sscs@
ieee.org. Together we can better serve
the SSCS.

IEEE SOLID-STATE CIRCUITS MAGAZINE

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Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Winter 2016