IEEE Solid-State Circuits Magazine - Spring 2016 - 7

c ircu it intu itions

Ali Sheikholeslami

Chopper Amplifier

W

Welcome to the ninth article in this
column series. As the title suggests,
each article provides insights and
intuitions into circuit design and
analysis. These articles are aimed
at undergraduate students but may
serve the interests of other readers as
well. If you read this article, I would
appreciate your comments and feedback, as well as your requests and
suggestions for future articles in this
series. Please e-mail your comments
to me at ali@ece.utoronto.ca.
The previous article in this series
presented two techniques for offset
cancellation. These techniques rely
on storing the dc offset voltage of
the amplifier on a capacitor that is
placed in series with the amplifier
so as to cancel the offset. However,
in doing so, the capacitors will end
up in the signal path, blocking the
dc content of the input signal and
attenuating its content at low frequencies. Obviously, this would be
OK if the signal is at higher frequencies, but there are many applications
in which the signal being amplified
is indeed at low frequencies (in the
order of a few hertz). For example,
temperature sensors and light sensors may produce signals that are in
sub-1-Hz frequencies. Yet, we need
to amplify these signals with amplifiers that inherently include offset.
How can we do this? In this article,
we explore a powerful technique
known as chopper stabilization.
The principle of operation of a
chopper amplifier is described using

the block diagram shown in Figure 1.
A chopper amplifier consists of a
regular voltage amplifier with a gain
of A and an (unwanted) offset voltage of VOS , flanked on both sides by
a modulator or chopper. The input
voltage to a chopper amplifier is multiplied (or chopped) by a square wave
periodic signal p (t) , which assumes
+ 1 and - 1 values. The chopped input
signal is then effectively added to VOS
before being amplified by A. The out-

Digital Object Identifier 10.1109/MSSC.2016.2548821

FigURe 1: A chopper amplifier and its sample waveforms. While the input signal vin(t) preserves its waveform at the output, the offset voltage, VOS, appears as chopped at the output.

Date of publication: 21 June 2016

1943-0582/16©2016IEEE

p (t )
vin(t )

put of the amplifier will be multiplied
by p (t) again to form the final output
of the chopper amplifier. In this process, the input waveform is chopped
twice, reproducing the input voltage
at the output node with a gain of A.
The offset voltage, VOS , however, is
chopped only once, hence appearing
chopped at the final output of the
chopper amplifier. In other words,
v out (t) = Ap 2 (t) v in (t) + AVOS p (t) .

p (t )

Vos
v1(t )

Modulator
(Chopper)

A

Amplifier with Offset
vin(t )

v2(t )

vout(t )

Modulator
(Chopper)
Vos
t

+1

p (t )
t

-1
vin(t )

vin(t )p (t )

Vos
t

vout(t )

Avin(t )p 2(t ) = Avin(t )

AVosp(t )
t

IEEE SOLID-STATE CIRCUITS MAGAZINE

s p r i n g 2 0 16

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