Cirrus Logic AN300 REV1 User Manual

AN300

CS3001/2/11/12 & CS3003/4/13/14 Chopper-stabilized

Operational Amplifiers

Jerome E. Johnston

1. INTRODUCTION

Cirrus Logic offers a variety of low-voltage CMOS chopper-stabilized amplifiers.

The chopper-stabilized amplifiers designed at Cirrus Logic are unique. These amplifiers offer performance benefits that combine the best features of bipolar amplifiers with the best features of chopper amplifiers. The intent of this application note is to understand Cirrus Logic's unique technology and to see how it can be applied in various measurement applications. But before the applications are discussed, the following provides a brief overview of the concepts involved in a chopper-stabilized amplifier.

2. CHOPPER AMPLIFIER AND CHOPPER-STABILIZED AMPLIFIER BASICS

Not everyone is familiar with chopper amplifiers and chopper-stabilized amplifiers. A look back at some history can help us understand how the chopper-stabilized amplifier operates.

Figure 1 illustrates the block diagram of a chopper-stabilized amplifier. A chopper-stabilized amplifier is a DC am-

plifier whose offset is stabilized by a chopping amplifier. The basic amplifier diagram in Figure 1 is called the Goldberg configuration, named after E. A. Goldberg, an engineer who designed and patented electron tube-based, chopper-stabilized amplifiers for RCA (Radio Corporation of America) in the 1940s and 1950s.

DC Amplifier

AMP1

out

AC Amplifier (Chopper)

AMP2

R3R2

Figure 1. Basic Chopper-stabilized Amplifier Block Diagram

The Goldberg configuration was later used in a transistorized chopper-stabilized amplifier designed and sold by Zeltex Corporation in the 1970s for about $125 (US). The Zeltex chopper-stabilized amplifier follows the basic block diagram of Figure 1. The input signal (e signal paths. (The Goldberg configuration amplifier could only be used as an inverting amplifier.) The first path is into Amplifier #1 of Figure 1. Components R1-C1 act as a high-pass filter that prevents the DC portion of the signal from passing directly into Amplifier #1. The second path is through a low-pass filter composed of R2 and C2. The R2-C2 filter limits the bandwidth of the signal to be chopped by the chopper amplifier. Amplifier #2 is the chopper

Oscillator

) at the inverting input of the amplifier travels through two different

Low-pass

Filter

S2S1

http://www.cirrus.com

JUL ‘08

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amplifier. The chopper amplifier is really a modulation system in which the DC and very low-frequency portion of the input signal is alternately turned on and off. This results in the signal being modulated at the carrier (chopping) frequency. An oscillator in the chopper amplifier toggles switches S1 and S2 on and off simultaneously. When switch S1 is conducting, the input to the chopper amplifier is ground. When switch S1 is off, the input signal is allowed to pass into Amplifier #2. The effect of chopping is to turn the DC input signal into a square wave (AC) signal whose amplitude changes between ground and the amplitude of the input signal. This signal is then amplified by the AC amplifier. Once the signal is amplified as an AC signal, it is restored to ground reference by the output chopping switch. This resulting signal, which is a ground-referenced square wave, is then filtered with a low-pass filter composed of resistors R5 and R6 in conjunction with capacitor C5. The corner frequency of this filter is very low – typically only fractions of a Hertz. The time constant must be very long to maintain the voltage on the filter capacitor over the half cycle when the chopped signal is not presented to the output filter.

Figure 2 illustrates the DC amplifier portion of the Zeltex device. This amplifier was constructed with a matched bi-

polar transistor pair as the front end. The open loop gain of the DC amplifier is about 94 dB.

Figure 3 illustrates the actual chopper amplifier portion of the Zeltex amplifier. The amplifier AC gain is about 3000,

but because the signal is on only 50% of the time, the effective gain is only half as much. Therefore the AC amplifier gain is about 1500, or about 63 dB. The chopping oscillator operates at about 200 Hz. Note that the chopper output is filtered by a low-pass filter with an extremely low corner frequency.

+15V

-IN

From AC

Amplifier

51k

1N459

0.1uFR1200k

0.005uF

2N2914

51k

2N4248

10M

500k 56k

10pF

2.4k

100k

-15V

Figure 2. Amplifier 1: DC Amplifier Portion of the Zeltex Module

E304

2N4248

680

OUT

2 AN300REV1

+15V

AN300

10k

150

2N4299

10k

22uF

150

10k

51k

0.033uF

2N3565

22uF

From

Oscillator

51kR5200k

S2 2N2946

fc = 0.009Hz

1N459

68uF

To DC

1N459

100k

From

Oscillator

100k

0.033uF

4700pF

S1 M5079

2.0M

3.9k

E305

18k

68uF

-15V

Figure 3. Amplifier 2: Chopper Amplifier Portion of the Zeltex Module

The gain transfer functions of the two amplifiers are multiplied together (or added when stated in dB). The combination of the two amplifiers produces an open loop gain of about 160 dB. One of the difficulties in the design of the chopper-stabilized amplifier combination is ensuring stability. The DC performance of the combined amplifiers is dictated by the performance of the chopping input. Input current of the Zeltex amplifier was dominated by charge injection in the chopping switch at the input of the chopper amplifier. Offset voltage and offset voltage drift of the amplifier combination was determined by how close the chopping switch at the input of the chopper amplifier approximated the ideal. The Zeltex amplifier achieved less than 100 pA of input bias current with an offset voltage drift less than 50 nV / °C.

Figure 4 illustrates a typical chopper-stabilized amplifier designed using electron tubes with the amplifiers connected

in the Goldberg configuration. The circuit has the same layout as the block diagram of Figure 1. The chopping switches were mechanical vibrating switches (about 400 Hz) manufactured by Airpax Corporation. The output filter exhibits an extremely long time constant.

E.A. Goldberg filed a U.S. patent (#2,684,999; assigned to RCA) for a tube-based chopper-stabilized amplifier on April 28, 1948. But the invention of the chopper architecture itself predates this. J. W. Milnor filed for a U.S. patent on a chopper amplifier on January 17, 1918.

AN300REV1 3

DC Amp

AN300

R2 330

C4 3nF

220k

350pF

100k

220k

10pF

+300V

10k

0.01uF

AC Amp

Figure 4. Chopper-stabilized Amplifier Using Electron Tubes and Mechanical Vibrating Swit ches

680k

R3 10k

2uF

R1 10M

5751

fc = 0.008 Hz

C6 2uF

R4 10M

V2A

2.2M

½ 5751

760k

Output

6AQ5

-250V

-400V

+300V

2.2M C7

0.01uF

2.2M

220k

0.1uF

5751

220k

0.1uF

1M 1M 1k

220k

½ 5751

0.1uF

NOTE: 5751 = Ruggedized 12AX7.

3. MODERN CHOPPER-STABILIZED AMPLIFIERS

Monolithic chopper-stabilized amplifiers became available in the late 1970s. Different chopping architectures have been promoted by various vendors. The various architectures will not be examined in detail here. Before discussing the particulars of the Cirrus Logic chopper-stabilized amplifiers, it will be beneficial to understand some differences found in bipolar versus MOS transistors.

For many years bipolar transistors dominated monolithic amplifier designs. Bipolar transistors have some distinct advantages over CMOS transistors for some performance parameters. The bipolar transistor provides higher transconductance (I/V gain) for a specific value of operating current. The bipolar device, because of its construction, also provides lower noise than can easily be achieved in a MOS transistor, for devices of similar silicon area. The wide band spot noise level is lower in the bipolar transistor. And a much lower 1/f noise corner can be achieved in a bipolar transistor. Bipolar transistors can also be better matched when manufactured together on the same silicon die.

Most recently, the most prominent semiconductor processes being developed to shrink the geometry size of devices is being applied to CMOS technology. CMOS is favored because it can provide lower power consumption for massive digital chips with millions of transistors. Smaller transistors mean the availability of more transistors in a given area of silicon. This results in smaller device package sizes. In the last decade, more analog and mixed signal devices have been designed to take advantage of the smaller geometry CMOS processes. This includes monolithic amplifiers based upon CMOS processes.

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As mentioned previously, bipolar transistors have long dominated the world of operational amplifiers. Let's look at some of the differences of parameters of bipolar transistors versus MOS transistors. Understanding the differences between these two technologies can help explain some of the constraints on the architectural choices used in the design of CMOS chopper-stabilized amplifiers.

The construction of bipolar transistors and MOS transistors are very different. The difference in construction results in large differences in some performance parameters. One area of significant difference is noise performance.

Figure 5 illustrates a noise plot for a bipolar transistor. This hypothetical but typical device exhibits a spot noise at 1

kHz of 4 nV/√Hz difficulty achieving this level of noise performance. Figure 6 illustrates a noise plot for a hypothetical but typical MOS transistor. This device exhibits a spot noise of 17 nV/√Hz

with a 1/f corner at 2.5 Hz. Because of the difference in construction, a MOS transistor would have

at 50 kHz with a 1/f corner at 2000 Hz.

Bipolar Transistor Noise

100

0.1 1 10 100 1k

Figure 5. Noise Performance Plot for a Bipolar Transistor

Frequency (Hz)

MOS Transisitor Noise

1000

100

10 100 1k 10k 100k

Frequency (Hz)

Figure 6. Noise Performance Plot for a MOS Transistor

Note from Figure 5 that the magnitude of the noise of the bipolar transistor at 0.1 Hz is about 20 nV/√Hz. The spot noise in the example MOS transistor does not match this 20 nV/√Hz

value until the frequency for the MOS transistor is at about 5 kHz. A MOS transistor can be designed to achieve a lower spot noise and a lower 1/f corner frequency than what is shown in Figure 6 by significantly increasing the size of the device and by increasing its

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