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1995 | Buch

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Frequency Compensation Techniques for Low-Power Operational Amplifiers

verfasst von: Rudy G. H. Eschauzier, Johan H. Huijsing

Verlag: Springer US

Buchreihe : The International Series in Engineering and Computer Science

Enthalten in: Professional Book Archive

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Über dieses Buch

Frequency Compensation Techniques for Low-Power Operational Amplifiers is intended for professional designers of integrated amplifiers, emphasizing low-voltage and low-power solutions.
The book bridges the gap between the professional designer's needs and available techniques for frequency compensation. It does so by explaining existing techniques and introducing several new techniques including Hybrid Nested Miller compensation, Multipath Miller Zero cancellation and Multipath Conditionally Stable compensation. All compensation techniques are treated in a stage-number-based order, progressing from a single transistor to circuits with six stages and more. Apart from discussing the mathematical basis of the compensation methods, the book provides the reader with the factual information that is required for practicing the design of integrated feedback amplifiers and many worked out examples. What is more, many bipolar and CMOS operational amplifier realizations, along with their measurement results, prove the effectiveness of the compensation techniques in real-life circuits.
The text focuses on low-voltage, low-power integrated amplifiers. Many of the presented bipolar circuits operate at supply voltages down to 1V, while several CMOS amplifiers that function correctly just slightly above this voltage are demonstrated. The lowest measured power consumption amounts to 17muW for a class AB CMOS opAmp with 120dB gain. Despite this attention to low voltage and low power, the frequency compensation strategies provided are universally applicable. The fundamental approach followed leads to efficient compensation strategies that are well guarded against the parameter variations inherent to the mass-fabrication of integrated circuits.
The book is essential reading for practicing analog design engineers and researchers in the field. It is also suitable as a text for an advanced course on the subject.

Inhaltsverzeichnis

Frontmatter

1. Introduction

Abstract

The rapidly increasing integration densities of I.C. technologies has forced some fundamental changes of direction in the field of analog integrated electronics. For years, the circuit design almost exclusively focussed on the improvement of the signal processing properties. Demands on accuracy, offset and noise, etc., led the appearance of new circuit generations. Although these properties remain of importance today, the design emphasis has shifted from the basic qualities towards the ability of circuits to operate at a low supply voltage, consuming a minimum amount of power. The lower supply voltage leads to drastic changes in the way circuits are built up, since many of traditional circuit solutions become obsolete and need replacement.

Rudy G. H. Eschauzier, Johan H. Huijsing

2. Properties of Feedback Circuits

Abstract

The essence of feedback is to redirect part of the output signal back to the input. There the returned signal is compared to the incoming signal. Any inconsistency between the desired and the actual output value produces ar error signal. The nature of the feedback loop helps reducing the error to minimum. In this way, the loop accurately controls the output to produce an —amplified or otherwise processed— replica of the input signal.

Rudy G. H. Eschauzier, Johan H. Huijsing

3. Stability of Feedback Circuits

Abstract

Feeding back the output signal to the input does not in all cases establish the equilibrium state expressed by the basic input-output relation

$$ G = \frac{A}{{1 + A\beta }} $$

(3–1)

We can understand this from considering the consequence of reversing the sign of the forward path. Setting its gain to -A results in the following alleged transfer function

$$ G = \frac{{ - A}}{{1 - A\beta }} $$

(3–2)

Although Eq. 3-2 is a valid solution, there is no change for an actual circuit to arrive in this state. The circuit will amplify a small error between the actual and the desired signal, caused for instance by noise or drift, and produce a large output signal. Because of the positive feedback, this in its turn leads to a greater error, resulting in an even larger output signal. The system is said to be instable, since any infinitesimal deviation from the equilibrium condition will cause the output signal to explode. This suggests the following definition for the stability of a linear system: stability is the condition that a bounded input signal results in a bounded response.

Rudy G. H. Eschauzier, Johan H. Huijsing

4. Basic Frequency Compensation of Integrated Circuits

Abstract

Monolithic IC technology puts severe limitations on the options available for frequency compensation. A major restriction is the fact that inductors of any significant value cannot be integrated, while these components play an important role in the conventional compensation practice. This leaves capacitors as the only frequency dependent element to shape the frequency response. Unfortunately, even their use is limited to small values —much smaller than demanded by classical theory.

Rudy G. H. Eschauzier, Johan H. Huijsing

5. Multistage Compensation Techniques

Abstract

In many situations the gain of a two-stage amplifier is not sufficient and more stages are necessary. The traditional approach is to add current follower or voltage follower stages to the two-stage circuits of the previous chapter. These followers do not change the high frequency behavior appreciably, but can nevertheless improve the low-frequency gain. A serious drawback, however, is that the cascodes and emitter (source) followers increase the minimum supply voltage. Since low voltage is an essential step towards low power, a higher supply voltage will directly hamper the obtainable bandwidth-to-power ratio.

Rudy G. H. Eschauzier, Johan H. Huijsing

6. Multipath Compensation Techniques

Abstract

In many of the compensation techniques described in the previous chapter, the addition of stages to the circuit leads to a reduction of the obtainable bandwidth. Going from simple two-stage Miller compensation to Nested Miller compensation with three gain stages, for instance, results in a fifty percent smaller bandwidth. Also Hybrid Nested Miller compensation suffers from this drawback. Although the bandwidth reduction effect is absent in Reversed Nested Miller compensation, the dependency of this compensation technique on the load capacitance makes it fundamentally unsuited for general purpose applications. Therefore we would like to eliminate the bandwidth reduction in Nested and Hybrid Nested Miller compensation, while retaining the high level of robustness inherent to these methods.

Rudy G. H. Eschauzier, Johan H. Huijsing

7. Realizations

Abstract

To evaluate their specific properties, this chapter gives several illustrative implementations of the various compensation methods presented in the preceding chapters. We will investigate ten amplifier realizations, distributed over a total of five chips. Four of the circuits are fabricated in a bipolar technology, while six of them are CMOS based.

Rudy G. H. Eschauzier, Johan H. Huijsing

Backmatter

Titel: Frequency Compensation Techniques for Low-Power Operational Amplifiers
verfasst von: Rudy G. H. Eschauzier
Johan H. Huijsing
Verlag: Springer US
Electronic ISBN: 978-1-4757-2375-5
Print ISBN: 978-1-4419-5154-0
DOI: https://doi.org/10.1007/978-1-4757-2375-5

Springer Professional

Über dieses Buch

Inhaltsverzeichnis

Frontmatter

1. Introduction

2. Properties of Feedback Circuits

3. Stability of Feedback Circuits

4. Basic Frequency Compensation of Integrated Circuits

5. Multistage Compensation Techniques

6. Multipath Compensation Techniques

7. Realizations

Backmatter