7.1: Introduction to Direct-Coupled Amplifiers

Operational amplifiers incorporate circuit configurations that may be relatively unfamiliar to the circuit designer with a background in other areas. An understanding of these special techniques is necessary for the most effective use of operational amplifiers.

One of the more challenging problems arises in the design of the input stage of an operational amplifier. One important consideration is that this stage provides gain to zero frequency. Thus the isual biasing techniques which incorporate capacitors that reduce low-frequency gain cannot be used. Circuits that provide useful gain at zero frequency are called direct-coupled or direct-current (d-c) amplifiers. The design of the direct-coupled input stage(It is obviously necessary that all stages of an operational amplifier be direct coupled if the complete circuit is to provide useful gain at zero frequency. Emphasis here is given to the input stage because it represents the most challenging design problem.) of an operational amplifier is further complicated by the fact that it should have low input current.

Direct-coupled amplifiers are also useful other than as the input stage of an operational amplifier. Applications include processing certain signals of biological or geological origin that may contain significant components at a fraction of a hertz. While bandpass amplifiers can theoretically be used for such signals, the various capacitors required may become prohibi­tively large or expensive. Furthermore, the recovery time associated with large capacitors following overload or turn on is intolerable in some appli­cations. In other cases, signals of interest contain frequencies of cycles per week, and response to zero frequency is mandatory in these situations. Alternatively, the designer may be interested in realizing a high-frequency amplifier, where minimization of capacitance to ground at certain critical nodes is of primary concern. If a large coupling capacitor is used, its stray capacitance to ground can deteriorate high-frequency performance.

The design of d-c amplifiers poses new problems because of the drift associated with such amplifiers. Drift is a phenomena whereby the output of an amplifier changes not because of a change in the input voltage applied to the amplifier but rather in response to changes in circuit elements. In direct-coupled circuits, it is not possible to distinguish between an output that is a result of an applied input signal and one that occurs in response to drift. For this reason, drift limits the minimum input signal that can be detected.

A new circuit technique is required for the design of an amplifier that provides sufficiently low drift to be useful in d-c applications. In this chapter we shall concentrate on one circuit, the differential amplifier, which is used almost exclusively for d-c amplification. This circuit is particularly valuable when realized with bipolar transistors, since their highly pre­dictable characteristics are readily exploited to yield low-drift performance. (A humorous comment on the difficulty of achieving acceptable d-c amplifier perform­ ance before modern bipolar transistors were developed is provided in L. B. Argumbau and R. B. Adler, Vacuum-Tube Circuitsand Transistors, Wiley, New York, 1956. Chapter III, section 15 of this book is titled "Direct-Voltage Amplifiers-Why to Avoid Building Them.")

The discussion in this chapter focuses on the techniques used to reduce the drift and input current of a d-c amplifier, and thus the techniques de­scribed are useful in a range of applications. Toward the end of expanding the applicability of the techniques described in this chapter, certain aspects are covered in greater detail than is necessary for a basic understanding of operational amplifiers. Thus, as is the case with the material on feed­back systems, operational amplifiers are used as a vehicle for illustrating technology valuable in a variety of electronic circuit and system design problems. The specific ways that these design techniques are incorporated into operational amplifiers are reserved for discussion in subsequent sections.