Amplifier requirement specification

Amplifier requirement specification#

In this chapter, we have discussed the modeling and characterization of amplifiers. We have found description methods for their ideal behavior and for their non-ideal behavior. Now, we will use this knowledge for the specification of amplifiers. In section Operational requirements, we will discuss the so-called operational requirements of application-specific amplifiers. Operational requirements describe the desired functionality, the performance measures, the cost factors and the environmental conditions that apply during the process of operation. This process, however, is not the only process that introduces design constraints. Generally, all life-cycle processes (see section Basic concepts) generate various requirements that need to be accounted for during design. As a matter of fact, even the requirements of the design process itself may seriously limit the set of possible solutions. A few remarks on relevant requirements that follow from those life-cycle processes will be made in section Requirements from other life cycle processes.

Operational requirements#

A complete list of performance aspects for application-specific amplifiers cannot be given. The definition of relevant performance aspects and limitations, as well as the definition of available resources and environmental conditions, needs to be extracted from a description of the application of the amplifier to be designed. This task is usually performed by experienced system architects. However, based on the knowledge acquired in this chapter, we can make a list of groups of items that usually need to be described.

As mentioned above, setting up the requirements for a design usually starts with describing the application of the amplifier. This comprises a description of:

  1. The signal processing task to be performed by the amplifier (also: fucntional specification)

  2. The quality level of the execution of these tasks (also: performance measures)

  3. The environment in which the signal processing takes place (also: environmental conditions)

  4. The resources that are available for the performance of this task (also: cost factors).

The description of the application needs to be translated into a clear list of measurable properties of the amplifier, including their test methods and test conditions. During the design of the amplifier, the performance measures of various design alternatives with their specific cost factors can be compared. Design choices can then be made on grounds of the performance-to-cost ratio.

Performance requirements#

The performance requirements describe the electrical properties of the amplifier that are required for its proper operation in the application, as well as their test methods. The following list of specification items is usually required (but not complete!).

  1. Input port requirement specification

    Relevant performance aspects for the input port are:

    1. Input port configuration (grounded, floating)

    2. Input impedance

    3. Input signal specification

    In many cases, these specifications need to be extracted from the source specification:

    1. Source configuration (grounded, floating)

    2. Source impedance

    3. Source signal specification (current or voltage , frequency spectrum, rate of change, peak values, etc.)

  2. Output port requirement specification

    Relevant performance aspects for the output port are:

    1. Output port configuration (grounded, floating)

    2. Output impedance

    3. Output signal specification

    In many cases, these specifications must be found from the load specification:

    1. Load configuration (grounded, floating)

    2. Load impedance

    3. Load signal specification (current or voltage, frequency spectrum, rate of change, peak values, …)

  3. Signal transfer specification (type, value and error budgets)

    Relevant performance aspects for the signal transfer are usually derived from the source and the load specification and from the specification of the environmental conditions. Usually, error budgets should be given for:

    1. Imperfect port isolation (common-mode port impedances, CMRR and PSRR)

    2. Noise addition

    3. Small-signal dynamic behavior (frequency range and filter characteristics)

    4. Static nonlinear behavior (offset, gain error, nonlinearity, voltage and current clipping)

    5. Nonlinear dynamic behavior (slew rate, overdrive recovery)

Operating conditions#

The operating conditions represent the environmental conditions under which the amplifier should perform according to its requirements. All environmental conditions that affect the operation of the amplifier should be specified. Amongst others, the operating conditions listed below are relevant to the functioning of electronics in general.

  1. Temperature (affects electrical properties of all electronic devices and thermal noise)

  2. Humidity (may cause parasitic current paths between devices)

  3. Shock and vibration (may cause defects in connections and in electronic devices)

  4. Electro Magnetic Interference (EMI: may cause degradation of signal quality or failure during operation)

  5. Power supply noise

  6. Electro Static Discharge (ESD: may cause degradation of the performance of devices or failure of devices)

Operating cost factors#

The operating cost factors are the resources that are required for the operation of the amplifier. Typical resources are:

  1. Power supply voltage(s) and current(s)

  2. The amount of space

  3. The maximum mass or weight

Reliability requirements#

Some examples of reliability requirements are:

  1. Mean Time To Failure (MTTF)

  2. Mean Time To Repair (MTTR)

  3. Mean Time Between Failures (MTBF)

Safety requirements#

Safety regulations for electronic products strongly depend on the application domain (consumer, automotive, industrial, medical, space) and on the nationality or region of use (e.g. CE compliance and UL compliance).

Requirements from other life cycle processes#

Although the operating process is the most important life-cycle process of the amplifier, other life-cycle processes may introduce serious design constraints. Life-cycle design is the name for a design process that accounts for requirements from all life-cycle processes. We will restrict ourselves to a few remarks on certain life-cycle processes.

Design#

The availability of design resources such as device models, device samples, design tools and design verification tools may seriously limit the solution space.

Production#

Electronic circuits are usually produced in standardized production processes. Such processes introduce design constraints that have to be accounted for. Design For Production (DFP) indicates that production aspects have been accounted for during design.

Test#

Test processes and the availability of test tools also introduce design constraints that have to be accounted for. Design For Testability (DFT) indicates that test aspects have been accounted for during design.

Transport#

Environmental conditions during transportation, as well as cost factors, may strongly differ from those during operation. This has to be accounted for during design.

Installation#

The availability of tools for installation may put specific constraints on the design of an amplifier.

Service#

The availability of tools, as well as the service environment, may introduce specific constraints that have to be accounted for during design.

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