Now that we discussed some of the basic applications for an op amp (operational amplifier) and developed one version of an instrumentation amplifier, let's take a look at some of the specifications that describe the performance of an op amp.

It is common for the first parameters in the specification table to be the input-stage DC characteristics. This list includes input voltage offset, offset drift, bias current, and offset current. The value of each of these parameters is set primarily by the design of the input stage. Consider the classical bipolar differential amplifier (diff amp) shown in Figure 1.

Figure 1: Input stage
(Click on image to enlarge)

Input voltage offset is defined as that voltage that must be applied to the input so as to drive the output to zero. In a perfect world, Q1 would be identical to Q2 and R1 equal to R2, which would result in an op amp with zero offset voltage. A great amount of effort is spent trying to make the input transistors identical; however, there is always a slight difference. The data sheet entry may call this parameter Vos (as in voltage offset), or Vio (as in voltage input offset), among other mnemonics.

Most of the device specifications for which the user sets the gain are referred-to-input or RTI values. Therefore, the output error magnitude for a particular application is that parameter value times the gain of the circuit, where this gain is determined by the design. Voltage offset is just such a parameter.

The voltage offset can be modeled as a signal applied to the non-inverting input. The voltage measured at the output of the circuit in Figure 2 will be 1001 times Vos.

Figure 2: Vos measurement circuit.
(Click on image to enlarge)

The forward voltage drop across a P-N junction, the base-emitter junction of the input stage transistors, changes with temperature. Unfortunately, each transistor changes at a slightly different rate; therefore, the voltage offset of an op amp will change with temperature.

This action is reported as Vos Drift in μV/°. A normal procedure is to measure and record Vos at three temperatures: minimum temperature, room temperature, and maximum temperature. The drift is then calculated by:

(Click on equation to enlarge)

While the bipolar junction transistor (BJT) model was used to illustrate Vos, similar actions occur in J-FET and CMOS devices.

Here's a special note regarding the single-supply op amp case: the voltage at the op amp's non-inverting input is the voltage to which the output stage will be driven when the input is zero. When the op amp is powered from a single supply, with the negative supply being connected to ground, the circuit in Figure 3A will not give correct results. The output of the op amp cannot be driven all the way to the negative supply.

When the op amp has a negative voltage offset, the output must be driven below the negative rail. In this single-supply case, it is necessary to bias the non-inverting input to a potential between the rails (see Figure 3B). The offset voltage will be measured with respect to that bias voltage.

Figure 3: Correct Vos measurement for single-supply operation .
(Click on image to enlarge)

In the next article, we'll cover bias current in an op amp.

About the author

William P. (Bill) Klein is a Senior Applications Engineer with the High Performance Analog group at Texas Instruments. Bill joined TI through its acquisition of Burr-Brown in August 2000. His experience as an analog circuit designer covers over 40 years in fields ranging from mineral exploration to medical nuclear imaging. One current role Bill has is hosting the Analog e-LAB Web Cast, presenting real world solutions to real world problems in analog circuit design. In addition to a BSEE from Arizona State University and registration as a Professional Engineer in the State of Arizona, he has authored numerous magazine articles, application notes and conference papers.

Previous installments of this series:

• "SIGNAL CHAIN BASICS Series (Part 5): Introduction to the Instrumentation Amplifier", www.planetanalog.com/features/showArticle.jhtml;?articleID=205208593, click here
• "SIGNAL CHAIN BASICS Series (Part 4): Introduction to analog/digital converter (ADC) types", www.planetanalog.com/features/showArticle.jhtml;?articleID=204803631, click here
• "SIGNAL CHAIN BASICS Series (Part 3): Analog and the digital world", www.planetanalog.com/features/showArticle.jhtml;?articleID=204400376, click here
• "SIGNAL CHAIN BASICS Series (Part 2): Op Amp--Basic operations", www.planetanalog.com/features/showArticle.jhtml;?articleID=203101699, click here
• "SIGNAL CHAIN BASICS: Operational Amplifier--The Basic Building Block", www.planetanalog.com/features/showArticle.jhtml;?articleID=202801320, click here