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22 September 2009
SIGNAL CHAIN BASICS (Part 7): Op Amp Performance Specifications--Bias Current

The Signal Chain Basics Series continues with a discussion of bias current, a key operational amplifier (op amp) parameter

By Bill Klein, Senior Applications Engineer, Texas Instruments
Planet Analog
February 2, 2008 (12:00 PM EST)

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(Editor's Note: There are links to the previous parts of this series at the end, below the author's biography.)

In Part 6 of this series, we discussed input voltage offset (Vos). The other significant operational amplifier (op amp) input stage parameter is bias current. This parameter is listed in data sheets as Ib or Iib. Other variations of this designation also are used: at times the trailing letters may be written as subscripts (IB or IIB).

All amplifiers have some current flow, termed bias current, through the input pins. In the BJT model used in Part 6, it was the base drive current. There may be a current cancellation circuit added to reduce this current. FET and CMOS input stages still suffer leakage current through their gate junctions.

In addition, almost all amplifiers have an internal circuit (cell) to protect the devices from electrostatic discharge (ESD) events. These cells will also suffer some current leakage. The magnitude of op amp bias currents ranges from femtoamps (10-15 A) to microamps (10-6 A). Since the two input circuits are not identical, there will be a difference in bias current between the inputs. This difference is termed the offset current or Ios.

For all but the smallest of bias currents, the circuit in Figure 1 provides all the data needed to determine these values.

Figure 1: Bias current test circuit

Start with both switches closed. Note that:

Open SW2 and the bias current for the non-inverting input will develop a voltage across R2. To determine Ib+, observe the change in Vout between SW2 open and SW2 closed. This value is the ΔVout term:

Close SW2 and open SW1, the bias current for the inverting input will develop a voltage across R1. To determine Ib-, you observe the change in Vout between SW1 open and SW1 closed. This value is the ΔVout term:

The offset current can be measured with a similar technique which relies on the op amp's ability to calculate a difference. Close both switches and note Vout, then open both switches and note the new value of Vout.

The subtraction of terms is done here by the differential input stage of the op amp. When R1 = R2, then

While the details of bias current measurement are straightforward, this characteristic of all amplifiers can have a subtle impact on some applications, and it is necessary to provide a DC path for the bias current.

Consider the circuit in Figure 2. Shown here, there is no path for the bias current. Therefore, without a path for the bias current, a voltage will continue to develop on C1. This voltage will appear to the op amp as an input signal, and be amplified until the output is saturated.

Figure 2: Bias current path

Click here to view a video clip which shows what happens when the DC bias current path is lost. (The circuit in Figure 2 is the same as in the video clip.) The loss of a path for Ib could be from omission in design, or from solder joint failure during, or occur after production.

The rate of output voltage change depends on the magnitude of the bias current and capacitor size. The defining equation is:

This action could be used to measure bias current in amplifiers with ultralow bias current.

(Note: The next part of this series will look at flash and pipeline architectures for A/D converters.)

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 BASIC Series (Part 6): Op Amp Input Voltage Offset", www.planetanalog.com/features/showArticle.jhtml;?articleID=205901111, click here
"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

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