Input Bias Current: Correction Methods

Key Takeaways

• The input bias current is the DC flowing into or out of an op-amp’s differential inputs during operation.

• Designers can add resistors to neutralize the input bias current; some packages have this functionality built-in.

• Depending on the intensity of the signal, designers can use resistors or capacitors with parallel switches to measure the input bias current.

Input bias current can contribute significantly to BJT amplifier performance.

One of the first things students learn when working with op-amps is the idealized form, which enables a general assessment without getting caught in real-world complexities. This kind of simplification is common throughout physics and engineering – frictionless ramps and pulleys, anyone? – but it does fall short when project cost and time come into focus. For certain amplifier technologies, it’s worthwhile to consider the effects of the DC input bias current flowing into or out of the op-amp’s differential inputs to ensure performance aligns with the expected results. In many cases, chip manufacturers use on-die circuitry to account for the input bias current; however, it may behoove designers to know how to set up external compensation or measure the effect directly.

Comparing Input Stages for Bipolar Transistors

The Input Bias Current’s Effect on Signal Integrity

While the basic approach to an op-amp is an idealized infinite impedance on the differential inputs, realistically, a small offset current  (fA ~ µA) passes through, with the exact value range dependent on the amplifier model. In hindsight, the observation is obvious but underscores the importance of incorporating non-ideal attributes to improve performance. The input offset voltage that arises from the ratio of the input offset current over the source impedance can impact the signal characteristics if the impedance is too low; like the input offset voltage, the input offset current is the difference between the current flowing (typically) into the inverting and noninverting outputs. It’s important to note that impedance matching the inputs plays an important role: an input offset current between unmatched inputs has little explanatory value. Fortunately, most voltage feedback amplifier networks adhere to a reasonable impedance matching on the inputs.

For this reason, many chip packages contain an input bias current cancellation on the input with two NPN transistors to ensure the current flowing into the amplifier inputs is the offset between the base and source current. This implementation improves the amplifier's stability electrically and thermally yet faces limitations in high-frequency applications, as noise from the source and transistor bases can add together and undermine signal integrity. In the presence of current compensation circuitry, the input bias current and input offset current have the same magnitude. However, not all op-amps have a built-in compensation circuit; designs need a straightforward and more general method to implement adjustments.

Using Ohm’s Law principles, designers can adjust the input bias current on the noninverting input to match the feedback line. Consider a simple resistor feedback network on the inverting op-amp that uses one feedback resistor and an input resistor on the inverting input: an input resistor on the noninverting input is necessary that provides the same impedance (and thus input bias current). The noninverting input resistance would simply be the parallel equivalent resistance of the feedback and inverting input resistors. Be wary that the noninverting input resistor will need a bypass capacitor in parallel to remove noise on the line once the total resistance becomes appreciable (~ 1 kΩ). 

Circuits to Measure an Input Bias Current

More involved treatments of an amplifier attempting to resolve the input bias current may require a measurement to establish a targetable current reading. While many approaches exist, two of the more common methods are switches or an integrator circuit for especially small input bias current values:

• Switch-selector - A variation on the feedback resistor network described above that adds a second high-value resistor to the input alongside a parallel switch. The idea is that the designer can isolate the input bias current by switching between four possible switch arrangements: both open, both closed, and one open. Suppose the input offset voltage is already known (the exact value is the result of material and processing imperfections and thus varies). In that case, it’s trivial to determine the change in input offset voltage with the additional resistance and work backward to find the input bias current. Note that the range for the high-value input resistor depends on the amplifier model, running from several kΩ to GΩ.

• Integrator circuit - Similar to the classic integrator circuit but adds parallel switches across the capacitors to quantify the rate of voltage change. Operators can alternate the open/closed switch between the differential inputs to measure the inverting or noninverting input bias current sequentially.

Because input bias current exhibits more strongly in bipolar op-amps than in FET-based models, designers may want to minimize BJT usage to avoid the issue. While this is a reasonable approach and can work in certain applications (while also reaping the power efficiency of CMOS amplifiers), BJT performance in high-frequency circuits greatly outstrips that of CMOS due to the latter’s large increase in losses with high-speed switching. The increasing prevalence of high-frequency designs for faster transfer speeds means designers cannot simply ignore the detrimental effects of input bias current without a compensation circuit, whether discrete or as-packaged.

Cadence Solutions for Practical Prototyping

The input bias current is just one of the many nonideal effects designers must contend with when dealing with bipolar op-amp technology. Idealized models are invaluable for estimating circuit behavior with a reasonably high degree of accuracy and speed, but practical concerns for manufacturing require a more rigorous approach. Simulating circuit performance with powerful modeling software can be the difference between an extra revision and the knock-on effect of time-to-market. Cadence’s PCB Design and Analysis Software suite gives design teams a comprehensive ECAD environment with powerful tools and customization options. Simulation results are then fed into OrCAD PCB Designer, ensuring DFM has never been easier or more thorough.

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