How do you provide a DC return path for the bias current? You should do this...
Latest update time：2019-12-18
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Have you ever had this experience? In the rush to design a circuit, you overlooked some basic issues, and the circuit did not function as expected. One of the most common problems in AC-coupled op amp or instrumentation amplifier circuit applications is not providing a DC return path for the bias current. Today, I will discuss this problem for you and propose a super practical solution.
Op amps:
How to provide a DC return path for bias current
Error demonstration
In Figure 1
, a capacitor is connected in series with the noninverting (+) input of an op amp. This AC coupling is a simple way to isolate DC voltages from the input voltage (V
IN
), which is particularly useful in high-gain applications. At high gains, even a small DC voltage at the amplifier input can affect the dynamic range of the op amp and may even cause the output to saturate. However, capacitive coupling into a high-impedance input without providing a DC path for the current in the positive input can cause problems.
Figure 1: Incorrect AC-coupled op amp circuit
The input bias current flows through the coupling capacitor, charging it until it exceeds the rated common-mode voltage of the amplifier input circuit or exceeds the output limit. Depending on the polarity of the input bias current, the capacitor charges either toward the positive supply voltage or toward the negative supply voltage. This bias voltage is amplified by the closed-loop DC gain of the amplifier.
This process can be lengthy. For example, for an amplifier with a field-effect transistor (FET) input, if its bias current is 1 pA and coupled through a 0.1-μF capacitor, its IC charge rate, I/C, is
10
-12
/10
-7
=10μV/sec
The total output drift is 600μV/min. If the gain is 100, the output drift is 0.06V/min. It can be seen that if an AC-coupled oscilloscope is used for a short test, this problem may not be detected, and the circuit will fail after several hours. In short, it is very important to avoid this problem.
Correct display
Figure 2
shows a simple solution. In this example, a resistor is connected between the input of the op amp and ground, providing a return path for the input bias current. To minimize the offset voltage caused by the input bias current, when using a bipolar op amp, consider the matching of the two inputs of the op amp, and usually set R1 to the parallel value of R2 and R3.
Figure 2: Proper method for AC coupling the inputs of a dual-supply op amp
However, it should be noted that this resistor will always contribute some noise to the circuit, so a trade-off needs to be made between the circuit input impedance, the size of the input coupling capacitor required, and the Johnson noise introduced by the resistor. Typical resistor values are generally between 100,000 Ω and 1 MΩ.
Instrumentation Amplifiers:
How to Provide a DC Return for Bias Current
Error demonstration
Figure 3
shows an instrumentation amplifier circuit that is AC-coupled through two capacitors and does not provide a return path for the input bias current. This problem is common in instrumentation amplifier circuits that use dual supplies (
Figure 3a
) and single supplies (
Figure 3b
).
Figure 3: Incorrect AC-coupled instrumentation amplifier circuit
As
shown in
Figure 4
, this problem can also occur in circuits utilizing transformer coupling if a DC return path to ground is not provided in the transformer secondary.
Figure 4: Incorrect transformer-coupled instrumentation amplifier circuit
Correct display
Figures 5
and
6
show simple solutions for this type of circuit. A high value resistor (
RA
,
Rb
) is added between each input and ground. This is a simple and practical solution for dual-supply instrumentation amplifier circuits. The resistors provide a discharge path for the input bias current. In the dual-supply example, both inputs are referenced to ground. In the single-supply example, the inputs can be referenced to either ground (VCM
is
grounded
) or to a bias voltage, which is usually half of the maximum input voltage range.
Figure 5: Proper method for transformer input coupling for instrumentation amplifiers
The same principle can be applied to transformer-coupled inputs (
Figure 5
)
, except that the transformer secondary winding has a center tap, which can be connected to either ground or V
CM
. In these circuits, there is a small offset voltage error due to resistor and/or input bias current mismatch. To minimize this error, connect another resistor with a value of about one-tenth the two resistors (but still large compared to the differential source resistance) between the two inputs of the in-amp (thereby bridging the two resistors).
Figure 6:
High-value resistors between each input and ground provide the required bias current return path
This article is reproduced from: Analog Devices
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