Analysis of Common Problems in Amplifier Circuit Design

We often use amplifier circuits. Therefore, this article summarizes some issues that may be involved in the application of amplifier circuits, hoping to help everyone use and apply them better.

1. Lack of DC bias current loop

One of the most common application problems is not providing a DC return path for the bias current in an AC-coupled op amp or instrumentation amplifier circuit. In Figure 1, a capacitor is connected in series with the noninverting (+) input of an op amp. This AC coupling is a simple method to isolate the DC voltage from the input voltage (VIN). This method is particularly useful in high-gain applications, where 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 at high gains. However, capacitive coupling into high impedance inputs without providing a DC path for the current in the positive input can cause some 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 field-effect transistor (FET) inputs, with a bias current of 1 pA coupled through a 0.1- μF capacitor, the charge rate of IC, I/C, is:

The total output is 600 V/min. If the gain is 100, the output drift is 0.06 V/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 may fail after several hours. In short, it is very important to avoid this problem.

Figure 2. Correct method for AC coupling at the input of a dual-supply op amp.

A simple solution to this common problem is shown in Figure 2. In this example, a resistor is connected between the op amp input 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 op amp inputs, usually setting R1 to the parallel value of R2 and R3.

However, it should be noted that this resistor will always introduce some noise into 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 and 1 MΩ.

Figure 3. Incorrect AC-coupled instrumentation amplifier circuit

[page] 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 circuit.

Figure 4: Incorrect transformer-coupled instrumentation amplifier circuit

Figures 5 and 6 show a simple solution 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. Correct way to couple the instrumentation amplifier transformer input.

The same principle can be applied to transformer-coupled inputs (Figure 5), except that the transformer secondary winding has a center tap that can be connected to either ground or VCM. 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 of the two resistors (but still large compared to the differential source resistance) between the two inputs of the instrumentation amplifier (thereby bridging the two resistors).

Figure 6 High-value resistors between each input and ground provide the required bias current return path