How do you design a feedback network to improve the CMRR of op-amp in a closed-loop configuration?

1 Feedback and CMRR

Feedback is a process of feeding back some portion of the output signal to the input of the op-amp, either in phase or out of phase. Feedback can affect the gain, bandwidth, stability, and CMRR of the op-amp. Depending on the polarity of the feedback signal, feedback can be classified as positive or negative. Positive feedback increases the gain and instability of the op-amp, while negative feedback reduces the gain and improves the stability and CMRR of the op-amp. Negative feedback is more common and useful for closed-loop applications.

2 Feedback network types

A feedback network is a circuit that connects the output and the input of the op-amp, providing a feedback signal. There are four basic types of feedback networks: voltage-voltage, voltage-current, current-voltage, and current-current. Each type has a different effect on the input and output impedances of the op-amp, as well as on its CMRR. For example, a voltage-voltage feedback network has a high input impedance and a low output impedance, and it improves the CMRR by reducing the differential gain and increasing the common-mode gain of the op-amp.

3 Feedback network components

The feedback network components are usually resistors, capacitors, or a combination of both. The choice of the components depends on the desired frequency response, gain, and CMRR of the op-amp. Resistors are simple and linear elements that can provide a constant feedback ratio. Capacitors are nonlinear and frequency-dependent elements that can provide a variable feedback ratio. A combination of resistors and capacitors can create a filter or a compensator that can enhance or attenuate certain frequency ranges. The feedback network components should be matched and stable to ensure a consistent and reliable CMRR.

4 Feedback network design

The design of a feedback network involves selecting the type, the components, and the values of the feedback network. The first step is to determine the required gain and bandwidth of the op-amp. The gain is inversely proportional to the feedback ratio, which is the ratio of the feedback signal to the output signal. The bandwidth is directly proportional to the gain-bandwidth product, which is a constant parameter of the op-amp. The second step is to choose the type of feedback network that suits the application and the input and output impedances. The third step is to calculate the values of the components using formulas or simulation tools. The fourth step is to test and verify the feedback network performance and CMRR.

5 CMRR measurement

The CMRR of an op-amp can be measured by applying two identical signals to both inputs of the op-amp, and measuring the output voltage. The output voltage should ideally be zero, since the op-amp should reject the common-mode signal. However, in reality, there will be some residual output voltage due to the mismatch and nonlinearity of the op-amp. The CMRR can be calculated by dividing the differential gain by the common-mode gain of the op-amp. The differential gain is the ratio of the output voltage to the input voltage when only one input is varied. The common-mode gain is the ratio of the output voltage to the input voltage when both inputs are varied. The CMRR is usually expressed in decibels (dB).

6 CMRR improvement

The CMRR of an op-amp can be improved by using negative feedback, choosing a suitable feedback network type, and selecting matched and stable components. Negative feedback reduces the differential gain and increases the common-mode gain of the op-amp, resulting in a higher CMRR. A voltage-voltage feedback network can improve the CMRR by lowering the output impedance and increasing the input impedance of the op-amp. Matched and stable components can minimize the variations and errors in the feedback network, ensuring a consistent and reliable CMRR.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?