The CMFB amplifier is used to ensure that the average of the outputs from a differential amplifier (diff-amp) closely matches a predetermined reference voltage, Vbiasp. Its primary role is to maintain the common-mode voltage of the outputs around Vbiasp without affecting the differential amplification of the diff-amp. One approach to achieve this is by employing a common-mode feedback amplifier (CMFB amplifier), as illustrated in Fig. 1. Here’s how this mechanism works:
CMFB Amplifier Function
The CMFB amplifier serves the purpose of amplifying the difference between the average of the diff-amp’s outputs and a reference voltage, Vbiasp. If the CMFB amplifier has a high gain, it ensures that the average voltage of the two outputs closely matches Vbiasp. Importantly, the CMFB amplifier’s output, VCMFB, is connected in common to both outputs of the diff-amp through transistors M1L and M1R.
The primary goal of the CMFB amplifier is to make sure that the outputs of the diff-amp vary around Vbiasp. It should not interfere with the differential amplification function of the diff-amp. When the inputs to the diff-amp are equal (neglecting any offsets), their outputs should be equal to Vbiasp.
Implementation of a CMFB amplifier
- As shown in Figure 2, When the positive (+) inputs (notably, the PMOS transistors connected to these inputs have half the width of the other PMOS transistors) are set to Vbiasp, it results in a current of 20 A flowing through transistors M1 and M2. This current mirroring action is crucial for the operation of the diff-amp.
- If the average voltage at the positive inputs moves above Vbiasp, the current through M2 decreases, while the current through the 200/1 PMOS increases. This change in current causes VCMFB to rise. Consequently, transistors M3L and M3R activate further, pulling both outputs of the diff-amp down until their average aligns with Vbiasp.
Limitation of a CMFB amplifier
- Stability: The use of feedback in the CMFB loop introduces concerns about stability. To address this, capacitors can be added to the outputs of the diff-amp (inputs to the CMFB amplifier) or to the CMFB amplifier’s outputs. These capacitors help stabilize the CMFB loop.
- Input Common-Mode Range: The input common-mode range is a critical consideration. If the positive inputs of the CMFB amplifier move significantly away from the input range, proper balancing may not be achieved. This can result in large signal swings on the diff-amp’s outputs, preventing them from balancing around Vbiasp.
In practice, it’s crucial to use a CMFB circuit that can effectively balance the outputs over the entire range of diff-amp output voltages, addressing concerns related to stability and common-mode range to ensure the circuit functions as intended.
How does the CMFB amplifier achieve its goal of regulating the common-mode voltage in the differential amplifier outputs?
The CMFB amplifier amplifies the difference between the average of the differential amplifier’s outputs and Vbiasp. If the gain of the CMFB amplifier is large, it ensures that the outputs’ average remains close to Vbiasp. Any variation in the CMFB amplifier’s output signal affects both outputs of the differential amplifier equally.
What is the significance of ensuring that the CMFB amplifier does not affect the differential amplification in the differential amplifier (diff-amp)?
It is crucial that the CMFB amplifier does not interfere with the differential amplification in the diff-amp. This ensures that the primary function of the differential amplifier, which is to amplify the difference between its inputs, remains unaffected by the presence of the CMFB circuit.
What challenges are associated with the stability of the CMFB loop, and how can stability be achieved in practice?
Achieving stability in the CMFB loop can be challenging, particularly when adding feedback to electronic circuits. To stabilize the CMFB loop, capacitors can be added to the outputs of the diff-amp or the outputs of the CMFB amplifier. These capacitors serve to stabilize the feedback loop.
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