Detailed explanation of the advantages and disadvantages of the application of MOS op amps and the three major factors to pay attention to

For the above-mentioned various types of active filters, the operational amplifier circuit can be either a MOS operational amplifier or a transistor operational amplifier. Compared with transistor op amps, MOS op amps have their own advantages and disadvantages.

Since MOS operational amplifiers have the advantages of low power consumption, high input impedance, and extremely small input current, high resistance resistors can be selected when designing active filters, so smaller capacitors can be used, which reduces the size of the filter. Small. At the same time, single-chip MOS four-op-amp circuit is selected to form various filters with excellent performance of multi-op-amp circuit structure, and to make them standardized and thick film. Application of MOS operational amplifier. Transistor op amps have low input impedance and large bias current. Therefore, when choosing transistor op amps to design active filters, in order to avoid the offset or noise caused by bias currents, the resistance value of the filter is usually low, so it must be increased. The capacity of large capacitors leads to an increase in volume. Compared with QTS op amps, MOS op amps have obvious shortcomings, namely, greater offset and low-frequency noise. Therefore, when designing an active filter composed of MOS operational amplifiers, requirements should be put forward for the offset and low-frequency noise of the MOS operational amplifiers. In addition, MOS operational amplifiers must also consider the following three factors:

1. The stability of MOS op amp

The MOS operational amplifier in the filter usually constitutes deep negative feedback, and deep negative feedback tends to produce high-frequency oscillations. In order to avoid high-frequency oscillations, MOS operational amplifiers with good stability should be selected or some phase compensation circuits should be added.

2. Open loop gain of MOS op amp

Choose a MOS op amp with a large open-loop gain as much as possible. Generally speaking, the larger the open-loop gain of the MOS operational amplifier, the closer the filter parameter index is to the design value, and the better the performance of the filter.

3. Bandwidth and slew rate of MOS op amp

When designing a filter with a higher operating frequency, the bandwidth and slew rate of the MOS operational amplifier should be required. The bandwidth and slew rate determine the highest frequency that can be reached under a certain output amplitude, and this highest frequency limits the working frequency of the filter.

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