Equivalent resistance circuit and calculation formula of MOS switch capacitor

In recent years, in MOS analog integrated circuits, switched capacitor circuits composed of MOS analog switches, MOS capacitors and MOS operational amplifiers have been widely used, such as switched capacitor filters, switched capacitor oscillators, etc. The main principle of these circuits is that, under the action of a clock signal, a switched capacitor circuit composed of an analog switch and a capacitor can be regarded as an equivalent resistance. The equivalent resistance composed of MOS analog switch and MOS capacitor, its circuit form is shown in Figure 4.2-8, M1 and M2 are n-channel analog switches, and the gates act as ΦaΦb clock signals respectively. When Φa is high level (Φb Low level) M1 is on, M2 is off, capacitor C1 is connected with signal source voltage V1, and V1 charges capacitor C1; MOS switch capacitor. When Φb is high (Φa is low), M2 is turned on, M1 is turned off, capacitors C1 and V2 are connected, and capacitor C1 discharges to V2 until the capacitor voltage drops to V2, so that the total amount of power flowing from power supply V1 to power supply V2 The amount of charge is:

If the period of the clock signal ΦaΦb is T, and suppose that the frequencies of the power supplies V1 and V2 are smaller than the frequency of the clock signal, Then the average current delivered by the power supply V1 to the power supply V2 in a cycle is:

The switched capacitor can be equivalent to a resistance, and its relational formula is:

If C=1 picofarad and f=100 kHz, then the equivalent resistance of the switched capacitor is 10 megohms, which occupies only 0.01 square millimeters of chip area. MOS switch capacitor. The formula (4.2-3) is an important relational formula for designing switched capacitor circuits. According to this relational formula, high-performance switched capacitor integrators, switched capacitor filters and switched capacitor oscillators can be designed.

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