Basic Features and Analysis of MOS Tube Constant Current Source Circuit

In MOS analog integrated circuits, the constant current source circuit is often used as a bias circuit and as an active load of the amplifier. The performance of analog integrated circuits is often closely related to the constant current characteristics of the constant current source circuit. The circuit form of the MOS field effect tube constant current source is basically similar to the bipolar transistor circuit. Now introduce a common MOS tube constant current source circuit.

(1) MOS tube constant current source circuit analysis

The basic but current source circuit is shown in Figure 2.2-1, where IR is the reference current and IO is the output current. MOS tube M1 works in the saturation region (VDS=VGS), if the MOS tube M2, Then M2 also works in the saturation zone.

According to the current equation in the saturation zone, the relationship between Io and IR can be obtained as:

Where W2/L2 is the width-to-length ratio of the M2 tube, W1/L1 is the width-to-length ratio of the M2 tube, and it is assumed that the μ,Same as λ.

It can be seen from the formula (2.2-1) that as long as the width-to-length ratio of the M1 and M2 tubes is changed, the required Io can be designed, if=

，W2/L2=W1/L1, then Io=Ix, because，And Vo is the output voltage, usually Vo is a variable overflow, and , Due to the existence of the channel length modulation effect, Io changes with the change of the output voltage Vo. MOS tube constant current source circuit. Obviously, only when λ=0, can Io be a constant value.

To improve the constant current characteristics of the constant current source circuit in Figure 2.2-1, increase the channel length LL of the M2 tube, reduce λ, and increase the output impedance ro (ro=1/λIo), The constant current characteristics are improved. In many applications, a larger output impedance and good constant current characteristics are required, and the above-mentioned constant current source circuit must be improved. The improved constant current source circuit is discussed below.

relevant information

Constant current source circuit with high output impedance

Wilson (Wilson) constant current source circuit

Constant current source circuit with high output impedance and high swing output

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