Basic knowledge and understanding of high output impedance constant current source circuit

High output impedance constant current source circuit

As shown in Figure 2.2-2. Compared with the constant current source circuit in Figure 2.2-1, it adds MOS tube M2, M 4. Due to the addition of M3, M4 tubes, the constant current source circuit It has good constant current characteristics and high output impedance ro. It can be seen from the picture that M3 and M4 tube to make M1 tube VDS, and The Vos of the M2 tube is the same or approximately the same.

Io and IR can be written as:

The above formula shows that the output currentI o has nothing to do with λ, that is, the output current Io does not follow the output The voltage changes and changes, so it has good constant current characteristics and higher output impedance.

Figure 2.2-2 The output impedance of the circuit o can be represented by the AC small signal equivalent circuit Figure 2.2-3 Find.

Vo can be expressed as:

and

Due to reference currentIR is constant, so under the condition of small communication signal,  . Thus, the above expression is:

and So the output impedance ro is:

Because of , So the above formula is approximately.

(2.2-4) formula shows that the output impedance of the constant current sourcero is larger than the output image impedance of the constant current source circuit in Figure 2.2-1 times. Usually the value of is about 50~100, therefore, the value of Figure 2.2-2 The output impedance of the constant current source circuit is 50-100 times larger than that of the constant current source circuit in Figure 2.2-1.

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