Cascode Current MirrorThe cascade connection achieves a very high output resistance. Since this is a desirable

characteristic for a current mirror, exploring the use of cascades for high-performance current mirrors is natural.

A bipolar-transistor current mirror based on the cascode connection is shown in Fig. below. Transistors M3 and M1 form a simple current mirror, MOS structure is preferred because it does not suffer from finite beta, effect.

the small signal output resistance is given as

Explanation: in this approach ( allen holberg) We are objective to minimize lambda. Cascading we lead to increase to Rout. Now Rout is Inversely proportional to lambda, so increasing Rout We decrease lambda. So that Iout=Iin .

Even with finite mismatch between Vds1 and Vds2.

The Widlar Current SourceResistor R in the Widlar current source circuit shown in the schematic in Fig. below

gives the designer an additional degree of freedom in adjusting the mirror ratio of the current mirror.

In this circuit, the difference between the gate-source voltages of transistors M1 and M2 appears across resistor R, and IO can be expressed as

Here the current equastion can be given as below

Now If IREF, R, and the W/L ratios are known, then above Eq. can be written as a quadratic equation in terms of

• MOS Widlar Source Output Resistance: the small-signal model for the MOS Widlar source is recognized as a common source stage with resistor R in its source. Therefore,

Wilson Current Mirror• The Wilson current sources shown in Figs. Below (a) and (b) use the same number

of transistors as the buffered current mirror but achieve much higher output resistance; it is often used in applications requiring precisely matched current sources.

• Figure (a) figure(b)

the output current is taken from the drain of M3, and M1

and M2 form a current mirror. During circuit

operation, the three transistors are all pinched-off and in

the active region. Because the gate current of M3 is zero,

ID2 must equal reference current IREF.

If the transistors all have the same W/L ratios, then

The current mirror require

and because IO = ID3 and ID3 = ID1, the output current is given by

Active Loads