Biasing Cascoded - SubThreshold.com

Member of: Circuits Attach:cascode_bias.png | Self-biasing cascode

How to generate a bias for cascoded structures.

Self-biased Cascode

This shows a low-voltage cascode being transformed into a self-biased cascode structure. (Razavi pp. 398)

Note that the voltage drop across the entire self biasing structure is V_gs2 + V_ds1 and the drop across the two transistors is V_gs1.

To keep M2 active
- $V_{gs2} + V_{ds1} = I_b R + V_{ds2} + V_{ds1}$ - KVL - two ways to express total voltage drop
- $V_{gs2} - I_b R = V_{ds2}$ - Cancel V_ds1 and re-arrange
- $V_{ds2} = V_{gs2} - I_b R > V_{gs2} - V_{th}$ - V_{ds sat} requirements
- $R < V_{th} / I_b$
To keep M1 active
- $I_b R = V_{gs2} + V_{ds1} - V_{gs1}$ - Voltage across R by subtracting node voltages
- $I_b R = V_{ds1}$ - The current in M1 & M2 is identical therefore assume V_gs1 = V_gs2
- $I_b R = V_{ds1} > V_{ds sat}$ - V_{ds sat} requirements
- $R > V_{ds sat} / I_d$
$V_{ds sat} / I_b < R < V_{th} / I_b$ or $V_{gs} - V_{th} < I_b R < V_{th}$

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