enter image description here

From http://www.ece.tamu.edu/~spalermo/ecen474/lecture17_ee474_fully_diff_amps_cmfb.pdf

I'm having some trouble understanding why M3 drops to linear region.

  1. First Question - The increase in Ib should only cause M3 to drop into linear/triode if that change in Ib was caused by an increase in abs(Vb) - correct? Then I can understand that the abs(Vds) > abs(Vgs-Vt) condition could be violated. So, what is causing the increase in Ib here?
  2. Second Question - Consider the following Id/Vds plot enter image description here

Now, let's say that M3 was in saturation when only Ib was flowing. How could an increased Ib be supported when M3 is in linear region? Surely, the linear region supports a lower current than in saturation?

TLDR: What causes the increase in Ib and why would M3 drop to linear region because of it even though linear region supports a lower current than saturation?

  • \$\begingroup\$ Assume fixed VB. A large tail current will keep the M3 in the saturated (high Rout) region. Whereas a small tail current is not sufficient to cause M3 to enter saturated region. \$\endgroup\$ – analogsystemsrf Mar 17 at 17:01
  • \$\begingroup\$ @analogsystemsrf I'm not sure if I understand what you mean. The slide claims that somehow Ib increases slightly and this increased Ib cannot be supported by the tail current source and thus M3 drops to linear region. I don't get how that forces M3 to drop into linear. \$\endgroup\$ – AlfroJang80 Mar 17 at 17:32

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