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I am trying to understand the role of M14 and M13 transistors in the paper "Compact silicon neuron circuit with spiking and bursting behavior" by Jayawan H.B. Wijekoon, Piotr Dudek (2008) Schematic

Usually a simple differential amplifier schematic would have a constant current source or a current mirror for current biasing. But the publication I'm referring to reads: "The voltage Vbias controls the bias current in the comparator." I cannot distinct between the source and the drain which make it harder. How does Vbias regulate the current? What is the role of M13?

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M13 is the normal current mirror that is used for current biasing of the differential amplifier. The confusion for you is probably because the diode in the current mirror is not shown in your schematic. Source and drain are interchangeable but for simplicity consider that the source of M13, M14 is the one connected to ground.

enter image description here

Coming to M14, it is connected in a diode configuration. i.e., gate is connected to drain. enter image description here

There is no current going through M14 when VGS<VTHN. When VGS>VTHN, the current through M14 increases proportional to the square of (VGS-VTHN). So, current will shoot up if the drain of M14 goes higher than it's threshold voltage. AS per the paper, when V crosses VTH, the drain of M14 will move up therefore, the current in M14 and hence the current through M10 will increase and VB will decrease further than the case where M14 is not present. So, VB amplitude increases during a spike event and the duration pulse width of VB will also increase because the amplitude is more.

Edit: M14 in summary improves the amplitude and duration of pulse on VB during a spike event without increasing the steady state current of the comparator.

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