Subject - Microelectronics

Topic - Active Load MOSFET Differential Amplifier

Difficulty - Medium

Marks - 10M

The Beta multiplier is an example of a circuit that uses positive feedback. The addition of the resistor kills the closed loop gain (a positive feedback system can be stable if its closed loop gain is less than one). However, if we decrease the size of the resistor, we increase the gain of the loop and push the feedback system closer to instability. An example of when this could occur is if the parasitic capacitance on the source of M2 to ground is large (effectively shorting M2's source to ground). If the resistor, for example,is bonded out off-chip to set the current, it is likely that this bias circuit will oscillate.

Figure shows how the reference currents through M1 and M2 vary with VDD. The minimum value of VDD can be estimated by looking at the minimum value of voltage across the drain-source of M3 and Ml. For M3 this is Vsd,sat or 250 mV. For M1 this is Vds1 = Vgs1 = 1.05 V. We can then write

VDDmin=VSD3,sat + Vgs1 = 1.3 V

Finally, the sensitivity of Iref is directly dependent on the output resistance of the MOSFETs. From the simulations

or almost an order of magnitude better than the previous reference circuits.