Schematic Diagram

Simple MOS Current Mirror Schematic

SPICE Simulations

Operating Point Analysis

Here, we are calculating the DC voltages (bias voltages) at every node of our circuit.

Relevant source code lines:

OP                      
print all               
* Output current over constant input current (with 1V load): should be 1 for best match.
print (v2#branch/v1#branch)

Results: Node DC measurements (re-formatted for display).

Node                    Measurements
----                    ------------
n1                      1.1049 V
n2                      1.0 V
n_pos                   3.0 V
v1#branch               -50.0 uA
v2#branch               -49.8939 uA
(v2#branch/v1#branch)   .9979

DC Analysis

In our DC analysis, we are measuring the variation of the mirrored output current under different loads.

We are applying a DC sweep to V2 (our load voltage) from 0 to 3V in 0.1V increments.

We are plotting the output current magnitude vs drain voltage. (our load voltage at n2)

Relevant source code lines:

DC V2 0V 3V 0.1V          ; Sweep Drain voltage from 0v to 3v in 0.1v increments.

gnuplot $filename (v2#branch*-1e+06) ylimit $ylow $yhigh title $title xlabel $xlabel ylabel $ylabel

Simple MOS Current Mirror Simulation DC

Results

Similar to the Widlar current mirror but now with MOS devices: a simple current mirror can be constructed by placing two back to back NMOS FETs. The first transistor M1 has it’s drain and gate tied together in a “diode connected” fashion, albeit this is a misnomer as there is no internal diode PN junction, instead the gate to source threshold voltage is largely determined by the geometry of the device.

For the Simple MOS Current Mirror in the reference textbook, and because the channel dimension were chosen to be quite large:

  1. The variation of the drain current with changes in load voltage is as follows:

    Error Measurement: Variation of 49.89uA to 51.39uA over an operating range of 1 to 3V.
    This is equivalent to an error of 1.5uA or 3% relative to our current reference.

2.The current mirror requires at least a 0.5V load voltage to be operational (this can be seen from the plot above). However lower compliance voltages can be achieved through the use of larger devices.

Note that this topology is symmetrical and can be inverted by using PMOS transistors connected to the positive supply rail instead.

Figures of Merit

  • Output Resistance: 750KR (from 1 to 3V linear range)

  • Compliance Voltage: 0.5V (from ground)

Source code

References



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