Schematic Diagram

Widlar 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 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                      665.4844 mV
n2                      1.0 V
n_pos                   5.0 V
v1#branch               -50.0 uA
v2#branch               -49.4646 uA
(v2#branch/v1#branch)   .9893

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 5V in 0.1V increments.

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

Relevant source code lines:

DC V2 0V 5V 0.1V

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

Widlar Current Mirror Simulation DC

Results

The Widlar current mirror is named after it’s inventor Bob Widlar, A renowned bright analog IC designer with an “interesting” character.

Given our diode-connected (collector-base-tied transistor) Q1 and our bias current I1, we should see a “diode” voltage drop from collector-base to emitter (V_be). Thus, because Q1 and Q2 share the same base-emitter voltage, therefore the collector current of Q2 should match identically that of Q1, except for two errors:

  1. From our simulation results above with our 1V load, we can see that currents v1#branch and v2#branch are very close, but not exactly identical. (and it gets worse as soon as we “introduce” a load)

    The reason for this has to do with the base currents of both Q1 and Q2.

    Given a current gain (or beta = i_collector/i_base) of roughly 100, we can expect 1% of I1 to go to the base of Q1 and 1% to go to the base of Q2, for the worst case.

    Error (worst case): 2% relative to the current source reference, or 1uA in our case.

  2. The early effect degrades the current matching performance of Q2 under different loads.

    While the collector voltage of Q1 remains fixed at V_be, the collector voltage of Q2 can change: Our load voltage can be anything. Now, due to the early effect for bipolar transistors, as the collector voltage increases, the collector current also increases and no longer matches that of I1.

    This means our current mirror does not accurately match the source current under different loads; We can see the extent of this error in our DC analysis plot.

    Error measurement: Variation of 48.8uA to 53uA over current mirror operating region (roughly above 0.3V collector voltage). This is equivalent to an error of 4.2uA or 8.4% relative to our current reference.

There are ways to compensate for these errors through the use of additional transistors. These will be explored in upcoming labs.

Figures of Merit

There are two important performance metrics (specifications) for current mirrors, these are:

Output Resistance

A measure of the (non-ideal) current variation due to changes in load voltage.

It can be calculated as the change in output voltage divided by the change in output current.

The greater the output resistance of the current mirror, the less current variation with collector voltage (same current regardless of load), the better the performance of the circuit.

For the Widlar current mirror:

R_out = (delta_v) / (delta_i)

R_out = (5V - 0.3V) / (53uA - 48.8uA)

R_out = 1.12 MR
  • Output Resistance: 1.12 MR (from 0.3 to 5V range)

Compliance Voltage (for current mirrors)

A measure of the minimum load (or output) voltage for which the current mirror can maintain a constant current.

The closer to the supply rails the current mirror can operate, the greater the compliance range, the better the performance of the circuit.

For the Widlar current mirror:

  • Compliance Voltage: 0.3 V (from ground)

Source code

SPICE model device parameters are referenced in the course syllabus

References




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