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 .6654 V n2 1.0 V n3 1.3312 V n_pos 5.0 V v1#branch -50.0 uA v2#branch -49.4016 uA (v2#branch/v1#branch) .9880
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 ; Sweep Collector voltage from 0v to 5V in 0.1v increments. gnuplot $filename (v2#branch*-1e+06) ylimit $ylow $yhigh title $title xlabel $xlabel ylabel $ylabel
The Wilson Current Mirror was invented by George Wilson, an Analog IC design engineer who worked as head of the Integrated Circuits Group at Tektronix. It is said that he invented the circuit over an overnight challenge with Barrie Gilbert to design an improved current mirror that would use only 3 transistors. (or so the story goes)
In this circuit the combination of the load, Q3 and Q2 form a Cascode stage. This stage introduces a negative feedback loop into the system and shields the simple current mirror (Q1 and Q2) from load voltage fluctuations.
Initially all transistors are OFF, shortly thereafter a small current starts to flow into the base of Q3 which starts to conduct, the emitter current of Q3 then biases the current mirror formed by Q1 and Q2. As Q1 starts to conduct (and given our constant current reference), it reduces the base current of Q3. In turn, Q3 emitter current decreases which leads to reductions in Q1 and Q2 collector currents, the reduction in Q1’s collector current makes more current available for the base of Q3 and the process repeats itself from the beginning until loop errors are reduced (i.e. subtracted from Q3 base current); a constant setpoint is reached when the load current matches closely the reference current. (see a detailed explanation of the circuit here)
Furthermore, it is worth noting that the base error previously found in the simple 2-transistor (Widlar) current mirror is minimized. I.e. while the base current for Q3 is still taken from the reference (1), it is amplified by the current gain of Q3 and a small fraction of Q3’s larger emitter current is fed-back to drive the bases of Q1 and Q2 (2). These effects compensate for each other, with the collector current of Q3 (the output current) greater than that of Q1 and more closely matched to our current reference.
For the Wilson current mirror in the reference textbook, the following are the error measurements:
Variation of 49.409uA to 49.467uA over an operating range of 1.1V to 5V.
This is equivalent to an error of 0.116% relative to the current reference.
The minimum voltage of the load (the voltage compliance) is given by the need to maintain a collector voltage for Q3 of: V_be for Q2, plus the (collector to emitter) saturation voltage for Q3 (see figures of merit).
The current mirror transistors Q1 and Q2 are not perfectly matched: the collector voltage of Q1 is at 2V_be while that of Q2 is at V_be. This error will be explored and minimized in upcoming lab reports.
Figures of Merit
Output Resistance: 67.24MR (from 1.1 to 5V linear range)
Compliance Voltage: 1.1V (from ground)
The compliance voltage of this circuit is rather high which limits the use of this topology outside of the scope of most low power applications
There is also a PNP equivalent Wilson current mirror which will be explored in the next lab report.
- Textbook (Hans Camenzind)
- Chapter 3 (page 3-4)
- Previous report
- Lab 3.4: Current Mirrors – Emitter Resistors (Negative Feedback)
- Next report
- Lab 3.6: Current Mirrors – PNP Wilson Current Mirror
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