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/50e-06)
Results: Node DC measurements (re-formatted for display).
Node Measurements ---- ------------ n1 4.2884 V n2 4.0 V n_pos 5.0 V v1#branch -99.3714 uA v2#branch 48.4859 uA (v2#branch/50e-06) .9697
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
One of the few advantages of lateral PNP transistors (see reference textbook page 1-22) is that you can split the collector of the device into multiple sections and achieve excellent matching between collector currents (albeit at small currents only): thus the appropriate use of this device in a current mirror topology.
In the circuit above, Q1 is the split-collector lateral PNP transistor. I1 biases the transistor and stablishes a “diode” voltage drop from emitter to base (V_eb), the emitter current is then divided “identically” into two branches. (due to the split collector layout). However, there are two errors:
There is a large collector current variation with changes in collector voltage (due to the early effect), moreover the effect is more pronounced for the lateral PNP current mirror than it was for the Widlar current mirror, mainly:
Error measurement: Variation of 53.5uA to 47.5uA over current mirror operating region (below 4.7V collector voltage). This is equivalent to an error of 6uA or 12% relative to our current reference.
The second error (due to the use of a lateral PNP transistor) has to do with substrate currents (There is a competing PNP transistor: emitter, base, substrate). During normal operation a current about half the base current flows from emitter to substrate and under saturation the current increases to be almost the same as that of the collector.
In the graph above, this happens at a collector voltage of 4.7V or about 0.3V from the 5V positive supply.
Figures of Merit
Output Resistance: 783.3 KR (from 0 to 4.7V range)
Compliance Voltage: 0.3 V (from positive supply)
- Textbook (Hans Camenzind)
- Chapter 3 (page 3-2)
- Previous report
- Lab 3.1: Current Mirrors – Widlar Current Mirror
- Next report
- Lab 3.3: Current Mirrors – Simple MOS Current Mirror
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