PROBLEM 022 · Easy · Mirror ratios
Double Vision
A mirror with two outputs: 1× and 2×
The problem
Build an NMOS current mirror with one diode-connected reference transistor and two output transistors. A 50 µA Current source from VDD feeds the reference. The first output (net “out1”) must carry 50 µA; the second (net “out2”) must carry 100 µA.
Mirror ratio is set by transistor size, not by the reference: two transistors with the same gate-source voltage carry currents in the ratio of their W/L (here, the multiplier M). Ratios of whole numbers match best because the extra transistors are just copies of the same unit.
The test bench
- Supply: VDD = 1.8 V.
- Your reference is the Current source you place: set it to 50 µA.
- The test bench holds both “out1” and “out2” at 0.9 V with voltage sources and measures the current each one draws. It sweeps “out1” from 0.1 V to 1.7 V.
Numbers to hit
- First output at 0.9 V between 48 µA and 52 µA
- Second output at 0.9 V between 96 µA and 104 µA
- First output’s change 0.5 V to 1.3 V at most 6.5 %
- Every transistor in saturation
Design it yourself
Draw the circuit on a schematic canvas, size every transistor, and see it simulated in your browser with ngspice and the SkyWater SKY130 models. Every transistor lights up green, gold or red by operating region as you move the sliders.
Sign in or create a free account →More on current mirrors
- Copy PasteCopy a 50 µA reference into a 100 µA output and keep it accurate as the output voltage moves.
- Copy Paste ProStack a second pair of transistors so the copy barely notices the output voltage.
- Mirror, MirrorA mirror that sources current from VDD: the building block of every active load.
- Ten to OneA resistor in one source makes the copy much smaller than the reference, and its output resistance much higher.
- WilsonThree transistors and one feedback loop give a cascode’s output resistance without a bias voltage.
- Resistor ArmorEqual resistors in both sources make the mirror less sensitive to mismatch and to the output voltage.