PROBLEM 027 · Medium · Turning a current around
Turn the Corner
NMOS mirror into PMOS mirror
The problem
A 50 µA Current source from VDD feeds an NMOS mirror (a diode-connected transistor and a copy). The copy’s drain feeds a PMOS mirror (a diode-connected PMOS with its source on VDD and a copy). The PMOS copy’s drain is “out”: it sources 50 µA into the load.
Each mirror reverses the direction of the current relative to ground: the NMOS pair takes a current from the supply and turns it into a current to ground; the PMOS pair turns that back into a current from the supply. This is how a bias current made near ground reaches circuits that need current from VDD.
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 the net named “out” at 0.9 V with a voltage source, sweeps it from 0.1 V to 1.7 V, and measures the current your mirror exchanges with it.
Numbers to hit
- Output current at 0.9 V between 47 µA and 53 µA
- Change between 0.5 V and 1.3 V at most 3 %
- Output stays within 2% up to at least 1.3 V
- 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.
- Double VisionOne reference, several copies: scale each copy by its width.
- 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.