PROBLEM 040 · Hard · Input common-mode range
Limbo
How low can the inputs go?
The problem
Build a differential pair with load resistors to VDD and an NMOS tail transistor (gate on an Input source named “vbt”). Tie both gates together to one Input source named “cm”, so the pair sees the same voltage on both inputs. The outputs are “outp” and “outn”.
Whatever the input common-mode voltage, the tail should keep carrying its current. When “cm” falls too low the tail transistor loses its drain-source voltage and drops into triode, the current collapses and the outputs rise. The test bench sweeps “cm” and measures the range over which “outp” stays within 3% of its value at your chosen DC level: the input common-mode range (ICMR).
The test bench
- Supply: VDD = 1.8 V.
- The test bench sweeps the source “cm” from 0 V to 1.8 V and plots “outp”. The range is where it stays within 3% of its value at the “cm” level you chose.
Numbers to hit
- Lowest input level where the pair still works at most 0.9 V
- Highest input level where the pair still works at least 1.5 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.
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- Two's CompanyTwo matched transistors share a tail current and turn a voltage difference into a differential output.
- Pair, FlippedFlip the pair upside down: PMOS inputs, a tail current from VDD, loads to ground. It works with inputs near ground.
- Tail RiskAn ideal tail current source rejects everything the inputs share. A transistor tail only does so if its output resistance is high.
- Slow DownAdd a resistor in each source: the gain drops, but the pair stays linear for a much larger input.
- Load BearingUse PMOS diodes as the loads: the gain is no longer a resistor ratio but a transistor ratio.
- Mirror TailReal tails come from mirrors. Take a 25 µA reference, scale it to 100 µA, and use it to bias the pair.