PROBLEM 043 · Medium · Current steering and the transfer curve
Tipping Point
Full steering: how little input switches the pair?
The problem
Build a differential pair with load resistors to VDD and a tail Current source: inputs “inp” and “inn”, outputs “outp” and “outn”. Make it switch fully with a small input while giving a large output swing.
For a large enough input, all the tail current flows through one transistor, and the differential output saturates at ±I·R_D. The input needed to get most of the way there is about 1.4·Vov: so a pair with a small overdrive (wide transistors) is a sharp switch. A pair with a big overdrive needs a bigger input to switch, but is more linear.
The test bench
- Supply: VDD = 1.8 V.
- The test bench hangs a 200 fF load capacitor from each output and applies the AC stimulus to “inp” only.
- It sweeps the source “inp” from 0.4 V to 1.4 V (“inn” stays fixed) and plots the differential output. The steering span is the input range over which the output goes from 10% to 90% of its full swing.
Numbers to hit
- Full differential output swing at least 0.7 V
- Input span from 10% to 90% of the swing at most 0.3 V
- Differential gain at least 12 dB
- Power at most 250 µW
- 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 differential pairs
- 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.
- LimboA pair only works while its tail transistor stays saturated. Find how much common-mode range your design really has.
- Load BearingUse PMOS diodes as the loads: the gain is no longer a resistor ratio but a transistor ratio.