PROBLEM 037 · Medium · The PMOS-input pair
Pair, Flipped
PMOS differential pair
The problem
Build a differential pair from two matched PMOS transistors whose sources join at a tail node fed by a Current source from VDD. Each drain has a load resistor to ground. The drains are the differential output, “outp” (from the transistor whose gate is “inp”) and “outn”.
An NMOS pair needs its inputs well above ground (at least Vgs + Vdsat of the tail); a PMOS pair needs them well below VDD. A PMOS pair is the standard choice when the input signal sits close to ground, and complements an NMOS pair when an amplifier must accept inputs over the whole supply.
The test bench
- Supply: VDD = 1.8 V.
- The test bench hangs a 200 fF load capacitor from each of “outp” and “outn” to ground and applies the AC stimulus to “inp” only. It measures the differential output, “outp” minus “outn”.
Numbers to hit
- Differential gain at least 12 dB
- Bandwidth (−3 dB) at least 40 MHz
- Output common-mode level between 0.25 V and 0.7 V
- Output DC imbalance at most 20 mV
- 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.
- 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.
- Mirror TailReal tails come from mirrors. Take a 25 µA reference, scale it to 100 µA, and use it to bias the pair.