PROBLEM 041 · Medium · Active loads that set the gain by ratio
Load Bearing
Pair with diode-connected PMOS loads
The problem
Build a differential pair of two NMOS with a tail Current source to ground, and load each drain with a diode-connected PMOS (source on VDD, gate and drain together on the output). The drains are “outp” and “outn”; the inputs “inp” and “inn”.
A diode-connected load has a resistance of 1/gm_p. The gain is therefore gm_n/gm_p, set by a ratio of transistor sizes: it tracks the process and the temperature, and it needs no resistor. To get a gain above 1 the PMOS must be much narrower than the NMOS, so that for the same current it has a smaller gm.
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.
Numbers to hit
- Differential gain between 8 dB and 16 dB
- Bandwidth (−3 dB) at least 100 MHz
- Output common-mode level between 0.4 V and 1.1 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.
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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.
- LimboA pair only works while its tail transistor stays saturated. Find how much common-mode range your design really has.
- Mirror TailReal tails come from mirrors. Take a 25 µA reference, scale it to 100 µA, and use it to bias the pair.