Problems
Learn analog design one circuit at a time. Each problem starts from an empty canvas.
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Start here · Device fundamentals
Bias a transistor for efficiency
Beginner
Device fundamentals
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How one transistor behaves: its curves, its gain, and the headroom it needs.
- 001Sweet SpotBias a transistor for efficiencyBeginner
- 002Self-TiedA diode-connected transistor as a 0.68 V referenceBeginner
- 003Upside DownBias a PMOS for 50 µABeginner
- 004Long Story ShortIntrinsic gain: how long should the channel be?Easy
- 005Fake ResistorA transistor as a resistorEasy
- 006Barely OnWeak inversion: the most gm per microampMedium
Single-stage amplifiers
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One transistor stage at a time: gain, load, bias and bandwidth.
- 007Common SenseCommon-source amplifierBeginner
- 008Diode TaxCommon-source with a diode-connected loadEasy
- 009Mid-Rail RebelSelf-biased CMOS inverter amplifierMedium
- 010Follow the LeaderSource followerEasy
- 011Gate KeeperCommon-gate amplifierEasy
- 012Stack AttackCascode amplifierMedium
- 013DegenerateCommon-source with source degenerationEasy
- 014Mirror UniversePMOS common-source amplifierBeginner
- 015Fat LoadCommon-source with a current-source loadMedium
- 016Double StackCascode amplifier with a current-source loadHard
- 017TelescopeTelescopic cascode amplifierHard
- 018Fold ItFolded-cascode amplifierExpert
Current mirrors
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Copying currents accurately, and how output resistance limits it.
- 019Copy PasteBasic current mirrorEasy
- 020Copy Paste ProCascode current mirrorMedium
- 021Mirror, MirrorPMOS current mirrorEasy
- 022Double VisionA mirror with two outputs: 1× and 2×Easy
- 023Ten to OneWidlar current source: 10 µA from a 100 µA referenceMedium
- 024WilsonWilson current mirrorHard
- 025Resistor ArmorCurrent mirror with source degenerationMedium
- 026Gain BoostRegulated (gain-boosted) cascode mirrorExpert
- 027Turn the CornerNMOS mirror into PMOS mirrorMedium
- 028One Percent ClubA mirror accurate to 1%Medium
References and bias
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Bias currents and voltages that ignore the supply, the process and the temperature.
- 029Poor Man's BiasA bias current from a resistorEasy
- 030Diode DuoA 1.4 V reference from two diode-connected transistorsEasy
- 031BootstrapSelf-biased current referenceExpert
- 032Just EnoughA bias voltage for a cascode: Vgs plus a littleMedium
- 033Cold DiodeA diode-connected PNP: a voltage that falls with temperatureEasy
- 034Hot CurrentA current proportional to absolute temperatureHard
- 035Thermal ZenA bandgap reference: 1.2 V that ignores temperatureExpert
Differential pairs
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Amplifying a difference and rejecting what the inputs share.
- 036Two's CompanyDifferential pair with resistor loadsMedium
- 037Pair, FlippedPMOS differential pairMedium
- 038Tail RiskA pair with a real tail: reject the common modeHard
- 039Slow DownSource-degenerated differential pairMedium
- 040LimboHow low can the inputs go?Hard
- 041Load BearingPair with diode-connected PMOS loadsMedium
- 042Mirror TailA pair biased by a current mirrorMedium
- 043Tipping PointFull steering: how little input switches the pair?Medium
Operational amplifiers
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Whole amplifiers: gain, speed, stability and power together.
- 044Five AliveFive-transistor OTAHard
- 045Miller TimeTwo-stage Miller OTAExpert
- 046FlippedOTA with a PMOS input pairHard
- 047Sip PowerMicropower OTAMedium
- 048Need for SpeedA fast OTAHard
- 049Gain HoarderA high-gain OTAMedium
- 050StargazerTelescopic cascode OTAExpert
- 051Fold & HoldFolded-cascode OTAExpert
- 052Heavy LifterA buffered OTA driving 1 kΩHard
- 053Twice as NiceA current-mirror OTA with a gain of twoExpert
- 054Speed LimitSlew rate of a five-transistor OTAMedium
Frequency response
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Poles, zeros, the Miller effect and how to compensate an amplifier.
- 055Miller's TaxThe Miller effect on a common-source stageMedium
- 056Zero HeroBandwidth from a zero: the source RC networkMedium
- 057Seventy DegreesPhase margin 70°: place the second poleHard
- 058Heavy CapStable into 10 pFExpert
- 059Two-Stage TurboA fast two-stage amplifierExpert
- 060Penny PincherA two-stage amplifier on 50 µWHard
- 061Peak PerformanceBandwidth from an inductor: shunt peakingHard
- 062Tune InA tuned amplifier at 2 GHzHard
Feedback and stability
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Closing the loop: accuracy, loop gain and phase margin.
- 063Times TenNon-inverting amplifier: gain of 10Beginner
- 064Short on GainHit 14 dB with only 40 dB of op-amp gainEasy
- 065Minus TenInverting amplifier: gain of −10Easy
- 066Ramp MakerAn integrator with a 100 kHz unity-gain frequencyMedium
- 067Light CatcherTransimpedance amplifier for a photodiodeHard
- 068No Ringing ZoneGain of 2 into 100 pF without ringingHard
Output stages and buffers
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Driving a load: followers, class A and class AB.
- 069Heavy LoadA source follower into 1 kΩMedium
- 070Stiff OutputA follower with an output resistance under 150 ΩMedium
- 071Level UpA PMOS follower: a level shifter that goes upMedium
- 072Gain & CarryA gain stage with a buffer for a 1 kΩ loadHard
- 073Push & PullA push-pull follower: drive 1 kΩ from a small quiescent currentHard
Regulators
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Turning a noisy supply into a quiet one.
Noise and linearity
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What limits the smallest and the largest signals an amplifier can handle.
Comparators
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Deciding which of two voltages is larger, quickly and without hesitation.
Oscillators
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Making a clock from nothing but gain and delay.
- 089Ring RingA three-stage ring oscillatorEasy
- 090High FiveA five-stage ring near 700 MHzMedium
- 091StarvedA current-starved VCOHard
- 092Slow RingA 200 MHz ring: slow it down with capacitorsMedium
- 093Lucky SevenA seven-stage ring near 400 MHzMedium
- 094Tank CircuitA cross-coupled LC oscillator near 2 GHzExpert
Switched-capacitor and converters
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Sampling, charge transfer and the building blocks of data converters.
- 095Switch OnAn NMOS sampling switchMedium
- 096Both WaysA transmission gate for a high inputMedium
- 097Tick TockA switched-capacitor integrator: +50 mV per cycleExpert
- 098Binary LadderA 3-bit R-2R DACEasy
- 099Eight StepsA resistor string for eight voltage levelsBeginner
- 100Charge ThiefA sampling switch with a dummyHard
Troubleshooting
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A circuit that almost works. Find the fault and fix it.
Ready for a real contest? Try the ranked challenge (expert: size a two-stage Miller OTA for the best figure of merit).
Each stage opens after you solve 3 problems in the one before it, or when you choose to skip ahead.