PROBLEM 010 · Easy · The source follower (common-drain stage)
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Source follower
The problem
Build a source follower: an NMOS with its drain on VDD, its gate driven by the input, and the output taken from its source. A current sink from the output to ground sets its bias; use a current source or a resistor.
The follower’s gain is below 1 because of the transistor’s body effect, and its output sits a gate-source voltage below the input. The point of the circuit is not gain but a low output impedance that can drive a load: here, a capacitor.
The test bench
- Supply: VDD = 1.8 V.
- The test bench hangs a 200 fF load capacitor on the net named “out”.
- It applies the AC stimulus to the input source named “in”; you set that source’s DC level.
Numbers to hit
- Gain between -2.5 dB and 0 dB
- Bandwidth (−3 dB) at least 400 MHz
- Output DC level between 0.3 V and 1.2 V
- Power at most 150 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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- Common SenseTurn one transistor and a resistor into a voltage amplifier.
- Diode TaxReplace the load resistor with a transistor and see how gain becomes a ratio of sizes.
- Mid-Rail RebelAn NMOS and a PMOS driven together make a high-gain stage, if you can bias it.
- Gate KeeperDrive the source instead of the gate: a non-inverting stage with a low input resistance.
- Stack AttackStack a second transistor on top to multiply the output resistance, then use it with a big load.
- DegenerateA resistor in the source trades gain for a gain that no longer depends on the transistor.