PROBLEM 008 · Easy · Common-source stage with a diode-connected load
Diode Tax
Common-source with a diode-connected load
The problem
Build a common-source amplifier whose load is a diode-connected PMOS: its source on VDD, and its gate tied to its own drain, which is the output. The NMOS driver has its gate on the input and its source on ground.
Resistors are expensive on a chip, and a diode-connected transistor is a load that sets its own operating point. The gain of this stage is the ratio of the driver’s transconductance to the load’s, so it depends on the two transistors’ sizes and not on a resistor you chose.
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
- Voltage gain at least 6 dB
- Bandwidth (−3 dB) at least 80 MHz
- Output DC level between 0.4 V and 1 V
- Power at most 100 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 single-stage amplifiers
- Common SenseTurn one transistor and a resistor into a voltage amplifier.
- Mid-Rail RebelAn NMOS and a PMOS driven together make a high-gain stage, if you can bias it.
- Follow the LeaderA buffer with gain just under one: bias it so it is fast and stays in saturation.
- 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.