Telescope

The problem

Build a telescopic cascode: from ground upward, an NMOS driver (gate on the signal), an NMOS cascode (gate “vbn”), a PMOS cascode (gate “vbp1”) and a PMOS current source (source on VDD, gate “vbp2”). The output is the node between the two cascodes, “out”.

The stage has so much gain that nothing could set its output voltage by hand, so it biases itself: a 1 GΩ resistor from the output to the gate lets the DC level settle by feedback, and a 10 µF capacitor couples the AC input to the gate while keeping the source’s DC level out of the way. The resistor is so large that the feedback has no effect on the signal.

Cascoding both sides multiplies both resistances by about gm·ro, so the gain reaches 60 to 80 dB from a single stage. The price is headroom: four transistors share the 1.8 V supply, so every bias voltage must be chosen with care.

The test bench

• Supply: VDD = 1.8 V.
• The test bench hangs a 200 fF load capacitor from “out” to ground and applies the AC stimulus to “in” only.
• Set the “in” source to 0 V: the 10 µF coupling capacitor blocks DC.

Numbers to hit

• ◆Gain at least 62 dB
• ◆Unity-gain frequency at least 150 MHz
• ◆Output DC level between 0.55 V and 1.1 V
• ◆Power at most 200 µW
• ◆Every transistor in saturation