Heavy Lifter

The problem

Build a five-transistor OTA (an NMOS pair on a tail Current source with a PMOS mirror load; “inp” on the diode side) and follow its output with a source follower: an NMOS with its gate on the OTA’s output node, its drain on VDD and its source on “out”, biased by a Current source to ground. The test bench connects a 1 kΩ load from “out” to ground, and its feedback resistor ties “out” to “inn” for biasing.

The OTA alone would see a 1 kΩ load against an output resistance of hundreds of kilohms: all the gain would be lost. The follower’s output resistance of a hundred ohms or so makes the load a small fraction of the stage’s gain loss. The price is that its input sits one Vgs above its output.

The test bench

• Supply: VDD = 1.8 V.
• The test bench connects a 1 kΩ load from “out” to ground, adds a 1 pF capacitor on “out”, and ties “out” to “inn” through 1 GΩ (with a 1 F capacitor from “inn” to a quiet source) so that the amplifier biases itself but is open-loop for AC. The AC stimulus is on “inp”.

Numbers to hit

• ◆DC gain into 1 kΩ at least 28 dB
• ◆Unity-gain bandwidth at least 5 MHz
• ◆Phase margin at least 50 °
• ◆Power at most 2200 µW
• ◆Every transistor in saturation