Adjustable Linear Range Operational Transconductance Amplifier with Noise Compensation

1
Adjustable Linear Range Operational
Transconductance Amplifier with Noise Compensation
Brian Ginsburg, Muyiwa Ogunnika

• Overall topology
• Basic WLR with bulk drive, gate degeneration,
  source degeneration
• Variable gain amplifier between drain and gate of
  the source degeneration transistor to lower
  source degeneration
• Gate of input transistor driven with a weighted
  sum of its drain (for gate degeneration) and the
  input voltage
• Current steering sets the VL and is compensated
  to keep noise low
• Theoretical linear range
• Actual linear range varies
• from 73mV to 1.25 V
• N varies from 5.5 to 13.7

2
Block Diagram
0ltAlt1
3
Variable Linear Range Performance

• Linear range varies from 73mV to 1.248V
• Each OTA has 45 transistors
• Common mode input range is from 1.25V to 3 V at
  highest current levels increases to 0.85V-3V at
  low bias current levels
• In resonant filter, f90 can vary from 100Hz to
  10kHz, and Q can be variable from 1 to 6, though
  not for all VL

4
Stability of the OTA

• No load capacitor
• 1kHz bandwidth
• 100mV linear range
• Unity-gain feedback
• lt5 overshoot in step response
• OTA also stable with 10 component mismatch
• Power with 5pF cap and 10kHz bandwidth lt3.47µW

5
Circuit Details
Gate Drive Weighted Adder
Variable Gain Amplifier
6
Noise Impact of VL Variation
Effective number of noise sources from gate drive
circuit
Theoretical N vs. VL
Dashed Fixed kb Solid Variable kb

• Observations
• For fixed VL, as IB drops, noise improves for
  fixed IBS
• As VL decreases, noise shoots up
• Solutions
• For fixed VL, have IBSkbIB
• Make kb inversely dependent on VL
• IBS(16(1-A))IB/20
• Power overhead varies from 30 to 130

Actual N vs. VL

• At Q2, 10kHz
• VL1.25, N5.4
• 21.3µW
• 0.4dB variation from unity in passband, gt50dB
  attenuation at high frequencies

7
Final Layout

• Fits in less than one quarter of the chip!
• Most of the additional circuitry is kept very
  small
• Conclusions
• Questions