An AER Analog Silicon Cochlea
Model using Pseudo Floating
Gate Transconductors
Master Thesis in
Electronics and Computer Science,
Microelectronics Programme,
University of Oslo
by:
Hans Kristian Otnes Berge
Thesis assignment

Design and produce an Analog VLSI
Cochlea with an Adress-Event
Representation (AER) output interface,
using Pseudo Floating-Gate Inverters
as transconductors.
The Human Ear



Sound waves set the
middle ear bones in
motion.
Amplification from the
tympanic membrane to
the oval window.
The cochlea converts the
waves into a neural
code.
The Cochlea


Oval window movement causes
a travelling wave on the Basilar
Membrane (BM).
The inner hair cell (IHC)
transforms the motion into a
neural signal.
Unrolled cochlea
Cochlea tuning curves



Large sensitivity range,
almost 6 orders of
magnitude.
A BM site responds best to
a Characteristic Frequency
(CF)
CF’s gradually change from
high at the base to low at
the apex.
Inner Hair Cell (IHC)
Transduction



Molecular chains open ion
channels in the tips of the
stereocilia.
Ions flow through the
channels and activate
voltage gated ion channels
along the IHC lateral wall.
Glutamate is released from
the base into synapses
leading to Auditory Nerves
(AN).
Circuit implementation



Cascaded second order filters approximate basilar membrane response.
Halfwave rectification and integrate&fire neurons approximate IHC&AN response.
Digital circuits handle the output spikes and transmit these on a common bus.
Second order filter


Similar response compared
to BM response curves.
Cascaded second order
sections provide pseudoresonance and steep roll-off.
Second order section using differential
transconductance amplifiers.
Simulation of 32 cascaded second order sections.
Normalized response of several cascaded ideal
second order sections with a gradient of fc’s.
Pseudo Floating-Gate
Second order filter
(Left) Varying the filter cut-off by
varying the bias voltages of the
first and second inverter by the
same amount.
(Right) The second order
section implemented with
Pseudo Floating-Gate
Inverters
(Left) Varying the filter Q by
adjusting the bias voltages of
the first and second inverter
independently.
(Right) Typical mean ac
simulation of 32 cascaded
second order sections.
Pseudo Floating-Gate (PFG)
Inverter
Symbol:



An inverting amplifier.
Transconductance and output
offset is tuned by a pair of
voltages Vp, Vn.
May have many inputs.
V1
VOut
V2
VP
C2
V2
V1
C1
C1
C2
Bias
Circuit
VN
Bias
Circuit
VOut
Pseudo Floating-Gate Inverter
Simplified Small Signal Model
First order filter:
VOut
V1
CL
May use C3 to
increase attenuation
in the stop-band
somewhat. It may
also be used to
attenuate noise at
Vp and Vn nodes.
Mismatch between
C1 and C2 may
affect low-frequency
gain and cut-off
frequency. The
amount of mismatch
shown here is +- 5%
Pseudo Floating-Gate Inverter
Monte Carlo Simulations

Simulating process and
mismatch variation with
zero device correlation
(worst case).
W/L = 50/0.35
W/L = 50/2
Output offset variation between two equally biased
PFG inverters. Short transistors have lower gain.
Variations in lowfrequency gain of a
PFG inverter.
W/L = 50/0.35
Cutoff (-3dB) ratio (fc1/fc2)
between two equally
biased PFG inverters.
The denominator cutoff is
along the x-axis.
W/L = 50/0.35
Pseudo Floating-Gate
Biasing Circuits
Input voltage(s)


Was a major difficulty with the
work in this master project.
Non-linear circuits may exhibit
chargepumping:
Capacitively
coupled inputs
V1
Bias
Circuit
Transistor w/ DC
Operating point
V+
V+
V-
V-
V2
Vn
V
(a)
V
V
(b)
V
V
(c)
(d)
Above: Some earlier proposed bias structures
implementable in CMOS. Below: Two proposed
structures, not directly implementable in CMOS.

For low Vth processes, the
channel current dominates for
a diode-connected transistor
at zero gate-source voltage.
V
Wide swing
V
Low swing
Pseudo Floating-Gate
Biasing Circuits

The employed bias circuit unfortunately failed to operate
correctly, although a very similar structure had been
tried&tested earlier. Later Monte-Carlo simulations showed
that the structure should have a yield of only 15%, mostly
due to Vth mismatch.
VBias
VStep
To readout
Amplifiers
Inner hair cell &
Auditory Nerve Circuit (1)
Voltage-to-Current Rectification
Inner hair cell &
Auditory Nerve Circuit (2)
Leaky integrate and fire neuron
Integration node
potential (Volts)
Simulation results constant input current.
(I&F neuron only).
Measurement results. (V2C rectifier + I&F neuron)
Integration node
potential (Volts)
Output is a spike-train:
Inner hair cell &
Auditory Nerve Circuit (3)
Measurements of V2C rectifier + I&F neuron
Spiking frequency as a function of input DC
voltage.
Maximum spiking frequency as a function of
V2C rectifier bias voltage.
Inner hair cell &
Auditory Nerve Circuit (4)
Measurements of V2C rectifier + I&F neuron
Response of the hair cell to a waveform input.
Adress-Event Representation
(AER) Arbiter



Hair-cell spikes are passed
through an arbitration
hierarchy, which decides
which spike to transmit first.
An adress, signifying the
location of the hair-cell, is
passed on the Data bus.
The transmission
is carried out as
fast as possible.
AER timing diagram
Full
Chip
Inverter
Transistors
Coupling
Capacitors
Bias Circuits
Second
Order
Section
Poly Resistor
Strips and Bias
Voltage Inputs
AER
Sender Logic
Integrate&Fire
Neuron
V2C Rectifier
Clamp
Hair Cell
and
Sender Logic
Capacitive
atttenuation
Results – Filter Cascades




One malfunctioning filter early in the cascade will
lead to a signal loss for the remainder of the cascade.
If we increase the number of filters per bandwidth the
delay increases.
Each filter generates noise, particularly for high Qvalues this may become a problem as the noise is
both amplified and accumulated.
Variations of transconductance, particularly as a
result of variations of threshold voltages, leads to
differences in cut-off and Q-values. This may affect
the pseudoresonance in the cascade strongly, and
also produce variations in the delay.
Results – PFG Inverter





A wide signal swing may be used.
May be used in a large frequency range (from a few
Hz to several GHz), although noise may be a
problem for low and high frequencies.
If more than one PFG inverter uses the same bias
voltages, one needs to control output offset, f.ex. by
using short and wide transistors.
The PFG Bias Circuit that was used for the
implementation did not work, it is recommended that
this design is avoided.
Two possible, improved PFG Bias Circuits have
been identified.
Results – Cochlea Models
Features of the projects scheme:
 Approximated Cochlea Behaviour
+ Filter bank has a low power consumption.
+ Small area required.
÷ Not fault-tolerant.
More Outcomes
 A VerilogA Diode Model suitable for
simulations under reverse bias operation
below breakdown was made and is included
in the thesis.
 An increased understanding of Pseudo
Floating-Gates bias structures was achieved.
A paper is in progress.