Improved Silicon Cochlea 
using 
Compatible Lateral Bipolar Transistors 
Andr6 van Schaik, Eric Fragnire, Eric Vittoz 
MANTRA Center for Neuromimetic Systems 
Swiss Federal Institute of Technology 
CH-1015 Lausanne 
email: vschaik@di.epfi.ch 
Abstract 
Analog electronic cochlear models need exponentially scaled filters. 
CMOS Compatible Lateral Bipolar Transistors (CLBTs) can create 
exponentially scaled currents when biased using a resistive line with a 
voltage difference between both ends of the line. Since these CLBTs 
are independent of the CMOS threshold voltage, current sources 
implemented with CLBTs are much better matched than current 
sources created with MOS transistors operated in weak inversion. 
Measurements from integrated test chips are shown to verify the 
improved matching. 
1. INTRODUCTION 
Since the original publication of the "analog electronic cochlea" by Lyon and Mead in 
1988 [1], several other analog VLSI models have been proposed which try to capture 
more of the details of the biological cochlear function [2],[3],[4]. In spite of the 
differences in their design, all these models use filters with exponentially decreasing cut- 
off frequencies. This exponential dependency is generally obtained using a linear 
decreasing voltage on the gates of MOS transistors operating in weak-inversion. In 
weak-inversion, the drain current of a saturated MOS transistor depends exponentially 
on its gate voltage. The linear decreasing voltage is easily created using a resistive 
polysilicon line; if there is a voltage difference between the two ends of the line, the 
voltage on the line will decrease linearly all along its length. 
6 72 A. VAN SCHAIK, E. FRAGNIfRE, E. VITTOZ 
The problem of using MOS transistors in weak-inversion as current sources is that their 
drain currents are badly matched. An RMS mismatch of 12% in the drain current of two 
identical transistors with equal gate and source voltages is not exceptional [5], even 
when sufficient precautions, such as a good layout, are taken. The main cause of this 
mismatch is a variation of the threshold voltage between the two transistors. Since the 
threshold voltage and its variance are technology parameters, there is no good way to 
reduce the mismatch once the chip has been fabricated. 
One can avoid this problem using Compatible Lateral Bipolar Transistors (CLBTs) [6] 
for the current sources. They can be readily made in a CMOS substrate, and their 
collector current also depends exponentially on their base voltage, while this current is 
completely independent of the CMOS technology's threshold voltage. The remaining 
mismatch is due to geometry mismatch of the devices, a parameter which is much better 
controlled than the variance of the threshold voltage. Therefore, the use of CLBTs can 
yield a large improvement in the regularity of the spacing of the cochlear filters. This 
regularity is especially important in a cascade of filters like the cochlea, since one filter 
can distort the input signal of all the following filters. 
We have integrated an analog electronic cochlea as a cascade of second-order low-pass 
filters, using CLBTs as exponentially scaled current sources. The design of this cochlea 
is based on the silicon cochlea described in [7], since a number of important design 
issues, such as stability, dynamic range, device mismatch and compactness, have already 
been addressed in this design. In this paper, the design of [7] is briefly presented and 
some remaining possible improvements are identified. These improvements, notably the 
use of Compatible Lateral Bipolar Transistors as current sources, a differentiation that 
does not need gain correction and temperature independent biasing of the cut-off 
frequency, are then discussed in more detail. Finally, measurement results of a test chip 
will be presented and compared to the design without CLBTs. 
2. THE ANALOG ELECTRONIC COCHLEA 
The basic building block for the filters in all analog electronic cochlear models is the 
transconductance amplifier, operated in weak inversion. For input voltages smaller than 
about 60 mVpp, the amplifier can be approximated as a linear transconductance: 
Iot = gm(Vra+ - Via.) 
(1) 
with transconductance gm given by: 
Io 
gm= 2nUT (2) 
where Io is the bias current, n is the slope factor, and the thermal voltage UT = kT/q = 
25.6 mV at room temperature. 
This linear range is usually the input range used in the cochlear filters, yielding linear 
filters. In [7], a transconductance amplifier having a wider linear input range is 
proposed. This allows larger input signals to be used, up to about 140mVpp. 
Furthermore, the wide range transconductance amplifier can be used to eliminate the 
large-signal instability shown to be present in the original second-order section [7]. This 
second-order section will be discussed in more detail in section 3.2. 
Improved Silicon Cochlea Using Compatible Lateral Bipolar Transistors 673 
The traditional techniques to improve matching [5], as for instance larger device sizes 
for critical devices and placing identical devices close together with identical 
orientation, are also discussed in [7] with respect to the implementation of the cochlear 
filter cascade. The transistors generating the bias current Io of the transconductance 
amplifiers in the second-order sections were identified as the most critical devices, since 
they have the largest effect on the cut-off frequency and the quality factor of each 
section. Therefore, extra area had to be devoted to these bias transistors. A further 
improvement is obtained in [7] by using a single resistive line to bias both the 
transconductance amplifiers controlling the cut-off frequency and the transconductance 
amplifier controlling the quality factor. The quality factor Q is then changed by varying 
the source of the transistor which biases the Q control amplifier. Instead of using two 
tilted resistive lines, this scheme uses only one tilted resistive line and a non-tilted Q 
control line, and therefore doesn't need to rely on an identical tilt on both resistive lines. 
3. IMPROVED ANALOG ELECTRONIC COCHLEA 
The design discussed in the previous section already showed a substantial improvement 
over the fu'st analog electronic cochlea by Lyon and Mead. However, several 
improvements remain possible. 
3.1 VT VARIATION 
The bias transistors have been identified as the major source of mismatch of the 
cochlea's parameters. This mismatch is mainly due to variation of the threshold voltage 
V-r of the MOS transistors. Since the drain current of a saturated MOS transistor in 
weak-inversion depends exponentially on the difference between its gate-source voltage 
and its threshold voltage, small variations in V-r introduce large variations in the drain 
current of these transistors, and since both the cut-off frequency and the quality factor of 
the filters are proportional to these drain currents, large parameter variations are 
generated by small VT variations. This problem can be circumvented by the use of 
CMOS Compatible Lateral Bipolar transistors as bias transistors. 
A CMOS Compatible Lateral Bipolar Transistor is obtained if the drain or source 
junction of a MOS transistor is forward-biased in order to inject minority carriers into 
the local substrate. If the gate voltage is negative enough (for an n-channel device), then 
no current can flow at the surface and the operation is purely bipolar [6]. Fig. 1 shows 
the major flows of current carriers in this mode of operation, with the source, drain and 
well terminals renamed emitter E, collector C and base B. 
Vsc < 0 
Sub B  ( E G C 
Isub Is VSE > 0 .Ic 
P 
electrons n 
G Sub 
o 
B 
Fig. 1.  Bipolar operation of the MOS transistor' carrier flows and symbol. 
6 74 A. VAN SCHAIK, E. FRAGNI.RE, E. VITTOZ 
Since there is no p+ buried layer to prevent injection to the substrate, this lateral npn 
bipolar transistor is combined with a vertical npn. The emitter current I. is thus split 
into a base current IB, a lateral collector current Ic and a substrate collector current Isub. 
Therefore, the common-base current gain ct = -Ic/I. cannot be close to 1. However, due 
to the very small rate of recombination inside the well and to the high emitter efficiency, 
the common-emitter current gain [5 = Ic/IB can be large. Maximum values of ct and [5 are 
obtained in concentric structures using a minimum size emitter surrounded by the 
collector and a minimum lateral base width. 
For Vc. = VBE'VBc larger than a few hundred millivolts, this transistor is in active mode 
and the collector current is given, as for a normal bipolar transistor, by 
Ic = Isb e Ur (4) 
where Isb is the specific current in bipolar mode, proportional to the cross-section of the 
emitter to collector flow of carriers. Since Ic is independent of the MOS transistor 
threshold voltage VT, the main source of mismatch of distributed MOS current sources is 
suppressed, when CLBTs are used to create the current sources. 
[ 
n+ poly-Si 
Fig. 2. CLBT cascode circuit (a) and its layout Co). 
A disadvantage of the CLBT is its low Early voltage, i.e., the device has a low output 
resistance. Therefore, it is preferable to use a cascode circuit as shown in fig. 2. This 
yields an output resistance several hundred times larger than that of the single CLBT, 
whereas the area penalty, in a layout as shown in fig 2b, is acceptable. 
Another disadvantage of the CLBTs, when biased using a resistive line, is their base 
current, which introduces an additional voltage drop on the resistive line. However, 
since the cut-off frequencies in the cochlea are controlled by the output current of the 
CLBTs and since these cut-off frequencies are relatively small, typically 20 kHz, the 
output current of the CLBTs will be small. If the common-emitter current gain [5 is 
much larger than 1, the base current of these CLBTs will be very small, and the voltage 
error introduced by the small base currents will be negligible. Furthermore, since the 
cut-off frequencies of the cochlea will typically span 2 decades with an exponentially 
decreasing cut-off frequency from the beginning to the end, only the first few filters will 
have any noticeable influence on the current drawn from the resistive line. 
3.2 DIFFERENTIATION 
The stabilized second-order section of [7] uses two wide range transconductance 
amplifiers (A1 and A2 in fig. 3) with equal bias current and equal capacitive load, to 
control the cut-off frequency. A basic transconductance amplifier (A3) is used in a 
Improved Silicon Cochlea Using Compatible Lateral Bipolar Transistors 675 
feedback path to control the quality factor of the filter. The voltage Vout at the output of 
each second-order stage represents the basilar membrane displacement. Since the output 
of the biological cochlea is proportional to the velocity of the basilar membrane, the 
output of each second-order stage has to be differentiated. In [7] this is done by creating 
a copy of the output current I of amplifier A2 at every stage. Since the voltage on a 
capacitor is proportional to the integral of the current onto the capacitor, Ia is 
effectively proportional to the basilar membrane velocity. Yet, with equal displacement 
amplitudes, velocity will be much larger for high frequencies than for low frequencies, 
yielding output signals with an amplitude that decreases from the beginning of the 
cochlea to the end. This can be corrected by normalizing I to give equal amplitude at 
every output. A second resistive line with identical tilt controlling the gain of the current 
mirrors that create the copies of I at each stage is used for this purpose in [7]. 
However, if using a single resistive line for the control of the cut-off frequencies and the 
quality factor improves the performance of the chip, the same is true for the control of 
the current mirror gain. 
Output 
Vf L 
Av. vout to next section 
V V __ I 
from pre . 
section 
Fig. 3. One section of the cochlear cascade, with differenfiator. 
An alternative solution, which does not need normalization, is to take the difference 
between Vot and V (see fig. 3). This can be shown to be equivalent to differentiating 
Vot, with 0dB gain at the cut-off frequency for all stages. This can be easily done with a 
combination of 2 transconductance amplifiers. These amplifiers can have a large bias 
current, so they can also be used to buffer the cascade voltages before connecting them 
to the output pins of the chip, to avoid charging the cochlear cascade with the extra 
capacitance introduced by the output pins. 
3.3 TEMPERATURE SENSITIVITY 
The cut-off frequency of the first and the last low-pass filter in the cascade can be set by 
applying voltages to both ends of the resistive line, and the intermediate filters will have 
a cut-off frequency decreasing exponentially from the beginning to the end. Yet, if we 
apply directly a voltage to the ends of the resistive line, the actual cut-off frequency 
obtained will depend on the temperature, since the current depends exponentially on the 
applied voltage normalized to the thermal voltage U-r (see(3). It is therefore better to 
create the voltages at both ends of the resistive line on-chip using a current biasing a 
CLBT with its base connected to its collector (or its drain connected to its gate if a MOS 
transistor is used). If this gate voltage is buffered, so that the current through the 
resistive line is not drawn from the input current, the bias currents of the first and last 
filter, and thus the cut-off frequency of all filters can be set, independent of temperature. 
6 76 A. VAN SCHAIK, E. FRAGNI.RE, E. VITTOZ 
3.4 THE IMPROVED SILICON COCHLEA 
The improved silicon cochlea is shown in figure 4. It uses the cochlear sections shown 
in figure 3, CLBTs as the bias transistors of each filter, and one resistive line to bias all 
CLBTs. The resistive line is biased using two bipolar current mirror structures and two 
voltage buffers, which allow temperature independent biasing of the cut-off frequencies 
of the cochlea. A similar structure is used to create the voltage source Vq to control, 
independent of temperature, the actual quality factor of each section. The actual bipolar 
current mirror implemented uses the cascode structure shown in figure 2a, however this 
is not shown in figure 4 for clarity. 
4' 4' Vdiff, 4' 
__4 CochlearL .Cochlear_. _lCochlear I 
Ic Section l ..... / Seaon / .... l Section I 
resistive line 
4. TEST RESULTS 
Fig 4. The improved silicon cochlea. 
The proposed silicon cochlea has been integrated using the ECPD15 technology at ES2 
(Grenoble, France), containing 104 second-order stages, on a 4.77mm X 3.21ram die. 
Every second stage is connected to a pin, so its output voltage can be measured. In fig. 5, 
the frequency response curves after on-chip derivation are shown for the output taps of 
both the cochlea described in [7] (left), and our version (righ0. This clearly shows the 
improved regularity of the cut-off frequencies and the gain obtained using CLBTs. The 
drop-off in gain for the higher frequency stages (right) is a border effect, since at the 
beginning of the cochlea no accumulation of gain has yet taken place. In the figure on 
the left this is not visible, since the first nine outputs are not presented. 
..., 10 10 
-I0 1 
-20 
-40 .40 
100 n-y (Hz) ! 0000 200 2o00 20o00 
Fig.5. Measured frequency responses at the different taps. 
In fig. 6 we show the cut-off frequency versus tap number of both chips. Ideally, this 
should be a straight line on a log-linear scale, since the cut-off frequency decreases 
Improved Silicon Cochlea Using Compatible Lateral Bipolar Transistors 677 
exponentially with tap number. This also clearly shows the improved regularity using 
CLBTs as current sources. 
100430 
1OO0 
10 15 20 25 30 
0 10 20 30 40 50 
Fig.6. Cut-off frequency (Hz) versus tap number for both silicon cochleae. 
5. CONCLUSIONS 
Since the biological cochlea functions as a distributed filter, where the natural frequency 
decreases exponentially with the position along the basilar membrane, analog electronic 
cochlear models need exponentially scaled filters. The output current of a Compatible 
Lateral Bipolar Transistor depends exponenfiMly on the base-emitter voltage. It is 
therefore easy to create exponentially scaled current sources using CLBTs biased with a 
resistive polysilicon line. Because the CLBTs are insensitive to variations of the CMOS 
threshold voltage Vx, current sources implemented with CLBTs are much better 
matched than current sources using MOS transistors in weak inversion. 
Regularity is further improved using an on-chip differentiation that does not need a 
second resistive line to correct its gain, and therefore doesn't depend on identical tilt on 
both resistive lines. Better independence of temperature can be obtained by fixing the 
frequency domain of the cochlea using bias currents instead of voltages. 
Acknowledgments 
The authors would like to thank Felix Lustenberger for simulation and layout of the 
chip. We are also indebted to Lloyd Watts for allowing us to use his measurement data. 
References 
[1] 
[2] 
[3] 
R.F. Lyon and C.A. Mead, "An analog electronic cochlea," IEEE Trans. Acoust., 
Speech, Signal Processing, vol. 36, pp. 1119-1134, July 1988. 
R.F. Lyon, "Analog implementations of auditory models," Proc. DARPA Workshop 
Speech and Natural Language. San Mateo, CA:Morgan Kaufmann, 1991. 
W. Liu, et. al., "Analog VLSI implementation of an auditory periphery model," 
Advances Res. VLSI, Proc. 1991 Santa Cruz Conf, MIT Press, 1991, pp. 153-163. 
[4] L. Watts, "Cochlear Mechanics: Analysis and Analog VLSI," Ph.D. thesis, 
California Institute of Technology, Pasadena, 1992. 
[5] 
[6] 
[7] 
E. Vittoz, "The design of high-performance analog circuits on digital CMOS 
chips," IEEE J. SolM-State Circuits, vol. SC-20, pp. 657-665, June 1985. 
E. Vittoz, "MOS transistors operated in the lateral bipolar mode and their 
application in CMOS technology," IEEE J. Solid-State Circuits, vol. SC-24, pp. 
273-279, June 1983. 
L. Watts, et. al., "Improved implementation of the silicon cochlea," IEEE J. Solid- 
State Circuits, vol. SC-27, pp. 692-700, May 1992. 
