Biomedical Engineering Reference
In-Depth Information
FIgURE 7.6:
First-stage amplifier.
(CMOS) process. Figure
7.6
shows the schematic of the preamplifier and the second stage is in
Figure
7.7
. The amplifier designs are based on the work of Reid Harrison [
49
].
The first-stage midband gain,
A
M,
is -
C
1
/
C
2
, the lower corner frequency is at ω
1
≈ 1/(
RC
2
),
and the higher corner frequency is at ω
2
≈
g
m
C
2
/(
C
L
C
1
), where
g
m
is the transconductance of the
operational transconductance amplifier (OTA) shown in the lower portion of Figure
7.5
[
50
]. To
obtain a low cutoff frequency, a large resistance is needed in the feedback loop provided by two
diode-connected transistors, Ma and Mb, acting as “pseudo-resistors” without sacrificing die area.
Noise performance is important in biological preamplifier designs, it is minimized by care-
fully choosing the width and length of the transistors. However, there is a trade-off between stabil-
ity and low noise. Because low noise is critical in this application, decreasing the transconductance
of M1, M2, M7, M8, and the four transistors of the two Wilson current mirrors can minimize the
thermal noise. Flicker noise, which is important in low-frequency applications, is minimized by
increasing the device area. Because p type metal oxide semiconductor devices exhibit less flick noise
than n type, p type is used in the input pair.
However, the typical neural signal amplitude is on the order of tens to hundreds of microvolts,
the output voltage of the preamplifier is still too small for signal processing by an A/D converter.
Therefore, a second-stage amplifier is required [
51
], in which
C
1
/
C
3
determines the midband gain,
