A
MOS
D
IFFERENTIAL
A
MPLIFIER
OSCILLATOR
Cher

Shiung Tsai
,
Jia

Ming Wu,
Ming

Yi Hsieh,
Chun

Chieh Liao,
Tien

Hung Chang,
Kwang

Jow Gan,
Dong

Shong Liang,
Yaw

Hwang Chen
,
Chia

Hung Chen
,
Chun

Ming Wen
Department of Electronic Engineering
Kun Shan Unive
rsity of Technology
Tainan
,
Taiwan
710, R.O.C.
A
BSTRACT
In this thesis, we present an oscillator
mainly composed by a MOS differential
amplifier. We use H

spice to verify the
differe
ntial amplifier oscillator at 9
8
0 MHz
s
uccessfully under CIC 0.18um

Si process
parameters. We also use discrete devices on
bread board to prove the circuit is an oscillator
circuit. The experiment shows such oscillator
c
an work stably
from 1.
0
5
volts to
3.3
volts
supply voltage.
W
hen supply vol
tage is
close to
3
.3
volts, the output frequency
will be more than
20
MHz. The differential amplifier oscillator can
start oscillat ing at low volta
ge
when supply
voltage is only 1.05
volts and
output
frequency
is
about
426
K
Hz. We use FFT (Fast Fourier
Tra
nsform) diagram to analyze the oscillator and
shows the oscillator is with low noise
characteristic. Finally, those experiment
al
results
reveal that
the oscillator is also a
n
excellent
voltage contro
l
l
ed
oscillator (VCO).
Keyword:
differential amplifier, V
CO, FFT.
1.
I
NTRODUCTION
We use the high input resistance, high
output resistance and high voltage gain
characteristics of MOS differential amplifier
[1

3] to create an oscillator. Such oscillator is
based on differential amplifier have two outputs,
one
output is high voltage state and the other
will be in low voltage state. We connect one
CMOS inverter and two CMOS inverters
after
two different outputs. It is an asymmetric
structure and most output waveform tends to be
square waveform. The oscillator fre
quencies are
decided by resistors values,
parallel NMOS
numbers
and CMOS inverter
s
time delay. In this
thesis, we present a different type oscillator and
use experimental results to prove such oscillator
is useful, easiness and flexibility in design.
2.
C
IRCUIT
THEOREM
AND
SIMULATION
The oscillator is
composed of
MOS
differential amplifier
by adding three CMOS
inverters
as
shown in Fig.1. A formal differential
amplifier needs a constant current source but we
use resistor R3 to replace constant current so
urce
for simplicity. According to differential amplifier
operation, transistor M1 and M2 can
’
t be off in
the same time. Transistors M1, M2 will both be
in on state
(
saturation)
or one is on and the other
is off.
In
the meanwhile
,
transistor M1 or
M2
can
’
t
be in t
riode
state.
Because we can
’
t
fabricate t wo completely equalized transistors
M1 and M2, so most conditions are M1 on
(saturation)
and M2 off, or M1 o
ff
and M2 is
on
(saturation)
.
In Fig.1 we assume M1 on and M2 off, so
voltage OP1 is in low state a
nd voltage OP2 is in
high state. In the meanwhile,
G
1 voltage is high
and
G
2 voltage is low. After CMOS inverter
(INV1) t ime delay, voltage
G
2 becomes high
state to turn on transistor M2, so voltage OP2
changes into low state. After CMOS inverters
(INV2, I
NV3) time delay, voltage
G
1 changes
into low state to turn off transistor M1. Base on
same analysis, M2 will be off and M1 will be on
in the next run. After a fixed period, M1 and M2
will toggle their states. Such on/off continuous
switching phenomena will
cause oscillation. The
nodes status in Fig.1 also shows H/L (High or
Low) state.
Fig.1
The
MOS
d
ifferential amplifier oscillator.
We use CIC 0.18um

Si
process par
ameters
to run simulation of MOS
differential amplifier
oscillator. Under
2 volts operation voltage and
resistor R1 is 1.2 K
Ω
, R3 is 0.22 K
Ω
then
the
output oscillat ion frequency is
merely
9
8
0 MHz
as shown in Fig.2.
Fig.2
Output w
aveform of simulation result.
3.
E
XPERIMENTAL
RESULTS
In this thesis, we use Tektronix TDS3034B
o
scilloscope to measure oscillator circu
it and
fast Fourier transform (FFT
) diagram. The
discrete devices are N
MOS
transistors M1
(
M2
),
resistors R1
(
R3
)
and
CMOS
inverter
s
.
We
can
’
t
buy
a discrete NMOS transistor
M1 (or M2)
. S
o
particularly
,
w
e take CMOS
i
nverter
(MM74HC04N)
out
put as
drain electrode
,
CMOS
inverter
in
put
as
gate electrode
, CMOS
inverter
ground as source
electrode
and let
CMOS
inverter
V
DD
open.
T
he transformation
is
shown in Fig.3
.
We put
all
these discrete devices
on bread board and measur
e output signals.
Fig.3
CMOS inverter transforms into NMOS
transistor M1 (or M2)
.
T
he output waveform
under
1.0
5
volts
supply voltage and oscillat ion frequency is
425.9
K
Hz
as shown in Fig.4
.
M1 (or M2) is composed
of
three
pa
rallel NMOS transistors.
Fig.4
Output waveform under 1.0
5 volts
.
(
V
DD
:
OPEN)
Gate
(Input)
Drain
(Output)
Source
(Ground
: OPEN
)
(
PMOS
: Idle)
NMO
S
M1
(
or
M2)
Vcc
R
1
R3
OP1
L/H
R
1
OP2
H/L
CMOS
INV1
H/L
CMOS INV
2
M1
M2
CMOS
INV3
H/L
G1
H/L
G
2
*L/H
OUTPUT
* :
Change
State
R1=4.7 K
Ω
R3=0.83 K
Ω
Fig.5
shows
oscillator with different
resistor under
3.3
volts supply voltage,
output
frequency is
20.51
MHz
and M1 (or M2) is
composed of
six
parallel NMOS transistors
.
Fig.5
Output
waveform under 3.3
volts.
Fig.6
is the
typical
fast Fourier transform
(FFT
) diagram of
the MOS differential amplifier
oscillator
.
Fig.
6
show
s the oscillator
with
low
no
ise characteristics from 0 to 9
MHz
and it
s
output frequency is about 4.2
MHz
. The hig
hes
t
signal is more than the other
signals
about
35
db
in Fig.
6
.
I
t means
the main
oscillat ion signal is
fifty

six
(10
3
5
/20
=
56.2) times stronger than the
other
signals. If
the
main signal is 1.5 volts then
the other
s
signals shall be s maller than 0.0
27
vol
ts.
But
most conditions are noise signals will
become larger as output frequency increases in
the
MOS differential amplifier oscillator.
Fig.6
Typical FFT diagram of
the
MOS
differential amplifier oscillator.
Fig.
7
s
hows resi
stor R3
(0.83
K
Ω
)
and
transistor M1 (M2) unchanged but R
1
is changed
from
3.3
K
Ω
to
4.7
K
Ω
.
M1 (M2) is composed
of
six
parallel NMOS transistors.
Fig.7 reveals
smaller R1 will have higher output frequency.
Fig.
7
Oscillator
output
frequencies
under
different
R1
and
supply
voltage
s
.
Fig.
8
s
hows resistor R1
(3.3K
Ω
)
and
transistor M1 (M2) unchanged but R
3
is changed
from
0.83
K
Ω
to
3.3
K
Ω
. M
1 (M2) is composed
of
six
parallel NMOS transistors.
Fig.8
reveals
smaller R3 will have higher output frequ
ency.
Fig.
8
Oscillator
output
frequencies under
different
R3
and
supply
voltage
s
.
Fig.9
sh
ows resistor R1
(4.7K
Ω
)
and R3
(0.83
K
Ω
)
unchanged but transistor M1 (
or
M2)
is
changed
. Their
parallel NMOS
transistor
s
M1
(
or
M2)
are
three
and
six
respectively.
The
six
parallel
transistor
s
ha
ve
great improvement in
output
frequency because of
much
larger drain
current
.
The effect of M1
(
or
M2)
is
more
useful
than those effects of R1 and R3.
R 1 = 3.6 2 K
Ω
R 3 = 0.7 K
Ω
R 1 = 4.2
K
Ω
R 3 = 1.6 9 K
Ω
Vcc=2.9 vol t s
M1 ( o r M2 )
= T h r e e
p a r a l l e l N MOS.
Fig.9
Oscillator
output
frequencies under
different
M1
(M2)
and
supply
voltage
s
.
From Fig.7 to Fig.9, t
he o
scillat ion
frequency increases
as supply volta
ge increases
.
I
t reveals the MOS
differential amplifier
oscillator is also a voltage control
led
oscillator
(VCO)
.
The
MOS differential amplifier
oscillator
s
hows
excellent VCO linearity
from
2.15 volts
to 3.05 volts supply
voltage
as shown
in
Fig.10
.
Fig.10
Voltage controlled o
scillator
(VCO)
characteristics of Fig.1
.
4.
CO
N
CLUSIONS
:
The MOS
differential amplifier oscillator
generates square wave not the same as
traditional oscillators, such as quarts oscillator or
ring oscillator can genera
te sinusoidal waves.
I
t is the same as general oscillators that
the
noise
s
of
MOS differential
amplifier oscillator
are
proportional to output frequencies
.
But the
MOS
differential amplifier oscillator still
has
low noise and
excellent
voltage control
led
(VCO)
characteristics. In our experiments,
reduce
R1,
R3
values
or
increase
parallel
transistor
M1 (M2)
numbers
can increase
output
fre
q
uency
. We think
the
time delay
of
CMOS inverter could be
another
dominant factor. We will improve the
situation
by IC implementation. Resistor R1, R3
will change into PMOS and NMOS
transistor
respectively, CMOS inver
ter will become short
time delay in CIC process
and NMOS M1 (or
M2) is large size in width
. All those bread board
discrete devices will become
CIC
0.35um

Si
process IC
devices. If we can implement MOS
differential amplifier oscillator into IC chips,
then w
e will achieve not only in frequency
response
to
Giga Hertz
but also in voltage
control and noise performan
ce.
A
CKNOWLEDGES
:
The authors would like to thank the
National Science Council of Republic of China
for their kind support
.
This work was supported
by the National Science Council of Republic of
China under the contract no. NSC9
3

221
8

E

168

00
2
.
R
EFERENCES
:
1.
Adel S. Sedra and Kenneth C. Smith,
“
Microelectronic Circuits,
”
5th edition, pp.
687

719, 2004.
2.
Randall L. Geiger,
Phillip E. Allen and Noel R.
Strader,
“
VLSI Design Techniques for Analog
and Digital Circuits,
”
pp. 431

454, 199
0.
3.
Richard C. Jaeger and Travis N. Blalock,
“
Microelectronic Circuit Design,
”
2
nd
edition,
pp. 1087

1108, 2003.
(
R1=3
.
.62
K
Ω
R 3 =0.7
K
Ω
)
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