A MOS DIFFERENTIAL AMPLIFIER 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 University of Technology
Tainan, Taiwan 710, R.O.C.
ABSTRACT
CMOS inverter and two CMOS inverters after
In this thesis, we present an oscillator
two different outputs. It is an asymmetric
mainly composed by a MOS differential
structure and most output waveform tends to be
amplifier. We use H-spice to verify the
square waveform. The oscillator frequencies are
differential amplifier oscillator at 980 MHz
decided by resistors values, parallel NMOS
successfully under CIC 0.18um-Si process
numbers and CMOS inverters time delay. In this
parameters. We also use discrete devices on
thesis, we present a different type oscillator and
bread board to prove the circuit is an oscillator
use experimental results to prove such oscillator
circuit. The experiment shows such oscillator
is useful, easiness and flexibility in design.
can work stably from 1.05 volts to 3.3 volts
supply voltage. When supply voltage is close to
2. CIRCUIT THEOREM AND SIMULATION
3.3 volts, the output frequency will be more than
The oscillator is composed of MOS
20 MHz. The differential amplifier oscillator can
differential amplifier by adding three CMOS
start oscillating at low voltage when supply
inverters as shown in Fig.1. A formal differential
voltage is only 1.05 volts and output frequency
amplifier needs a constant current source but we
is about 426 KHz. We use FFT (Fast Fourier
use resistor R3 to replace constant current source
Transform) diagram to analyze the oscillator and
for simplicity. According to differential amplifier
shows
noise
operation, transistor M1 and M2 can’t be off in
characteristic. Finally, those experimental results
the same time. Transistors M1, M2 will both be
reveal that the oscillator is also an excellent
in on state (saturation) or one is on and the other
voltage controlled oscillator (VCO).
is off. In the meanwhile, transistor M1 or M2
Keyword: differential amplifier, VCO, FFT.
can’t be in triode state. Because we can’t
the
oscillator
is
with
low
fabricate two completely equalized transistors
1. INTRODUCTION
M1 and M2, so most conditions are M1 on
We use the high input resistance, high
output
resistance
and
high
voltage
gain
(saturation) and M2 off, or M1 off and M2 is on
(saturation).
characteristics of MOS differential amplifier
In Fig.1 we assume M1 on and M2 off, so
[1-3] to create an oscillator. Such oscillator is
voltage OP1 is in low state and voltage OP2 is in
based on differential amplifier have two outputs,
high state. In the meanwhile, G1 voltage is high
one output is high voltage state and the other
and G2 voltage is low. After CMOS inverter
will be in low voltage state. We connect one
(INV1) time delay, voltage G2 becomes high
state to turn on transistor M2, so voltage OP2
In this thesis, we use Tektronix TDS3034B
changes into low state. After CMOS inverters
oscilloscope to measure oscillator circuit and
(INV2, INV3) time delay, voltage G1 changes
fast Fourier transform (FFT) diagram. The
into low state to turn off transistor M1. Base on
discrete devices are NMOS transistors M1 (M2),
same analysis, M2 will be off and M1 will be on
resistors R1 (R3) and CMOS inverters. We can’t
in the next run. After a fixed period, M1 and M2
buy a discrete NMOS transistor M1 (or M2). So
will toggle their states. Such on/off continuous
particularly,
switching phenomena will cause oscillation. The
(MM74HC04N) output as drain electrode,
nodes status in Fig.1 also shows H/L (High or
CMOS inverter input as gate electrode, CMOS
Low) state.
inverter ground as source electrode and let
Vcc
CMOS
INV1
H/L
R1
OP1
L/H
R1
we
take
CMOS
inverter
CMOS inverter VDD open. The transformation is
shown in Fig.3. We put all these discrete devices
CMOS INV2
OUTPUT
OP2
H/L
on bread board and measure output signals.
(VDD: OPEN)
G1
M1
H/L
M2
G2
CMOS
INV3
*L/H
(PMOS: Idle)
H/L
R3
* : Change State
Gate
(Input)
Drain
(Output)
NMOS
M1 (or M2)
Source
(Ground: OPEN)
Fig.1 The MOS differential amplifier oscillator.
We use CIC 0.18um-Si process parameters
to run simulation of MOS differential amplifier
Fig.3 CMOS inverter transforms into NMOS
transistor M1 (or M2).
oscillator. Under 2 volts operation voltage and
The output waveform under 1.05 volts
resistor R1 is 1.2 KΩ, R3 is 0.22 KΩ then the
supply voltage and oscillation frequency is 425.9
output oscillation frequency is merely 980 MHz
KHz as shown in Fig.4. M1 (or M2) is composed
as shown in Fig.2.
of three parallel NMOS transistors.
R1=4.7 KΩ
R3=0.83 KΩ
Fig.2 Output waveform of simulation result.
3. EXPERIMENTAL RESULTS
Fig.4 Output waveform under 1.05 volts.
different
Fig.7 shows resistor R3 (0.83 KΩ) and
resistor under 3.3 volts supply voltage, output
transistor M1 (M2) unchanged but R1 is changed
frequency is 20.51 MHz and M1 (or M2) is
from 3.3 KΩ to 4.7 KΩ. M1 (M2) is composed
composed of six parallel NMOS transistors.
of six parallel NMOS transistors. Fig.7 reveals
Fig.5
shows
oscillator
with
smaller R1 will have higher output frequency.
R1
17
16.5
16
15.5
R1 = 3.3K
15
MHZ
14.5
14
R1=3.62 KΩ
R3=0.7 KΩ
R1 = 4.7K
13.5
13
12.5
Fig.5 Output waveform under 3.3 volts.
12
2.2
2.3
2.4
2.5
2.7
2.6
2.8
2.9
3
V
Fig.6 is the typical fast Fourier transform
Fig.7 Oscillator output frequencies
different R1 and supply voltages.
under
(FFT) diagram of the MOS differential amplifier
Fig.8 shows resistor R1 (3.3KΩ) and
oscillator. Fig.6 shows the oscillator with low
transistor M1 (M2) unchanged but R3 is changed
noise characteristics from 0 to 9 MHz and its
from 0.83 KΩ to 3.3KΩ. M1 (M2) is composed
output frequency is about 4.2 MHz. The highest
of six parallel NMOS transistors. Fig.8 reveals
signal is more than the other signals about 35 db
smaller R3 will have higher output frequency.
in Fig.6. It means the main oscillation signal is
R3
17
fifty-six (1035/20=56.2) times stronger than the
16.5
16
other signals. If the main signal is 1.5 volts then
15.5
the others signals shall be smaller than 0.027
volts. But most conditions are noise signals will
15
MHZ
R3 = 0.83K
14.5
14
become larger as output frequency increases in
R3 = 3.3K
13
the MOS differential amplifier oscillator.
12.5
FFT
0
R1=4.2 KΩ
13.5
R3=1.69 KΩ
12
2.2
Vcc=2.9 volts
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
V
Fig.8 Oscillator output frequencies
different R3 and supply voltages.
under
-20
Fig.9 shows resistor R1 (4.7KΩ) and R3
db
(0.83 KΩ) unchanged but transistor M1 (or M2)
-40
is changed. Their parallel NMOS transistors M1
(or M2) are three and six respectively. The six
-60
parallel transistors have great improvement in
M1 (or M2) = Three parallel NMOS.
output frequency because of much larger drain
-80
0
2
4
6
8
10
MHZ
Fig.6 Typical FFT diagram of the MOS
differential amplifier oscillator.
current. The effect of M1 (or M2) is more useful
than those effects of R1 and R3.
M1(or M2)
10
are proportional to output frequencies. But the
9
MOS differential amplifier oscillator still has
8
low noise and excellent voltage controlled (VCO)
7
MHZ
characteristics. In our experiments, reduce R1,
Six parallel NMOS
6
R3 values or increase parallel transistor M1 (M2)
5
numbers can increase output frequency. We think
4
the time delay of CMOS inverter could be
3
Three parallel NMOS
2
another dominant factor. We will improve the
1
situation by IC implementation. Resistor R1, R3
0
1.7
1.75
1.85
1.8
1.9
1.95
2
2.05
2.15
2.1
2.2
V
Fig.9 Oscillator output frequencies
different M1 (M2) and supply voltages.
under
will change into PMOS and NMOS transistor
respectively, CMOS inverter will become short
time delay in CIC process and NMOS M1 (or
From Fig.7
oscillation
M2) is large size in width. All those bread board
frequency increases as supply voltage increases.
discrete devices will become CIC 0.35um-Si
It reveals the MOS differential amplifier
process IC devices. If we can implement MOS
oscillator is also a voltage controlled oscillator
differential amplifier oscillator into IC chips,
(VCO).
amplifier
then we will achieve not only in frequency
oscillator shows excellent VCO linearity from
response to Giga Hertz but also in voltage
2.15 volts to 3.05 volts supply voltage as shown
control and noise performance.
The
to
Fig.9,
MOS
the
differential
in Fig.10.
ACKNOWLEDGES :
VCO
18
(R1=3..62 KΩ
17
R3=0.7 KΩ)
The authors would like to thank the
16
National Science Council of Republic of China
15
for their kind support. This work was supported
MHZ
14
by the National Science Council of Republic of
13
China under the contract no. NSC93-2218-E12
168-002.
M1(or M2) = Six parallel NMOS
11
10
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
3.1
3.2
V
Fig.10 Voltage controlled oscillator (VCO)
characteristics of Fig.1.
4. CONCLUSIONS :
REFERENCES:
1.Adel S. Sedra and Kenneth C. Smith,
“Microelectronic Circuits,” 5th edition, pp.
687-719, 2004.
The MOS differential amplifier oscillator
2. Randall L. Geiger, Phillip E. Allen and Noel R.
generates square wave not the same as
Strader, “ VLSI Design Techniques for Analog
traditional oscillators, such as quarts oscillator or
and Digital Circuits,” pp. 431-454, 1990.
ring oscillator can generate sinusoidal waves.
3. Richard C. Jaeger and Travis N. Blalock,
It is the same as general oscillators that the
“Microelectronic Circuit Design,” 2nd edition,
noises of MOS differential amplifier oscillator
pp. 1087-1108, 2003.