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. 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