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Transcript
Guided exercise_5: Investigation of frequency response of basic single stage amplifiers through simulation. Exercise 5.1: Consider the circuit shown in Fig.5.1. Given that: µnCox=100 µA/V2, VTN0=0.5V, µpCox=40 µA/V2, VTP0=-0.6V, 0.35µm CMOS technology Problems: A. Specify the name (the type) of the amplifier. B. Find the sizes W and L of the Mn Fig.5.1 and Mp in order to assure Au>10. C. Using PSpice examine the transfer characteristic of the amplifier, determine the operating area, choose the biasing of the transistor Mn, and read the operating current. D. Calculate the expected Au, BW and GBW of the circuit. E. Using PSpice examine the frequency characteristic of the amplifier and compare the obtained results with the calculations. Solution: A. The figure presents a CMOS amplifier with p-MOS active resistor as a load. B. The gain Au is given by formula Au g mn n Cox ( WnLn ) . g mp pCox ( Wp / Lp ) After replacing with known quantities we can find Au n Cox ( WnLn ) 100e 6( WnLn ) 10 pCox ( Wp / Lp ) 40e 6( Wp / Lp ) Wn / Ln 0.4 *100 40 Wp / Lp For minimal area it is recommended to choose (explain why!) Wn / Ln 1 7.5 40 Wp / Lp Finally we can determine Wn=Lp=15um and Ln=Wn=2um. C. To examine the transfer characteristic we must change VAC with VDC source and specify DC sweep between 0V and 3.3V. After simulation we will find the following plots 3.0V 2.0V 1.0V Vmin SEL>> 0V Vmax Vbias V(Vout) 20uA 15uA 10uA 5uA Vbias 0A 0V 0.1V 0.2V 0.3V 0.4V 0.5V 0.6V 0.7V 0.8V 0.9V 1.0V 1.1V 1.2V 1.3V 1.4V ID(Mn) V_VDC From upper plot we can find that the operating area (the linear part of the characteristic) is between 500mV and 660mV. We choose 585mV biasing voltage for Mn (in the middle of the linear region) and from lower plot we can read that the operating current is about 4.6uA. D. The calculations for expected Au, BW and GBW are Au BW n Cox ( WnLn ) 100e 6 15 / 2 11.85 21.5dB pCox ( Wp / Lp ) 40e 6 (2 / 15) 1 2routCL g mp 2CL 2pCox Wp / Lp IDp 2CL 2 40e 6 (2 / 15) 4.6e 6 7.07e 6 111.5kHz 2 10e 12 62.8e 12 GBW Au BW 11.85 *111.5 1.32MHz E. To examine the frequency characteristic of the amplifier VAC source is connected to the input. The parameters of the source are: DC=585mV, ACMAG=1mV, ACphase=0. In PSpice schematics screen specify Analysis Setup Analysis setup: AC sweep AC sweep type: Decade Sweep parameters: Pts/decade Start freq. 10Hz End freq. 10MHz After the simulation the following plot can be obtained 25 Au = 21.4 dB 20 15 10 5 BW = 108.8 kHz 0 10Hz 30Hz DB(V(Vout)/V(VAC:+)) 100Hz 300Hz 1.0KHz 3.0KHz 10KHz 30KHz 100KHz GBW = 1.27 MHz 300KHz 1.0MHz 3.0MHz Frequency The comparison between calculated and simulated results is made in the table. Au BW calculated 21.5 dB 111.5 kHz simulated 21.4 dB 108.8 kHz GBW 1.32 MHz 1.27 MHz Exercise 5.2: Consider the circuit shown in Fig.5.2. Given that: µnCox=100 µA/V2, VTN0=0.5V, µpCox=40 µA/V2, VTP0=-0.6V 0.35µm CMOS technology Problems: A. Specify the name (the type) of the amplifier. B. Find the sizes W and L of the Mn and Mp transistors in order to assure Fig.5.21 GBW>5MHz. C. Using PSpice examine the transfer characteristic of the amplifier, determine the operating area, choose the biasing of the transistors and read the operating current. D. Using PSpice examine the frequency characteristic of the amplifier. Instructions B. To determine the sizes of the transistors use the following formulas: GBM g mn I g mn g mp 2CL ; 2I ; Veffn pCox Wp n Cox Wn Veffn Veffp 2 Ln 2 Lp Take into consideration that for maximum linearity usually Veffn Veffp and obtain the approximate value of the DC biasing ( it is around VDD/2). After determine the current I , Wn/Ln and Wp/Lp. C., D. Proceed as in the previous exercise. If the obtained simulation result does not fulfill the requirement for the GBM, change the current and resize the transistors. Fig.5.3 Exercise 5.3: Consider the circuit shown in Fig.5.3. Given that: Wn/Ln=50/2, Wp/Lp=Wb/Lb=125/2, µnCox=100 µA/V2, VTN0=0.5V, µpCox=40 µA/V2, VTP0=-0.6V 0.35µm CMOS technology Problems: A. Specify the name (the type) of the amplifier. B. Using PSpice examine the transfer characteristic of the amplifier, determine the operating area, choose the biasing of the transistor Mn and read the operating current. C. Using PSpice examine the frequency characteristic of the amplifier for R=1and R=500kPresent the results in the table and explain the differences. Au R=1 R=500k BW GBW Exercise 5.4: The circuit in Fig.5.4 is used as an output stage of analog circuit. Given that: Wn/Ln=2/2, Wp/Lp=Wb/Lb=50/2, µnCox=100 µA/V2, VTN0=0.5V, µpCox=40 µA/V2, VTP0=-0.6V 0.35µm CMOS technology Fig.5.4 Problems: A. Specify the name (the type) of the output amplifier. B. Using PSpice, perform transient analysis of the circuit and determine graphically slew rate (SR+ and SR-). C. How we can increase the slew rate if it is necessary? Prove with experiment. Instructions B. To specify the transient analysis in PSpice schematics screen we specify Analysis Setup Analysis setup: Transient… Transient analysis: Final time: 15s. The parameters of Vpulse are: DC=0; AC=0; V1=-1.65V; V2=1.65V; TD=5u; TR=1n; TF=1n; PW=5u; PER=10u After the simulation we obtain the following plots 2.0V 1.0V 0V -1.0V SEL>> -2.0V V1(Vpulse) 2.0V 1.0V 0V -1.0V -2.0V 0s 1us 2us 3us 4us 5us 6us 7us 8us V(Vout) Time 9us 10us 11us 12us 13us 14us 15us To determine of SR+ use the zoomed plot, read the values of deltaVout and deltaTime and apply the formula SR delta Vout 2.55V 2V / s delta time 1.26s 1.97V 1.00V 0V -1.00V -2.00V V1(Vpulse) 2.0V 1.0V delta Vout Vout=Vmax-Vmin delta time 0V 0.9Vout -1.0V 0.1Vout SEL>> -2.0V 9.4us 9.6us V(Vout) 9.8us 10.0us 10.2us 10.4us 10.6us 10.8us 11.0us Time The method to obtain SR- is a similar. 11.2us 11.4us 11.6us 11.8us 12.0us 12.2us 12.4us