Download Investigation of frequency response of basic single stage amplifiers

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
2routCL

g mp
2CL

2pCox Wp / Lp IDp
2CL

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
2CL
;
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=1and R=500kPresent 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: 15s.
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 1.26s
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