CHAPTER 13 JFETs

Example 13-1, Page 428

In [1]:
VG=20                 #Gate voltage(V)
IG=1*10**-9           #Gate current(A) 

Rin=VG/IG             #input resistance(Ohm)

print 'input resistance of JFET Rin = ',Rin/10**6,'MOhm'

input resistance of JFET Rin =  20000.0 MOhm

Example 13-2, Page 430

In [1]:
Vp=6                 #Pinch off voltage(V)
IDSS=100*10**-3      #current drain to source with shorted gate(A)

RDS=Vp/IDSS          #Ohmic resistance(Ohm)
VGS_off=-Vp          #gate-source cutoff voltage(V)

print 'Ohmic resistance RDS = ',RDS,'Ohm'
print 'Gate-source cutoff VGS(off) = ',VGS_off,'V'

Ohmic resistance RDS =  60.0 Ohm
Gate-source cutoff VGS(off) =  -6 V

Example 13-3, Page 431

In [8]:
VGS_off=-4                #gate-source cutoff voltage(V)
IDSS=5                    #current drain to source with shorted gate(mA)

VGS=VGS_off/2             #gate voltage(V)
ID=IDSS/4.0               #drain current(mA)

print 'Gate voltage VGS = ',VGS,'V'
print 'Drain current ID = ',ID,'mA'

Gate voltage VGS =  -2 V
Drain current ID =  1.25 mA

Example 13-4, Page 432

In [9]:
VGS_off=-8                 #gate-source cutoff voltage(V)
IDSS=16                    #current drain to source with shorted gate(mA)

VGS=VGS_off/2             #gate voltage(V)
ID=IDSS/4.0               #drain current(mA)

print 'Gate voltage VGS = ',VGS,'V'
print 'Drain current ID = ',ID,'mA'

Gate voltage VGS =  -4 V
Drain current ID =  4.0 mA

Example 13-5, Page 433

In [2]:
Vp=4               #pinchoff voltage(V)
Vin=-10            #input voltage(V)
RD=10.0            #drain resistor(KOhm)
IDSS=10.0          #IDSS (mA)
VDD=10.0           #Drain supply voltage(V)

VGS_off=-Vp        #VGS cutoff voltage(V)
VD=-Vin            #drain voltage(V)
ID_sat=VD/RD         #saturation drain current(mA)
RDS=Vp/IDSS          #Ohmic resistance(Ohm)
VD=VDD*RDS/(RDS+RD)  #drain votage(V)

print 'Drain voltage VD = ',round(VD,2),'V'

Drain voltage VD =  0.38 V

Example 13-6, Page 436

In [22]:
Vp=4               #pinchoff voltage(V)
RD=2.0             #drain resistor(KOhm)
RS=400             #source resistance(Ohm)
IDSS=10.0          #IDSS (mA)
VDD=30.0           #Drain supply voltage(V)

RDS=1000*Vp/IDSS          #Ohmic resistance(Ohm)
ID=IDSS/4.0               #drain current(mA)
VD=VDD-(ID*RD)            #drain votage(V)

print 'Ohmic resistance RDS = ',RDS,'Ohm'
print 'drain voltage VD = ',VD,'V'

Ohmic resistance RDS =  400.0 Ohm
drain voltage VD =  25.0 V

Example 13-7, Page 437

In [29]:
IDSS=20               #IDSS (mA)
RS=270                #Source resistance (Ohm)

VGS=-IDSS*RS             #gate-source voltage(V)
VGSQ_min=-0.8            #Q point minimum gate-source voltage(V)
VGSQ_max=-2.1            #Q point maximum gate-source voltage(V)
IDQ_min=2.8              #Q point minimum drain current(V)
IDQ_max=8.0              #Q point maximum drain current(V)
VGS_off_min=2            #VGS(off) minimum(V)
VGS_off_max=6            #VGS(off) maximum(V)
ID_min=8.0               #ID minimum(mA)
ID_max=20.0              #ID maximum(mA)
Rs_min=1000*VGS_off_min/ID_min    #Minimum value for Rs(Ohm)
Rs_max=1000*VGS_off_max/ID_max    #Maximum value for Rs(Ohm)

print 'Maximum value for Rs =',Rs_max,'Ohm'
print 'Minimum value for Rs =',Rs_min,'Ohm'
print 'Choose approximately mid point between these two.'

Maximum value for Rs = 300.0 Ohm
Minimum value for Rs = 250.0 Ohm
Choose approximately mid point between these two.

Example 13-8, Page 440

In [38]:
VS=10           #source voltage (V)
RS=2.0          #source resistance(KOhm)
VDD=30          #Drain supply voltage (V)
RD=1.0          #Drain resistance(KOhm)

ID=VS/Rs           #Drain current(mA)
VD=VDD-(ID*RD)     #Drain voltage(V)
VDS=VD-VS          #Drain to source voltage (V)
ID_sat=VDD/(RD+RS) #ID(dc-saturation) (mA)
VDS_cut=VDD        #VDS(cutoff) (V)

print 'ID (saturation) = ',ID_sat,'mA'
print 'VDS (cutoff) = ',VDS_cut,'V'

ID (saturation) =  10.0 mA
VDS (cutoff) =  30 V

Example 13-9, Page 441

In [93]:
IDSS=20                     #IDSS (mA)
RS=270                      #Source resistance (Ohm)
VDD=30                      #Drain supply voltage (V)
RD=1.0                      #Drain resistance(KOhm)
R1=1*10**6                  #Gate resistor1(Ohm)  
R2=2*10**6                  #Gate resistor2(Ohm) 

VG=-VDD*R1/(R1+R2)          #gate-source voltage(V)
ID=VG/RS                    #current for second point(mA)

VGSQ_min=-0.4            #Q point minimum gate-source voltage(V)
VGSQ_max=-2.4            #Q point maximum gate-source voltage(V)
IDQ_min=5.2              #Q point minimum drain current(V)
IDQ_max=6.3              #Q point maximum drain current(V)

print 'Maximum value for VGS =',VGSQ_max,'V'
print 'Minimum value for VGS =',VGSQ_min,'V'
print 'Maximum value for ID =',IDQ_max,'mA'
print 'Minimum value for ID =',IDQ_min,'mA'

Maximum value for VGS = -2.4 V
Minimum value for VGS = -0.4 V
Maximum value for ID = 6.3 mA
Minimum value for ID = 5.2 mA

Example 13-10, Page 443

In [41]:
RS=3.0                      #Source resistance (KOhm)
VDD=15                      #Drain supply voltage (V)
RD=1.0                      #Drain resistance(KOhm)

ID=VDD/RS                    #drain current(mA)
VD=VDD-(ID*RD)               #drain voltage(V)

print 'Drain current ID = ',ID,'mA'
print 'Drain voltage VD = ',VD,'V'

Drain current ID =  5.0 mA
Drain voltage VD =  10.0 V

Example 13-11, Page 444

In [44]:
RS=2.0                      #Source resistance (KOhm)
VDD=10                      #Drain supply voltage (V)
RD=1.0                      #Drain resistance(KOhm)
VBE=0.7                     #BJT transistor drop(V)

ID=((VDD/2)-VBE)/RS          #drain current(mA)
VD=VDD-(ID*RD)               #drain voltage(V)

print 'Drain current ID = ',ID,'mA'
print 'Drain voltage VD = ',VD,'V'

Drain current ID =  2.15 mA
Drain voltage VD =  7.85 V

Example 13-12, Page 447

In [52]:
IDSS=5.0                #IDSS current (mA)
gm0=5000.0              #transconductance at VGS=0(uS)
VGS=-1.0                #VGS (V)
VGS_off=1000*-2*IDSS/gm0   #VGS (off) (V)
gm=gm0*(1-(VGS/VGS_off))   #gm at VGS=-1V

print 'VGS (Off) = ',VGS_off,'V'
print 'gm = ',gm,'uS'

VGS (Off) =  -2.0 V
gm =  2500.0 uS

Example 13-13, Page 449

In [3]:
RD=3.6                #drain resistance(KOhm)
RL=10                 #Load resistance(KOhm)
gm=5000               #transconductance (uS)
Vin=1                 #input(mVpp)

rd=RD*RL/(RD+RL)      #ac drain resistance(KOhm)
Av=gm*rd/1000         #voltage gain
Vout=Av*Vin           #Output voltage(V)

print 'ac drain resistance rd = ',round(rd,2),'KOhm'
print 'voltage gain Av = ',round(Av,2)
print 'output voltage Vout = ',round(Vout,2),'mVpp'

ac drain resistance rd =  2.65 KOhm
voltage gain Av =  13.24
output voltage Vout =  13.24 mVpp

Example 13-14, Page 450

In [4]:
RS=1.0                  #source resistance(KOhm)
RL=1.0                  #Load resistance(KOhm)
gm=2500.0*10**-6        #transconductance (S)
Vin=1                   #input(mVpp)

rs=1000*RS*RL/(RS+RL)        #ac source resistance(Ohm)
Av=gm*rs/(1+(gm*rs))         #voltage gain
Vout=Av*Vin                  #Output voltage(V)

print 'ac source resistance rs = ',round(rs,2),'Ohm'
print 'voltage gain Av = ',round(Av,2)
print 'output voltage Vout = ',round(Vout,2),'mVpp'

ac source resistance rs =  500.0 Ohm
voltage gain Av =  0.56
output voltage Vout =  0.56 mVpp

Example 13-15, Page 450

In [74]:
Ra=780                #Adjustaed resistance(Ohm)
RS1=220               #source resistance(Ohm)
VDD=30                #Drain supply voltage(V)
RL=3                  #Load resistance(KOhm)
gm=2000.0*10**-6      #transconductance (S)

RS=(RS1+Ra)/1000             #total source resistance(KOhm)
rs=1000*RS*RL/(RS+RL)        #ac source resistance(Ohm)
Av=gm*rs/(1+(gm*rs))         #voltage gain

print 'Ac source resistance rs = ',rs,'Ohm'
print 'voltage gain Av = ',Av

Ac source resistance rs =  750 Ohm
voltage gain Av =  0.6

Example 13-16, Page 451

In [5]:
RS=2.2                #source resistance(Ohm)
VDD=30                #Drain supply voltage(V)
RL=3.3                #Load resistance(KOhm)
gm=3500.0*10**-6      #transconductance (S)
R1=2*10**-6           #Base resistor 1(Ohm)
R2=1*10**-6           #Base resistor 2(Ohm) 

VD=VDD*(R2/(R1+R2))   #drain voltage(V)
ID=VD/RS              #drain current(mA)
rs=1000*RS*RL/(RS+RL) #ac source resistance(Ohm)
Av=gm*rs/(1+(gm*rs))  #voltage gain

print 'Ac source resistance rs = ',rs/1000,'KOhm'
print 'voltage gain Av = ',round(Av,2)

Ac source resistance rs =  1.32 KOhm
voltage gain Av =  0.82

Example 13-17, Page 454

In [6]:
RD=10.0               #Drain resistance(KOhm)
R1=0.2                #resistance(KOhm)
IDSS=10.0             #IDSS current(mA)
VGS_off=-2            #VGS(off) (V)
gm=3500.0*10**-6      #transconductance (S)
Vin=10.0              #input voltage (mVpp)

RDS=-VGS_off/IDSS        #Ohmic resistance(Ohm)
Vout_on=Vin*RDS/(RD+R1)  #Output voltage when JFET is on(V)
Vout_off=Vin             #Output voltage when JFET is off(V)
ratio=Vout_off/Vout_on   #on-off ratio

print 'output voltage Vout = ',round(Vout_on,2),'mVpp'
print 'on-off ratio = ',ratio

output voltage Vout =  0.2 mVpp
on-off ratio =  51.0

Example 13-18, Page 455

In [7]:
RD=10.0               #Drain resistance(KOhm)
R1=0.2                #resistance(KOhm)
R2=10*10**6           #resistance(Ohm)
IDSS=10.0             #IDSS current(mA)
VGS_off=-2            #VGS(off) (V)
gm=3500.0*10**-6      #transconductance (S)
Vin=10.0              #input voltage (mVpp)

Vout_on=Vin*(RD/(RD+R1))         #Output voltage when JFET is on(V)
Vout_off=Vin*(RD/R2)             #Output voltage when JFET is off(V)
ratio=Vout_on/Vout_off/1000      #on-off ratio

print 'output voltage Vout when on = ',round(Vout_on,2),'mVpp'
print 'output voltage Vout when off = ',Vout_off*10**6,'uVpp'
print 'on-off ratio = ',round(ratio,2)

output voltage Vout when on =  9.8 mVpp
output voltage Vout when off =  10.0 uVpp
on-off ratio =  980.39

Example 13-19, Page 455

In [8]:
fout=20           #frequency(KHz)
RDS=50.0          #Ohmic resistance(Ohm)
RL=10*10**3       #Load resistance(Ohm)
Vin=100           #input voltage (mV)

Vpeak=Vin*RL/(RL+RDS) #peak voltage(V)

print 'Outout Vpeak = ',round(Vpeak,2),'mV'

Outout Vpeak =  99.5 mV
In [ ]: