written 2.4 years ago by |
The circuit diagram for cs - cs Amplifier is as shown below.
Let $A_2$ is gain of BJT Ampli
" $A_1$ " " " FET "
Taking $A_2 = 50 A_1$ = 10
$\therefore$ A = $A_1 x A_2$ = 10 x 50 = 500.
Designing of 2nd stage.
1) Selection of biasing ckt - we will use voltage divider biasing.
2) Selection of Rc
Rc = $\frac{|Av| . hie} {hfe}$
= $\frac{50 x 4.5k}{290}$
Rc = 0.775k$\Omega$
$\therefore$ taking Rc = 810 $\Omega$ / 1/4 w
3) Selection of Q point
Given Vcc = 6V
VCEQ = $\frac{Vcc}{2}$ = 3V
VRE = $\frac{vcc}{10}$ = $\frac{6}{10}$ = 0.6 v.
$V_RC = V_CC - V_CE - V_RE$
= 6 - 3 - 0.6
VRC = 2.4 v
$\therefore$ ICQ Rc = 2.4
ICQ = $\frac{2.4}{RC}$ = $\frac{2.4}{0.81}$ = 3MA
4) Selection of RE
VRE = ICQ RE
RE = $\frac{VRE}{ICQ}$ = $\frac{0.6v}{3mA}$ = 200 $\Omega$
$\therefore$ Selecting RE = 200 $\Omega$ / 1/4 w.
5) Selection of biasing Resi.
S = $\frac{(1+\beta)}{1+\beta\frac{RE}{rth}}$
8 = $\frac{1+290}{1+\frac{290 x 0.2}{RTH}}$
8 + $\frac{464}{RTH}$ = 291
RTH = $\frac{464}{283}$ = 1.63k
$\frac{R_1 R_2}{R_1 + R_2}$ = 1.63k ------ (A)
$V_B = V_BE + V_RE$
= 0.7 + 0.6
VB = 1.3v
$\frac{vcc R2}{R_1 + R_2}$ = 1.3
$\frac{R_2}{R_1 + R_2}$ = $\frac{1.3}{6}$
$\frac{R_2}{R_1 + R_2}$ = 0.21k ------ (B)
Putting the value of B in (A)
$R_1$ x 0.21 = 1.63
$R_1$ = 7.76 k $\Omega$
$\therefore$ Selecting $R_1$ = 8.1k / 1/4w
Putting selected value of $R_1$ in (B)
$\frac{R_2}{8.1 + R_2}$ = 0.21
0.79 R_2 = 1.7
$R_2$ = 2.15 k
$\therefore$ Selecting $R_2$ = 2.2 k / 1/4 w
Designing of 1st stage
6) We will design JFET Ampli for ZTD using self bias method.
| Vasq | = | vp | - 0.63
| VasQ | = 3.37 v
$\therefore$ VasQ = -3.37 v
7) Calculation of ID and gm
ID = IDss ( 1- $\frac{Vasq}{vp})^2$
= 7 ( 1 - $\frac{-3.37}{-4})^2$
ID = 0.18 MA
gm = gmo ( 1- $\frac{vasq}{vp})$
= 5000 ( 1 - $\frac{-3.37}{vp})$
gm = 800 $\mu$ $\mho$
$\therefore$ gm = 0.8 m $\mho$
8) Selection of R4
$\because$ we want Ri > 1 M $\Omega$ $\therefore$ we will select
R4 = 1M $\Omega$ / 1/4 w
9) Selection of Rs.
for self bias ckt
Rs = | $\frac{vasq}{IO}$ |
= $\frac{3.37}{0.18}$
Rs = 18.72 k
$\therefore$ Rs = 20k / 1/4 w.
10) Selecting of RD.
RD = $\frac{lAil}{gm}$
= $\frac{10}{0.8}$
RD = 12.5 k
$\therefore$ Selecting RD = 15k / 1/4 w.
11) Selection of capacitors.
Cc1 = $\frac{1}{2\pi fc Rg}$
$\because$ fl is not given assuming 20 Hz
Cc1 = $\frac{1}{2 IT 20x 1m}$ = 0.793 nf
$\therefore$ Selecting Cc1 = 0.79nf/25v
Cs = $\frac{10}{2\pi flRS}$ = $\frac{10}{2x\pi x 20 x 20 x 10^3}$
Cs = 3.97 x $10^-6$ f
Selecting Cs = 3.9 $\mu$ f / 25 v
CE = $\frac{10}{2\pi flRE}$
= $\frac{10}{2 x \pi x 20 x 200}$
CE = 397 $\mu$ f
$\therefore$ Selecting CE = 390 = $\mu$ f / 25v
Cc2 = $\frac{1}{2 \pi fl Req}$
Req = RO1 + Ri2
Ro1 = RD = 15k
Ei2 = R1 || R2 || hie
= 8.1 || 2.2 || 4.5
= 1.25k
Cc2 = $\frac{1}{2 x \pi x 20 x 16.25 x 10^3}$
Cc2 = 48.9 $\mu$ f
$\therefore$ Selecting Cc2 = 50 $\mu$ f / 25v.