Question

Figure 3. Variable RC circuit A variable time constant circuit is shown above. The potentiometer (R2) can change its resistance from 0 Ohms to 750 Ohms. R1 has a fixed value of 70 Ohms. The capacitor has a capacitance of 35 pF. a) Use the supplied MATLAB™ code Calc RC and function RC_Circ to find the time variation of voltage across the Capacitor, Ve, and the fixed resistor R1 for a steady voltage V1 = 5 Volts which is turned on at time = 0. Use R2 = 350 Ohms. You should turn in your edited MATLAB™ code and a plot showing these voltages as a function of time. Your plot must have labeled axes and a legend. Note: The supplied MATLAB" code is for a series RC circuit with one resistor (like discussed in class). b) Use the supplied Matlab code Calc_RC and function RC_Circ to find the time variation of voltage across the Capacitor, V., for a time varying voltage which switches between V1 = 5 Volts (on) and V1 = 0 Volts (off) with a square wave frequency of 10 MHz. Do this simulation at the following values for R2 = 0, 350, and 750 Ohms and plot V. for all cases on the same plot. You should turn in your edited MATLAB™ code and a plot showing these voltages as a function of time as well as the input voltage square wave. Your plot must have labeled axes and a legend.

Answer 1

a)

%matlab function

function Vc = RC_Circ(R,C,V0, VInf,t)
T=R*C;
Vc=(VInf)+(V0-VInf).*exp(-t./T);
end

-----------------------

%matlab script

R1=70;
R2=350;
R=R1+R2;
C=3.5e-11;
V0=0;
VInf=5;
t=0:1e-9:1e-7;
Vc=RC_Circ(R,C,V0,VInf,t);
plot(t,Vc);xlabel('t');
ylabel('V_c(t)');

--------------------------

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0. 9 1 x 10-7

-----------------

b)

%matlab

R1=70;
R2=350;
R=R1+R2;
C=3.5e-11;
V0=5;
VInf=0;
T=1e-7;
t1=0:1e-9:(0.5*T-1e-9);
t2=(0.5*T):1e-9:(T-1e-9);
Vc1=RC_Circ(R,C,V0,VInf,t1);
Vc1(end)
Vc2=RC_Circ(R,C,Vc1(end),V0,(t2-0.5*T));
t=[t1 t2];
Vc=[Vc1 Vc2];
plot(t,Vc);
xlabel('t');
ylabel('V_c(t)');

----------------------------

N 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0. 9 1 x 10-7