bjtnpn
In any transistor, we plot the output characteristics, i.e. effect of change in Vce on Ic.
Next, we write the KVL equation on the transistor, i.e. Vcc = IcRc + Vce and plot the load line from this equation.
So, now we have 2 relations between Vce and Ic – output characteristics and load line.
Why do we 2 plots for the same 2 variables, when will each be used and what all do they signify?
Karan, the bipolar transistor is a two-port device that is not free of return effects. That means:
While measuring input characteristics (input port) , the conditions at the output port (voltage, current) play a certain role and, therefore, should be kept constant.
Likewise: While measuring output data (at the output port), the conditions at the input port must be kept constant.
In practice, we require to keep all the DC conditions (voltage, current) at the respective ports constant. This is identical to the following requirements (which sometimes are misunderstood):
(1) DC voltage constant means: ac voltage zero, short circuit in the small-signal equivalent diagram.
(2) DC current constant means: ac current zero, open circuit in the small-signal equivalent diagram.
I hope this helps.
The following two schematics are equivalent:
simulate this circuit – Schematic created using CircuitLab
Where \$V_{TH}=V_{CC}\frac{R_2}{R_1+R_2}\$ and \$R_{TH}=\frac{R_1\cdot R_2}{R_1+R_2}\$.
From the above, and assuming \$I_B^{'}=\mid I_B\mid\$ and \$V_{BE}^{'}=\mid V_{BE}\mid\$, you can compute:
$$\begin{align*} V_{TH}+ I_B^{'}\cdot R_{TH}+ V_{BE}^{'} + I_B^{'}\cdot\left(\beta+1\right)\cdot R_E&=V_{CC}\\\\ \therefore\quad I_B^{'}=\frac{V_{CC}-V_{TH}-V_{BE}^{'}}{R_{TH}+\left(\beta+1\right)\cdot R_E} \end{align*}$$
Given your values, I get \$I_B^{'}\approx 9.6\:\mu\textrm{A}\$.
The start of shallow entry into saturation occurs right when \$V_{BC}=0\:\textrm{V}\$ or when \$V_B=V_C\$:
$$\begin{align*} V_C&=V_B\\\\ I_C\cdot R_L &= V_{TH}+I_B^{'}\cdot R_{TH}\\\\ \beta\: I_B^{'}\cdot R_L &=V_{TH}+I_B^{'}\cdot R_{TH}\\\\ \therefore \quad R_L &=\frac{V_{TH}+I_B^{'}\cdot R_{TH}}{\beta\: I_B^{'}}=\frac{1}{\beta}\left(R_{TH}+\frac{V_{TH}}{I_B^{'}}\right) \end{align*}$$
From which I get \$R_L\approx 2130\:\Omega\$.
Best Answer
The transistor characteristics say what the transistor does. The load line says what the resistor does. The intersection of the transistor characteristic lines with the resistor load line says what the amplifier does.