# The BJT characteristic curve

Lets say you want to operate at 20uA base current. To move along that 20uA graph, you need to vary Vce. But to vary Vce, you either vary Vb (voltage at base) or change the values of the resistors at emitter or collector or the Vc (voltage at collector), is this correct? • Your question is unclear, first because you mention both $20\mu\mathrm{A}$ and then $30\mu\mathrm{A}$, and then because you're talking about what you need to do to stay at the $20\mu\mathrm{A}$ point. To the extent that the collector current is constant, you can vary the collector voltage at will and the collector current won't change. – TimWescott Nov 5 '18 at 23:32
• To expand on what Tim said: A BJT in forward-active mode with a fixed base current (such as when you have fixed base and emitter resistors and fixed base input voltage) acts like a current source, with the same $I_C$ regardless of other conditions. – Hearth Nov 5 '18 at 23:44
• What does the y intercept give? This is the max collect current? But for a fixed base current, between 20 to 80uA, none of them can give this max collector current... – user97449 Nov 6 '18 at 1:30
• The load-line is based upon the circuit, which you don't show, and the definition for $V_\text{bb}$ which is obviously different from the one shown here. The meaning of the y-intercept of this load line awaits a clarity not yet provided from you. But I can, at least, tell that the load line is for a $2\:\text{k}\Omega$ resistance, which I gather from the y-axis equation must be $R_\text{C}+R_\text{E}$. Write more, and/or reference other questions. Make each question self-contained. – jonk Nov 6 '18 at 11:49
• In reality those IB curves shouldn't be flat, they should have an upwards slope to the right because of Early Effect – James Mar 13 at 12:27

You are correct in your assumption that to vary Vce you must adjust the resistor values. The equation for the collector current (Ic) is as follows: Now solve for Vce and you get: Plug in Ic = Beta*Ib: We know that beta and base current are generally constant (in your question), so as long as Vcc is also constant the only variable that you can adjust is the resistance.

• I am afraid, there is a misunderstanding. Of course the first sentence is not wrong - however, it does not reflect practical aplication aspects. Of course, the resistors remain fixed and the base current varies acording to the input signal. And now the load line comes into the game: For different base currents only points along the load ne are possible, which means: VCE is varying. And this gives the signal ouput! – LvW Nov 6 '18 at 8:17