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why does the base current decreases as the collector current increases? And as the base current increases the collector current also increases. Why and how?

Thank you. I really need some help and understanding in this area.

  • 3
    \$\begingroup\$ You have to show us the schematic you're using to evaluate this. There is an Rv mentioned, I do not know how you're using it. Also, beta = Ic/Ib must be constant over a large range of currents. I suspect your Vce is not constant and that could explain the results you see. \$\endgroup\$ – Bimpelrekkie Oct 2 '15 at 9:13
  • \$\begingroup\$ I think your transistor is hot. When the temperature increases, the gain of the transistor increases. Turn off the circuit. Wait until the cooling of the transistor. Include a scheme for a short time. Quickly make measurements. \$\endgroup\$ – user87926 Oct 2 '15 at 10:52
  • \$\begingroup\$ You really need to narrow this down and show what understanding you already have. \$\endgroup\$ – David Oct 2 '15 at 17:13
  • \$\begingroup\$ user87932, because you did not mention HOW you have changed the currents, I have assumed that you did increase VCE, correct? \$\endgroup\$ – LvW Oct 2 '15 at 17:14
  • \$\begingroup\$ We apply a DC voltage Vdc = 20 v to the circuit, our base current is measured with interrupted collector line (potentiometer removed) and entered in table 8.1 for Ic=0. Then the potentiometer is inserted and used to set the collector current values specified to Table 8.1.The dependence of the base current om the collector current (at constant base/ emitter voltage) is shown in graph form in Figure 8.3. Then the circuit is modified as shown in figure 8.2. \$\endgroup\$ – user87923 Oct 2 '15 at 20:34

You have measured the results of the so-called Early effect. Increasing the voltage Vce (and, thus, the voltage Vcb for a fixed Vbe) causes an increase in Ic and a (small) decrease of Ib. This is because the sum of both (Ic+Ib=Ie) must be constant (Vbe determines Ie and remains constant).

Why does Ic increase and Ib decrease? Answer: Because an increased Vcb increases the width of the depletion layer region and a corresponding width reduction of the B-E junction (which allows the base current). Hence, more charged carriers arrive at the collector (Ic increases, Ib decreases correspondingly). This effect is responsible for the well-known finite slope of the Ic=f(Vce) characterisics.

For reducing Ic back to the former value you must somewhat reduce Vbe. If the Ic increase was caused by temperature the Vbe reduction value is -2mV/K.

  • \$\begingroup\$ At table 8.1 I calculated the Beta(B) where B=Ic/Ib, i noticed that Beta is increasing as the current in the terminal c (collector) increases. Then the Beta on the table 8.2 is decreasing as the current in terminal B (Base) increases. \$\endgroup\$ – user87923 Oct 2 '15 at 14:05

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