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There is a question I can't solve. sat

I know that \$\beta\ =50\$ and \$V_{ce}\ (sat)\$ \$ =0.2 \$.

I need to find the value of \$ V_1 \$ that the BJT is in saturation.

I've tried some KVL/KCL equation using the \$V_{ce}\ (sat)\$ value but I can't get an answer (\$V_1\$=3.1V).

What is the way to approach things like that?

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    \$\begingroup\$ Did you mean to say that \$V_{ce}(sat)=-0.2\$? Is should be the case if the transistor is PNP. \$\endgroup\$ – Vasiliy Oct 7 '13 at 14:12
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what is the way to approach things like that?

Since \$V_E = 0 \$, the transistor will be saturated when \$V_C = -0.2V\$. Thus

\$I_C = \dfrac{4.8V}{2k \Omega} = 2.4mA\$

\$I_B = \dfrac{2.4mA}{50} = 48uA\$

Then, by KVL:

\$V_1 = V_{EB} + 48uA \cdot 50k \Omega = V_{EB} + 2.4V\$

So, assuming \$V_{EB} = 0.7V \$ (a reasonable assumption)

\$V_1 = 3.1V \$

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  • \$\begingroup\$ OK, that assumption was what I was missing. great solution, much apprecieted! \$\endgroup\$ – YNWA Oct 7 '13 at 16:21
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The preliminary condition for BJT to be in saturation is that it is not cut-off. This means that you begin by assuming:

$$V_{EB} \approx 0.6V$$

Assumed that, and knowing \$V_{EC_{Sat}}\$, you're calculating the range of collector current such that:

$$V_{EC} < V_{EC_{Sat}}$$

Since you know \$\beta\$, you can derive the range of base current from the above range of collector current.

Knowing the base current, it is straightforward to calculate the \$V_1\$. The answer you quoted is, probably, the lowest limit of \$V_1\$ such that the BJT is in saturation.

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