No, it's the collector current that is dependent on the base current, not the other way around. No matter what the collector current is, the base current is \$\dfrac{V_{MCU} - V_{BE}}{R}\$.

Keep in mind that \$V_{BE}\$ will be twice the value of another transistor, as there are two junctions between base and emitter.

But it's true that the collector current is what you want in the end. So to find the resistor value (don't just pick 1k), you calculate \$I_B=\dfrac{I_C}{H_{FE}}\$. If you want \$I_C\$ = 2A and \$H_{FE}\$ = 400, then your \$I_B\$ will have to be \$\dfrac{2A}{400}=5mA\$. This is a value your microcontroller will be able to deliver, but always check the datasheet.

To put it all together, \$R=\dfrac{H_{FE}}{I_C}\times (V_{MCU} - V_{BE})\$.

**edit**

Olin is right about the resistor value being the maximum, i.e. the base current being minimum. For many parameters in a datasheet you'll find more than one value, like typical and maximum or minimum. You should always calculate for worst case conditions, and it may require some logical thinking to find out whether worst case is minimum or maximum for a particular parameter.

Take \$H_{FE}\$. In my example I picked a value of 400. As higher is usually better datasheets often mention a minimum value. What if it's higher? The base current won't be different, so the collector current will be higher. If you drive the transistor in saturation \$I_C\$ will no longer be determined by the transistor, but by the load's impedance will be a limiting factor. So, while the transistor would very much like to draw a larger collector current, it won't change. So you think you're safe; the minimum specified \$H_{FE}\$ is fine, higher is still OK. There's something else to consider, however: \$H_{FE}\$ is not constant, it varies with \$I_C\$, and the datasheet should have a graph for this. So check this for the wanted collector current.

\$V_{BE}\$. Two PN junctions, so that's 2 x 0.65V = 1.3V. Olin found that a 300\$\Omega\$ base resistor should be fine, in fact leaves some margin. But when I look at the datasheet for the TIP110 it says \$V_{BE}\$ may be as high as 2.8V! That would result in a base current of \$\dfrac{3.3V - 2.8V}{300 \Omega}=1.7mA\$, and that's too little to get the wanted \$I_C\$ of 2A: \$400 \times 1.7mA\$ is only 670mA.

You're getting the idea. Don't simply use typical values, but make sure that your circuit still works with components with extreme parameter values. This is not so much of a problem with projects where you only build 1 device: you can see what's wrong and adjust. For production you have no choice: always design for worst case.

Yes, that's essentially correct.

However, you should actually over-drive the base of the transistor in order to make sure that it stays in saturation even if conditions change a bit (particularly transistor current transfer ratio, which is difficult to control precisely). This is called a "design margin".

So, assume that in the worst case, the h_{FE} of your transistor is 1/2 or even 1/3 the "nominal" value, and redo your resistor calculation accordingly. There's no harm in introducing more current than needed (within limits, of course) to the base of a transistor.

## Best Answer

No, not really. The collector of a bipolar transistor acts like a current source (or sink) whose value is determined by the base current and the h

_{FE}of the device. However, the external circuit can limit the current to something less than this value, in which case theeffectiveh_{FE}is lower.Yes. The actual value varies considerably from device to device, even from the same manufacturing batch, and it also varies somewhat with the operating parameters (voltage, temperature, etc.) of the device. You really can't depend on having a particular (or even a constant) value, so you design your circuits so that they work over a range of values.

This is part of the circuit design. When you're creating a voltage amplifier, you use the collector current of the transistor to develop the desired voltage across the external resistor. This resistor is called the "load resistor", and it gives you a definite value of output impedance — the transistor by itself has a very high effective output impedance.

Example:

OK, assuming you mean that 9V is applied to the base through a 10K resistor, 9V is applied to the collector through a 330Ω resistor, and that the emitter is grounded, the steps are as follows:

The base current is \$I_B = \frac{V_{BB} - V_{BE}}{R_B} = \frac{9.00 V - 0.65 V}{10k \Omega} = 0.835 mA\$

Assuming the transistor is not saturated, the collector current \$I_C = h_{FE} \cdot I_B = 100 \cdot 0.835 mA = 83.5 mA\$

The voltage across the collecor resistor should be \$I_C \cdot R_C = 83.5 mA \cdot 330 \Omega = 27.5 V\$

Since that value is higher than our supply voltage, the assumption made in the second step must be false — the transistor

issaturated. Therefore, the collector current is determined entirely by the collector resistor and the collector supply voltage: \$I_C = \frac{V_{CC} - V_{CE(SAT)}}{R_C} = \frac{9.00 V - 0.3 V}{330 \Omega} = 26.4 mA\$