Electronic – Is the current gain value constant for a single BJT

amplifiercurrenttransistors

hfe of a npn bjt transistor is given by collector current / base current so to calculate hfe you need to know the collector current and base current (which can be set using resistors) . I have to ask , what is the point of doing this ? You can simply calculate the emitter current using Kirchhoff current rule . This is obviously pointless since the transistor acts like a resistor between collector and emitter and unless you know the resistance of the resistor you can't find the current .

I see in transistor datasheets a maximum and minimum value of hfe . This leads me to believe that hfe is a constant and I do not undersand the point of a varying constant ; I think for a single transistor only a single value of hfe exists which must be measured using a multimeter and the collector current is dependent of the base current so the collector current can be measured .

And also , if the collector current is entirely dependent on the base current (Ic = Ib*hfe) then what is the point of the adding a resistor to the collector end of the transistor ? Surely there must be some change in current if a greater resistance is added to the collector end . I checked this with a multimeter with base resistance of 10k ohms and collector resistance of 330 ohms and I found no change in current when I connect the multimeter in common emitter topology (I think , I connect the emitter to ground and tested the current flowing through the collector )

I think there is a big hole in my understand or a big misunderstanding in my not understanding . Please help

Thank you

Best Answer

"... since the transistor acts like a resistor between collector and emitter ... "

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 hFE of the device. However, the external circuit can limit the current to something less than this value, in which case the effective hFE is lower.

"I see in transistor datasheets a maximum and minimum value of hfe."

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.

"... then what is the point of the adding a resistor to the collector end of the transistor?"

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:

collector emitter voltage is 9 v ,Hfe = 100 , base emitter voltage is 9 , a resistor at the collector has resistance of 330 ohms and one at the base has resistance 10k ohms, tell me the current at the collector with steps.

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 is saturated. 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\$