# Calculate Ohmic Resistance of Bipolar Junction Transistor

• Faisal Moshiur
In summary, the ohmic resistance of a PN junction can be calculated by measuring the voltage drop across the junction and extrapolating the straight line.
Faisal Moshiur
How am I supposed to calculate ohmic resistance from a non-linear electronic device component?
Say, for example, I need to calculate ohmic resistance of a pn junction of bipolar junction transistor from its forward I-V characteristics curves. (It can be either Base-Emitter junction or Base-collector junction.)

A non-linear device (like a pn junction) has no "ohmic resistance" per definition.
For such a non-linear V-I characteristic we can define only (a) a static resistance or (b) a dynamic/differential resistance.
Both resistances depend (vary) on (with) the corresponding DC operating point (Q point).

I am not sure exactly what you are asking. Perhaps the curve is fairly linear during certain operating ranges, therefore it could be approximated as a resistor for certain calculations.

It depends on your definition. The slope of the V-I curve at any point is a resistance. But the resistance changes for every different point on the curve.

Similarly, pick any two points on the V-I curve, draw a straight line between them. The slope of the line is the average resistance in that region.

That may not be useful, but it is one way to define it.

If that is not what you mean, then tell us how you want to define ohmic resistance.

anorlunda said:
It depends on your definition. The slope of the V-I curve at any point is a resistance. But the resistance changes for every different point on the curve.
.
Yes - that´s what we call differential resistance r=dV/dI..[/QUOTE]
More than that, pick any point on the V-I curve and draw a straight line between this point and the origin.
This gives you the static resistance R=Vo/Io (but it is not an "ohmic resistor" because this value depends on the selected point Vo,Io- the value R is not a constant as required for an ohmic characteristic).

Faisal Moshiur said:
How am I supposed to calculate ohmic resistance from a non-linear electronic device component?
Pass a small current such as 10 uA through the forward biassed PN junction. Measure the voltage drop across the junction, plotting the voltage across the junction against the Log10 of the current as you go. Increase the current in steps to 100 uA, 1 mA, 10 mA … At the start you should get a straight line, but at higher currents the measured voltage will begin rise faster than the extrapolated straight line. That extra voltage above the straight line is due to the ohmic resistance of the junction. You know the current flowing, and you know the extra voltage above the straight line, ohms law will then give you the ohmic resistance.
Keep the junction at a stable temperature. Apply higher currents for shorter periods and watch out for heating effects which will change the junction voltage.

Faisal Moshiur and Asymptotic

## What is a bipolar junction transistor (BJT)?

A bipolar junction transistor is a type of electronic device that is used for amplification and switching of electrical signals. It is made up of three layers of semiconductor material, and has two types of charge carriers (electrons and holes) that flow through the device.

## How do you calculate the ohmic resistance of a BJT?

The ohmic resistance of a BJT can be calculated using the formula: R = V/I, where R is the resistance, V is the voltage across the device, and I is the current flowing through the device. For a BJT, the resistance can vary depending on the voltage and current, so this calculation is typically done using a graph of the device's characteristics.

## What factors affect the ohmic resistance of a BJT?

The ohmic resistance of a BJT is affected by a few factors, including the type of semiconductor material used, the doping levels of the different layers, the size and shape of the device, and the temperature. These factors can all impact the flow of charge carriers through the device, and therefore affect its resistance.

## How does the ohmic resistance of a BJT impact its performance?

The ohmic resistance of a BJT can have a significant impact on its performance. A higher resistance can lead to a decrease in the device's ability to amplify and switch signals, and can also cause excess heat to be generated. It is important to carefully consider and manage the resistance when designing and using BJTs in electronic circuits.

## Are there any limitations to calculating the ohmic resistance of a BJT?

While calculating the ohmic resistance of a BJT can provide valuable information, there are some limitations to keep in mind. The resistance can change depending on the operating conditions of the device, and it may also vary between different BJTs of the same type. Additionally, the calculation is based on ideal conditions and may not accurately reflect the actual resistance in a real-world circuit.

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