# Bipolar Transistors connected in parallel

Bipolar Transistors (BJTs) can be connected in parallel if balancing resistors are placed in series with the emitter.

BJTs generally tend to become more conductive as their temperature increases. The example below from MMBT2222A datasheet shows one how this device’s typical gain can change by a factor of 3 to 5 times as its temperature varies over the allowed operating range. This causes thermal instability in BJTs operated in parallel, because even if the two transistors and their associated base resistors are perfectly identical (which is impossible), any slight temperature difference between them will cause one of them to begin conducting a greater portion of the shared current, which will cause it to become hotter, and carry even more of the shared current, and become even hotter still, until something eventually breaks.

Typically, BJTs can be more reliably operated in parallel if balancing resistors are placed in series with the emitter, rather than the base. Even a small resistance here will strongly counteract the tendency for imbalance, because an increase in current flowing through any one transistor causes the applied base-emitter voltage to decrease. This in turn reduces the amount of base current that flows, and therefore the portion of the shared current that the affected transistor carries. This technique is commonly known as “Emitter Degeneration.”

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@Barley_Li Do you have any links or references on how to calculate the value of balancing resistors. thanks.

Hi bharathshanmuganatha,
This Analog Devices resource about “Bipolar Transistor circuit design” would like to share to you.

It is faster and easier though to observe that a current imbalance results in different voltages across these resistors, and that this difference in voltage compared to the Vbe of the transistors used is what makes the addition of these resistors something that improves the circuit’s balance. A good starting place then, is to choose resistor values that produce voltage drops similar in scale to the transistors’ Vbe for the expected current flow.

In practice, matching the resistor values and transistor characteristics (including temperature) is just as important to achieving an accurate balance as the value of the resistance chosen.

Thank you very much, it was very helpful.