How to Run Bipolar Transistors in Parallel

Understand how to double up transistors reliably to increase their capacity while not falling into the design pitfalls and traps.

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In transistor circuit design, or circuit design using any device, running two active devices in parallel is not often undertaken.

There can be issues to running two transistors in parallel, but if it is addressed correctly, then it can be achieved very successfully.

Issues with running transistors in parallel

in many instances, running two active devices, and in this case bipolar transistors may not be advisable, and indeed it is not often seen in many designs.

This is because of the issues with sharing the "load" equally between the two devices. It can be a bit like doubling contacts in a connector, and it ends up with one taking all the current, and the other virtually none.

For bipolar transistors it can be even worse. BJTs become more conductive as their temperature rises, and the gain increases.

This can be a major issue because one transistor is bound to take more current than another because the manufacturing processes mean that gain and the like can never be exactly the same. This means that one transistor will take more current than its pair. As this happens it will heat up more, and take even more current, and so the proces will evolve until one transistor takes all the curent and the other takes virtually none.

This can be disastrous because one transistor will take virtually allt he current and the whole reason for having two transistors will be nullified.

The solution

It is possible to use two bipolar transistors connected in parallel if the circuit is designed correctly.

Some designs show sharing resistors in the base circuit, but this is not correct.

The way to enable two transistors to run in parallel and share the "load" is to incorporate two identical balancing resistors, one in each of the emitter circuit of each transistor.

How to connect two bipolar transistors in parallel

Placing the resistors in the emitter, effectively introduces some negative feedback and means that the current drawn is not totally governed by the rather variable parameters of the transistor itself.

The balancing resistors work because if one transistor starts to draw more current, the voltage across the resistor will increase and in turn this will reduce the base emitter voltage across that transistor - note that both base connections are connected together and they will have exactly the same base voltage.

As the base emitter voltage decreases, so the current drawn by the transistor will decrease - in fact the system is self stabilising and provides the overall circuit with the sharing that is required.

This is a very simple solution to an issue that is often encountered in electronic circuit design, especially where the power limitations of single devices might limit the operation of a circuit.