BJT transistor calculation

Thread Starter

Matung Bechok

Joined Jan 6, 2017
2
"I wonder about BJT maximum collector current, maximum base current, and maximum emiter current, or maximum voltage ,and whether or not these values can be use to calculate the needed minimum current or voltage to find all resistors and other parts?"
 

MrChips

Joined Oct 2, 2009
30,711
"I wonder about BJT maximum collector current, maximum base current, and maximum emiter current, or maximum voltage ,and whether or not these values can be use to calculate the needed minimum current or voltage to find all resistors and other parts?"
No. The max current and voltages tell you the values you want to stay away from if you want your circuit to survive.
You use the operating current and voltages and you desire and the behavior of the circuit to calculate the resistor values.
 

dl324

Joined Mar 30, 2015
16,845
Welcome to AAC!

Maximums represent values that should not be exceeded, except in the manner(s) specified in the datasheet. For example, maximum collector current is stated as a continuous value and a higher peak current is allowed as long as certain conditions are met.

These guide you in selecting a device that will satisfy your requirements, but have nothing to do with the minimums you choose.
 

Thread Starter

Matung Bechok

Joined Jan 6, 2017
2
Welcome to AAC!

Maximums represent values that should not be exceeded, except in the manner(s) specified in the datasheet. For example, maximum collector current is stated as a continuous value and a higher peak current is allowed as long as certain conditions are met.

These guide you in selecting a device that will satisfy your requirements, but have nothing to do with the minimums you choose.
Thank you for your feedback! For clarification, Ic, Ib, and Ie seemed to have come with maximum currents predicted by their manufacturers, so, if I take half of each I would be ok?
 

MrChips

Joined Oct 2, 2009
30,711
Thank you for your feedback! For clarification, Ic, Ib, and Ie seemed to have come with maximum currents predicted by their manufacturers, so, if I take half of each I would be ok?
That is not the correct approach. Forget the specifications or even the number of the transistor.

Firstly, look at the circuit application.

What is the circuit required to do?
What are your supply voltages?
What are the input, output, frequency, gain, and power requirements and restrictions?

Design the circuit first. Then you select an appropriate device.
 

BobTPH

Joined Jun 5, 2013
8,813
Agree with Mr Chips.

For instance, the max emitter to base voltage of a 2N3904 is listed at 6V, but it most applications that voltage will be between 0 and -0.7V.

If you design for 3V, you will have a transistor that will be permanently off!


Bob
 

crutschow

Joined Mar 14, 2008
34,283
Generally you design the transistor circuit to use the minimum voltages and current that will allow the circuit to perform the intended application.
You always start with the requirements for the circuit application and work from there.
Then you select a transistor whose maximum values are well within those requirements.
 
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