That is an excellent question.I’ve noticed that on a datasheet for a specific transistor that the beta (hfe) min and max values are given. How are you supposed to design when this varies so much? I need some help, please. Thanks.
So is the design value somewhat arbitrary? The reason I ask about beta in my post is that I need it to calculate the base current and resistor to get a certain Ic. How can this calculated resistor be correct if beta is a variable?That is an excellent question.
Current gain beta of any transistor will always vary from one device to another (from the same batch and part number) and for different operating conditions. You want to design the circuit so that it functions reliably under all likely values of beta, not the beta in the spec sheets. In other words, the circuit must work independent of what transistor is mounted on the board.
If, of example, the datasheet gives beta = 300, use a design value of 100.
If the transistor is to be used as a switch, use a design value of 10.
Collector current Ic is determined by your circuit elements, not by the beta of the transistor.So is the design value somewhat arbitrary? The reason I ask about beta in my post is that I need it to calculate the base current and resistor to get a certain Ic. How can this calculated resistor be correct if beta is a variable?
The details depend on the circuit. A well-designed circuit will yield a desired Ic as long as the base circuit is capable of providing some minimum base current, with the actual base current being what is needed to actually produce the desired Ic. This is done through a variety of feedback techniques, commonly relying on the base-emitter voltage being relatively constant over a wide range of operating conditions.So is the design value somewhat arbitrary? The reason I ask about beta in my post is that I need it to calculate the base current and resistor to get a certain Ic. How can this calculated resistor be correct if beta is a variable?
I see now how there is self control in the circuit via the emitter resistor!The base of most common-emitter transistors is not biased from a single resistor from the supply voltage, instead they have the base fed a voltage and current from a voltage divider (one resistor from the supply and a second resistor to ground).
Then the transistor has a series emitter resistor that provides negative feedback.
If the transistor has a high hFE and/or a high temperature and/or has a low Vbe then the collector current will try to be too high but the extra current in the emitter resistor reduces Vbe which reduces the collector current.
If the transistor has a low hFE etc. then the opposite occurs and the collector current is increased by the negative feedback.
Here is the circuit:
So the 100 and 10 are beta?Collector current Ic is determined by your circuit elements, not by the beta of the transistor.
You can calculate base current as
IB = Ic / 100
or
IB = Ic / 10 for switching applications.
This gives you a minimum design criteria for IB. Don't make it lower than this.
Thanks for the link.Hi quad,
Consider them as hfe and hFE, look over this link.
E
https://sites.google.com/a/davidmor...stors/troubleshooting-transistors/what-is-hfe
Clip:
The difference between hFE and hfe is: FE is for a fixed DC bias and a fixed DC current gain (and that must be specified for each value of hFE). hfe is the small signal AC current gain (and it is also specified for a given bias). hfe is frequency dependant. hFE is only good at DC.
Here is the crux of my problem:So the 100 and 10 are beta?
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