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.

 

A good designer will never do a design that depends on the value of a specific parameter like beta (hfe). Consider the operational amplifier where the gain of the circuit depends on the values of external components whose value can be tightly controlled. In particular it does NOT depend on the precise value of the open loop gain of the amplifier, it just needs to be larger than some value.

Go back and consider the Common Emitter amplifier circuit. Derive or lookup the expression for the voltage gain. Does it involve hfe? I think not. It involves the load impedance, the emitter resistor, and the emitter current.

 

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.

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, for 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.

 

Beta (hFE) may dynamically change in process of BJT work.
See how hFE depends on base current in particular circuit:

 

In addition to the beta (hFE range of numbers), the base-emitter voltage and the temperature also have ranges of numbers and affect the common-emitter transistor circuit.
I do not buy thousands of transistors, test them all then select only the few that work properly in a simple circuit. Instead I design a transistor with negative feedback that cancels most of the bad effects of the ranges of specs. Then all of the circuits I design and make work perfectly and the same.
Negative feedback also reduces the high distortion of a simple common emitter transistor and extends its bandwidth.

 

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.

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?

 

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?

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.

 

Here is an example of a common emitter amplifier.
Make sure R1 can supply more than the calculated IB.



Ic = Vs / ( Rc + RE)
IB = Ic / 100

 

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.

If your circuit requires you to know beta in order to determine what base current you need to make happen in order to achieve a desired collector current, then it is a bad design.

Simple circuits that require this approach are common in coursework when first working with transistors, because the focus is on the fundamental relationships and how to work with "the big picture" and a lot of ideal behavior is assumed -- just like how the world has no friction when you first start learning physics.

 

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:

 

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:

I see now how there is self control in the circuit via the emitter resistor!

 

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.

So the 100 and 10 are beta?

 

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.

 

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.

Thanks for the link.

 

So the 100 and 10 are beta?

Here is the crux of my problem:
Transistor as a Switch Example No2
Again using the same values, find the minimum Base current required to turn the transistor “fully-ON” (saturated) for a load that requires 200mA of current when the input voltage is increased to 5.0V. Also calculate the new value of Rb.

Transistor Base current:

Ib=Ic/beta

Transistor Base resistance:

Rb= (Vin-Vbe)/Ib

Transistor switches are used for a wide variety of applications such as interfacing large current or high voltage devices like motors, relays or lamps to low voltage digital IC’s or logic gates like AND gates or OR gates.

Here, the output from a digital logic gate is only +5v but the device to be controlled may require a 12 or even 24 volts supply. Or the load such as a DC Motor may need to have its speed controlled using a series of pulses (Pulse Width Modulation). transistor switches will allow us to do this faster and more easily than with conventional mechanical switches.


HOW CAN THIS BE VALID WHEN THE BETA IS VARIED?

 

Hi quad,
Your images are not visible.??
E

 

You are posting example questions taken from text books on how transistors work. Textbooks often use theoretical scenarios but often fail to show actual practical applications.

In your example, a transistor might show hFE = 300. However, this is for a given collector current Ic.
hFE will be different at a different Ic.

Let us look at the datasheet of a common transistor 2N2222.

You will notice that hFE values shown are minimum specifications.
Thus at Ic = 150mA, we can use hFE = 50 in our example question.
In a real world application, you want to derate this even further. The rule of thumb for switching applications is to use hFE = 10.
In practice, hFE in the range 10-20 is usually adequate.

 

It is VERY important to note that the quoted Beta values in a datasheet are for transistors in the active or linear operation. Meaning that VCE has at least a full volt across.
As the transistor approaches saturation, the Beta is reduced dramatically. It is not uncommon to see saturated Beta values one-tenth or less of its linear Beta values.

 

Which is exactly what transistor curves will show.

 

Lets look at it this way.

Suppose you have a load that will take 100 mA at 10V. And at that current, the beta of a transistor is anywhere from 12 to 15. (the beta does decline with current).

If you calculate the base current needed for the 15 beta and you get a transistor that is 12, it will not turn the transistor on fully with a 100 mA load.

So lets calculate it based on the 12 beta. Now what happens if your transistor actually has a beta of 15? Does it now supply more than 100 mA to the load?

No! With the 10V supply, the current cannot go above 100 mA.

Supplying extra base current does not change anything for the collector current. So you simply supply a little more current than the lowest beta transistor will need, and the higher beta ones are fine.

That is, of course over simplified. In reality, the transistor never tuns on fully. There is a voltage drop from the collector to emitter (Vce). In saturation this is usually around 200 to 300 mV. Higher base currents will actually lower the Vce a bit.