# Datasheet does not provide beta of transistor!!!

#### babaliaris

Joined Nov 19, 2019
115
My starters pack just arrived and I'm doing some tests. I'm currently trying to create this circuit to test a transistor:

The manual of the transistor can be found here. The thing is that the manual does not say anything about b? How am I going to calculate Ib, Ic, and Ie?
The manual says that the transistor operates at 100mA max 200mA. So what I'm trying to do here is requiring that Ic = 100mA (just enough to operate) the voltage of the led according to the manual can operate at 2-3.6V and 20mA, so I require that Vl=3v (enough brightness) and now I somehow need to figure out R1, R2, R3.

The main problem right now is that I don't know b. Where can I find it? From class, the professor told us that b is a characteristic of the transistor so shouldn't be given by the manufacturer?

Thank you.

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#### SamR

Joined Mar 19, 2019
4,194

#### Papabravo

Joined Feb 24, 2006
18,403
β is related to both the small signal gain $$h_{fe}$$ and the DC or large signal gain $$h_{FE}$$ both of which are contained in the datasheet. The use of h-parameters is a more modern and consistent way of showing the parameters.

#### babaliaris

Joined Nov 19, 2019
115
This is the one. So how do I calculate it exactly?

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#### SamR

Joined Mar 19, 2019
4,194
β=hFE

#### babaliaris

Joined Nov 19, 2019
115
So it is 420? (since my model is 547C) . But why does it have a min and a max value?

#### Papabravo

Joined Feb 24, 2006
18,403
The datasheet will give you a range of values for the parameter. The range includes a typical value which you can treat as if it were the mean of a normally distributed random variable. It will also give you a minimum which you can consider the lower 3σ point of the normally distributed random variable and it might give you a maximum which is the upper 3σ value. what this tells you is that 99% of the parts will have a value in the range.

To compute the value experimentally:
For the large signal gain you measure Ic and Ib and take the ratio at fixed value of Ib and Ic
For small signal gain you need to at frequency as a part of the equation because there is a frequency at which the gain is 1, but it still involves instantaneous values of ic and ib

#### dl324

Joined Mar 30, 2015
14,458
But why does it have a min and a max value?
Beta is not a tightly controlled parameter.

#### SamR

Joined Mar 19, 2019
4,194
Lots of manufacturing variances and temperature effects on transistors. A cheap chinese M-Tester will give you a good reading on the individual transistors Vb and hFE. At least a decent ballpark reading.

#### dl324

Joined Mar 30, 2015
14,458
I'm currently trying to create this circuit to test a transistor:
Where did you get that circuit? It's more typical for the LED to be placed on the collector of an NPN transistor so the voltage required to turn it on is fairly constant. The way you have it connected, the voltage drop across the LED and it's current limit resistor determine the voltage needed on the base to light the LED.

#### BobaMosfet

Joined Jul 1, 2009
1,979
My starters pack just arrived and I'm doing some tests. I'm currently trying to create this circuit to test a transistor:

View attachment 193134

The manual of the transistor can be found here. The thing is that the manual does not say anything about b? How am I going to calculate Ib, Ic, and Ie?
The manual says that the transistor operates at 100mA max 200mA. So what I'm trying to do here is requiring that Ic = 100mA (just enough to operate) the voltage of the led according to the manual can operate at 2-3.6V and 20mA, so I require that Vl=3v (enough brightness) and now I somehow need to figure out R1, R2, R3.

The main problem right now is that I don't know b. Where can I find it? From class, the professor told us that b is a characteristic of the transistor so shouldn't be given by the manufacturer?

Thank you.

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

#### dl324

Joined Mar 30, 2015
14,458
So it is 420? (since my model is 547C) .
Probably not. It depends on how you have the transistor biased and we can't determine that without values for the resistors.

If you're operating the transistor in saturation mode, the datasheet gives a beta of 20.

#### Papabravo

Joined Feb 24, 2006
18,403
It is a huge mistake to design a transistor circuit that DEPENDS on the specific value of ANY parameter, because unless you are willing to pay a person to test each part that goes into production it is essential that the specific value should be a don't care. Can you do this reliably? People have been doing just that since I was born (1948)

#### SamR

Joined Mar 19, 2019
4,194
Just to basically test a transistor build a very basic switching circuit on a breadboard with an LED to indicate switching. Google around and look at the various transistor switch circuits to get an idea and test it out. It is probably time to get familiar with LTSpice and use its ability to model and test circuits.

#### babaliaris

Joined Nov 19, 2019
115
It is a huge mistake to design a transistor circuit that DEPENDS on the specific value of ANY parameter, because unless you are willing to pay a person to test each part that goes into production it is essential that the specific value should be a don't care. Can you do this reliably? People have been doing just that since I was born (1948)
So in other words, you are saying I don't need to know b and the transistor currents?

Where did you get that circuit? It's more typical for the LED to be placed on the collector of an NPN transistor so the voltage required to turn it on is fairly constant. The way you have it connected, the voltage drop across the LED and it's current limit resistor determine the voltage needed on the base to light the LED.
I think I'm getting what you're saying. I'm not experienced in practice but I do take circuit classes (I've seen a lot in theory).
So when I'm using a transistor as a switch, the rest of the circuit should be connected through the collector (and emitter just goes to ground), and when I'm trying to make an amplifier or something, then I'm connecting stuff to the emitter?

#### SamR

Joined Mar 19, 2019
4,194
Try it and see That's what breadboards are for and how to learn. Switching circuits are pretty basic. All you need is a Vbe just above 0.7V and current limiting for Vce to prevent overloading the transistor.

#### dl324

Joined Mar 30, 2015
14,458
So when I'm using a transistor as a switch, the rest of the circuit should be connected through the collector (and emitter just goes to ground), and when I'm trying to make an amplifier or something, then I'm connecting stuff to the emitter?
Yes. something like this is more typical:

The circuit fragment is contrived because the transistor serves no purpose and can be replaced with S1.

#### babaliaris

Joined Nov 19, 2019
115
Will I burn the transistor with the following? In the University Lab, we made a signal amplifier but we had a resistance in the base to limit the current go into the base (At least 1KΩ and higher). There is probably a reason for that. I suspect this because $$I_{C} = bI_{B}$$ so if $$I_{B}$$ is high enough it will burn the transistor right? So should I put a resistor instead of connecting the source directly to the base?

#### dl324

Joined Mar 30, 2015
14,458
Will I burn the transistor with the following?
Yes. The only thing that would protect the BE junction is the series resistance of the 5V source.

So should I put a resistor instead of connecting the source directly to the base?
Yes.

#### ericgibbs

Joined Jan 29, 2010
15,515
hi,
you must add a series Base resistor.
E