Transistor calculation

[Rumieus]

In books which im currentyly studying.. all the transistor calculation for Ic and Ib using β=Ic/Ib. I copy the circuit from the book to multisim and measure it, and i got different result. Why is that so?

 

[Diver300]

β=Ic/Ib is a guide, and the gain varies with collector voltage and temperature. Transistors are also not linear so the gain varies with base current. There is also a large differences in gain between different transistors of the same type.

That is why transistor specifications are stated at a particular collector voltage and current. There is also a range of gains, often a minimum and a typical value.

Also if the collector voltage is too small, the transistor is in a condition known as saturated and the collector current falls.

Can you tell us what circuit you are simulating and what results you get?

 

[audioguru]

On another forum somebody said that Multisim uses the Max value of current gain instead of the "typical" value.

 

[Rumieus]

i copy the circuit from book which im currently studying.

In the book, it use β to calculate Ic and Ib..

I try to measure it using multisim and i got the measurement as follow..

If i calculate it using V=IR, the answer is different between β=Ic/Ib.

So what the use of β?
Em pls try to keep it simple when explaining..
Im not good in english nor electronic..
Just getting started and review electronics because i finish study in electronic and know nothing about it..

 

[audioguru]

The 100k resistor has 9.3V across it because the base-emitter of the transistor is a forward-biased 0.7V diode. So the base current is 9.3V/100k= 93uA.

When the hFE is 50 then the collector current is 93uA x 50= 4.7mA.
Multisim uses an hFE value of 104 so it shows a collector current of 9.73mA.

 

[Rumieus]

what do you mean when hFe = 50?
hFe=β right?
so the β=Ic/Ib?

 

[audioguru]

hFE is beta. It is Ic/Ib. At low current it is from 35 to about 100 for a 2N4014 transistor.

 

[Rumieus]

TQ AudioGuru. I'll try refer to my books.

 

[crutschow]

The 100k resistor has 9.3V across it because the base-emitter of the transistor is a forward-biased 0.7V diode. So the base current is 9.3V/100k= 93uA.

When the hFE is 50 then the collector current is 93uA x 50= 4.7mA.
Multisim uses an hFE value of 104 so it shows a collector current of 9.73mA.

Since the transistor has less then 0.3V at the collector and is in saturation, its measured hFE is less than it would be at a higher collector voltage where hFE is typically specified. Removing the collector resistor would give the hFE at 10V collector voltage.

 

[Diver300]

β is a good guide for how much base current you have to supply.

In your circuit, if you want to put nearly the whole 10V on R2, there will be about 10 mA in the collector. So you take the worst gain (35) and use that.

That means you need 10 mA/35 = 285 µA in the base, so you chose a resistor of 10/.000285 = 35 kΩ, so use 33 kΩ as a standard value.

If you are just turning transistors on and off, it is quite easy as you can just use more current than you need and it works OK.

For analogue circuits, the circuits are arranged so that the gain must be more than the minimum for the transistor (in this case 35). The circuit must work OK even if the gain is a lot larger.

 

[audioguru]

The saturation voltage loss of a transistor is spec'd when its base current is 1/10th the collector current for most transistors and 1/20th for very high gain transistors.

if you use a base current that is 1/35th the collector current then the transistor will not saturate very well.