Introduction: Beta Meter
One day you wanted to become a nerd, studied transistor, came to know about a variable beta (current gain) of transistor, you became curious and bought one but did not afford to buy a measuring device that tells you the beta value of the transistor.
This project measures the beta value of the transistor with an accuracy of ±10.
Follow the steps!
you will require some maths :)
Step 1: Theory
When you became a nerd the first thing you would learn in transistor is base is the boss.
ie,. the base current determines the collector current(dc) given by the equation:
Ic=β*Ib β:current Gain now by omhs law across the resistor(R4) we get Ic=V/R4 V:potential across R4
V=β*Ib*R4 Now if we measure V with a mili-voltmeter keeping Ib*R4=10^-3V the reading will be β mV.
Step 2: Choice of Ib and R4
As there are 2 variables and one equation, we need to have some more information or parameters to choose the values of resistor and capacitor.
We take into consideration the power dissipation in the transistor which should not exceed its capacity, viz. 250mW** (the worst case power dissipation, when BJT goes to saturation).
keeping that in mind take R4=100 Ω, accordingly Ib=10 μA.
**contact for more info.
Step 3: Making Constant Current Source
This part in itself is a very good use of transistor.
Again another basic characteristics of p-n junction is the potential drop across the junction in forward bias is constant and is generally 0.7 V for silicon sub states.
taking that into account the base voltage Vb is constant 0.74 V (experimentally) and base-emitter voltage is
0.54 V so the potential across R2 is 0.2 V (0.74-0.54) which is constant.
As the potential across the resistor R2 is constant current will also be constant given by 0.2/R2 A.
the required current is 10 μA, R2=20 kΩ.
This current source is independent of Rl (load resistance) and the input voltage V1.
Step 4: Final Assembly
In place of Rl connect the base of the transistor which is to be examined.
NOTE: The values in the above circuit diagram is different because the transistor in the current source part is not same. So, do not blindly use the resistors as given in the circuit diagram, measure and calculate.
Step 5: Result
After all the connections apply a constant voltage source e.g. 1.5V, 3V, 4.5V, 5V(recommended) , 9V.
measure the potential across R4(collector resistance =100Ω) using a mili-voltmeter or multimeter.
The Value measured will be β (current gain) of the transistor.
Step 6: 2nd Version
For a more robust β meter design follow: