Introduction: Β Meter Version II (more Stable and Accurate)
The version I β meter was quiet accurate but the current source was not constant with the input voltage (Vcc).
Version II β meter is quite stable ie., the current value does not change much with change in input voltage (Vcc).
Step 1: Whats the Difference in Version I and II ?
!.Version I worked on the forward bias region which is an exponential curve so as the current through diode increase the potential drop also increase.
Version II works on the break down region, the curve is lot more steeper in the breakdown region ie., potential drop across diode does not change much with change in current through it.
To ensure the diode to be in breakdown region the reverse bias current through the diode must be at least 5mA
By simple kvl we get R1=540 Ω. This will the the boundary point in breakdown region. We take R1=330Ω for the diode to be in breakdown region completely.
2. The Biasing Dc point of second transistor is also different now we are working on ib=1 uA and Rc=1 KΩ, Rather than ib=10 uA, Rc=100 Ω. The reason for doing this that the % change in current source with Vcc is constant so choosing a lesser ib value will give a lesser change in ib.
Step 2: Circuit Diagram
Selection of R2 is done by calculating the potential difference between R2 which is constant so a constant current should flow through R2, the value of R2 will decide the value of current.
You will find the calculations here: http://bit.ly/2S7KNjz
set ib=1uA and get R2
Although experimentally the value of R2 to be used will be a little different than the calculated, because of the tolerance in resistors.
Step 3: 1uA Current Source
Taking R2 to be around 2.7mΩ at 5 V (Vcc) I got a current source of 1 uA. This value varies from 0.9 uA to 1.1 uA if Vcc varies from 3.5V to 15V. The circuit does not work below 3.5 V because below this voltage the diode will not remain in breakdown region.
Step 4: Β = 264
The potential across R3 is measured in mV, 256mV is the reading, this is the β value of the npn transistor.
Step 5: Making
Step 6: Report
Link for Lab report : http://bit.ly/2MQKVyV