PSU output into BJT Collector current

[gjpollitt]

I am trying to calculate the hFE of a BC108 transistor using a tutorial book. One of the exercises is to measure the current at the collector output based on the current input at the base.

ie 5ua at base gives x at collector
10ua at base gives x at collector

upto 100ua and then plotting these on a graph for loadline etc

The tutorial states to set a bench top psu to 5v to keep constant voltage at the collector. I havent got one of these yet (I will build my own soon as my first proper project) so am taking the collector voltage from the circuit supply voltage (9v) and a 1k pot set to 5v.

My question : Is using a 1k pot set to 5v going to give me a different set of results on the above compared to using a psu voltage ie in terms of collector current. I can calculate the current through the 1k pot and change that to alter the current but what about a bench top psu, what would that do the current at the collector, would I have to put a resistor in series with it to give me a set current?

Secondly when I have calculated the hFE gain and the transistor is swapped with a different one would I have to do this again, especially if my circuit was beta specific? I have read that the hFE can vary considerably on each device. And if so how do factory processed boards compensate for this?

thanks
Graham

 

[JimB]

Yes you can use a pot to give a lower voltage, but be aware that the voltage will drop as the current supplied by the pot increases, you will have to keep checking the voltage and re-adjust as necessary.

A different transistor of the same type usually has different characteristics, you will get different results.

Circuits should be designed so that they are not device dependant. There are various biassing schemes which wil compensate for the variability of individual transistors.

JimB

 

[audioguru]

Hi Graham,
You can maintain 5V between the collector and emitter of a transistor by changing the collector load resistance with a pot that goes to a higher supply voltage (like you have been doing) or by varying the supply voltage to a fixed load resistor.
Another problem with testing hFE is temperature. The transistor will heat-up at higher currents so manufacturers test hFE with a very short pulse to the base and measure the resulting collector current pulse with an oscilloscope.

Biasing schemes to reduce the effect of different amounts of hFE in a circuit are:
1) Use a voltage divider to feed the base instead of just a single resistor to the supply.
2) Bias the base with a resistor from its collector. This form of negative feedback works fairly well but reduces the transistor circuit's input impedance. If this transistor circuit is fed from a resistance then the overall gain is reduced.
3) Use an emitter resistor in series with the emitter to the negative supply (for an NPN transistor). This form of negative feedback also reduces the gain of the transistor circuit. The emitter resistor can be bypassed with a capacitor to maintain AC gain.
4) Use overall negative feedback in an entire transistor circuit (like an amplifier) from the output to an inverting input. The circuit's AC gain can be maintained by using a voltage divider for the negative feedback that connects to a capacitor to ground.
5) A few or all of the above. :lol: