Understanding Transistor Gain

Understanding Transistor Gain 3/29/2015

A little background

I’ve read a little about transistors and I’ve even used the in some simple circuits that I’ve tinkered with.  However I really have never experimented with them.  In all cases I’ve used a transistor as a switch, either turning it on or turning it off, basically I’ve been using the transistor in only two of its three possible operating zones.  Those zones are:

1. Cutoff:                The transistor is off, no current is flowing
2. Active:                The transistor is active, some current is flowing, which is dependent on the base current
3. Saturation:          Maximum current is flowing

I’ve been working either in Cutoff or Saturation.  I want to now experiment in the active zone.

Experiment

I’ve created a simple circuit using two 1KΩ  resistors, a 10KΩ variable resistor, and an NPN transistor, P/N C9014.  My goal is to characterize the transistor by measuring the Emitter and Collector currents for varying Base currents.  The figure below shows the circuit I created to measure the transistor currents.   The 10KΩ variable resistor is used to vary the base current.  I varied the base resistance from the lowest setting of 1.45KΩ up to 10KΩ in increments of 1KΩ.  The 1.45KΩ was the only non-integer resistance value, the remaining values were taken at whole numbers starting with 2KΩ.

The drawing below shows the setup I used.

 

Below is a graph showing the base, emitter, and collector current.  The graph shows that as the base current increases from the first measurement point (point 1) to the last measurement point (point 10) the collector current remains pretty consistent.  At the first measurement point the base current starts out at 10 mA, the base resistance is 1.45KΩ and the collector current is 15.6 mA.  At the last measurement point the base current is .34 mA, the base resistance is 10KΩ and the collector current is 15.98 mA. The table below the graphs shows the Base resistance values and the resulting Base, Emitter, and collector current values.

 

 

 

The results show that even with a base current as low as .34 mA the collector current is still around 16 mA.  I would like to see what happens if I continue to lower the base current, at what point will I see a significant drop in the collector current?  In my next experiment I will setup the same circuit with a variable resistor that lets me decrease the base current to values small than .34 mA to see what happens to the collector current.  Stay tuned for Part 2 of this experiment.