How do I test a bipolar transistor for DC Forward Current Gain on my curve tracer?
DC Forward Current Gain - DC Beta or hFE
What It Is:
DC forward current gain is the ratio of IC to IB under specified operating conditions. Because of variability with temperature, VC and IC values must be obtained at specific operating points. Values at different operating points are often given.
On the curve tracer, VC is applied by the Collector Supply. Forward IB is applied by the Step Generator, and IC is sensed at the collector terminal. With the Step Generator providing base drive, the curve will be displaced upward from the horizontal axis as the base drive causes a proportional collector current.
What The Display Shows:
The display shows VC on the horizontal axis, and the resulting IC on the vertical axis.
The specification is met when, at either the specified IB or the specified IC, the value of (IC/IB) falls within the specified min/max range.
How To Do It:
1. Set controls:
A: Max Peak Volts to the lowest setting above the specified VC
B: Max Peak Power Watts to the lowest setting that satisfies (IC x VC)
C: Horizontal Volts/Div to display VC between the 5th and 10th horizontal divisions
D: Vertical Current/Div to display IC between the 5th and 10th vertical divisions or, if IC isn’t specified, to display (IB/hfe) between the 5th and 10th vertical divisions
E: Collector Supply Polarity to (+DC) for NPN or (-DC) for PNP
F: Number of steps to minimum (zero)
G: Step Generator to Current
H: Step Generator Polarity to apply forward bias (+ for NPN), (- forPNP)
I: Step/Offset Ampl to approx 1% of the specified base current or, if none given, to approx 50% of the value of (IC/hfe)
J: Configuration to (Base/Step Gen, Emitter/Common)
K: Variable Collector Supply to minimum % (full ccw)
L: DotCursor ON
2. Apply power to the transistor:
A: Position the Left/Right switch as appropriate
B: Slowly increase the Variable Collector Supply % until the specified VC is reached
3. Adjust to parameters:
Press and hold Offset Aid until an appreciable vertical displacement of the curve occurs. It will be necessary to readjust Variable Collector % to maintainVC. Continue adjusting Step Offset and VC alternately until the specified operating point is reached.
Note: If the displayed curve drifts, thermal effects are present. To see how serious it is, use Vert Envelope Acq Mode and observe the drift. If the problem is severe, it may be impossible to obtain an accurate reading, in which case....
....To minimize thermal effects, use the Step Generator's Pulsed mode (Long or Short). The idea is to reduce the On/Off duty cycle. In pulsed mode, it may not be possible to exactly match the specified collector or base current.
4. Calculate hFE:
Read hFE directly from the cursor readout
5. Compare to data sheet specifications:
Check that the value is within specifications
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