How do I test a bipolar transistor for Small Signal Current Gain on my curve tracer?
Small SignalCurrent Gain - Small Signal Beta or hfe
What It Is:
Small signal current gain is the ratio of a change in IC to a change in IB, with the initial IC value specified.
On the curve tracer, hfe is checked by measuring the difference in IC between two values of IB. The Collector Supply drives the collector and the Step Generator drives the base. The Step Generator provides two values of IEB - the first to provide the specified initial value of IC, the second to provide (IC x 2). The change in collector current is divided by the change in base current to arrive at hfe.
What The Display Shows:
The display shows VCE on the horizontal axis, and IC on the vertical axis. With the Step Generator providing base drive, two values of IC will be displayed - the first at specified initial value of IC and the second at (IC x 2).
The specification is met when hfe is between the specified min/max limits.
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: Collector Supply Polarity to (+DC) for NPN or (-DC) for PNP
E: Vertical Current/Div to display (IC x 2) between the 5th and 10th vertical divisions
F: Configuration to (Base/Step Gen, Emitter/Common)
G: Step Generator to Current
H: Step Generator Polarity to apply forward bias (+ for NPN), (- forPNP)
I: Step Mult .1X to On
J: Number of steps to zero
K: Step/Offset Ampl to approx 1% of the specified IC
L: Variable Collector Supply to minimum % (full ccw)
M: Dot Cursor 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. Apply base drive:
A: Press and hold the Offset Aid button until the specified initial value of IC is attained and note the value of IB (name it IB1)
B: Press and hold the Offset Aid button until (IC x 2) is attained and note the value of IB (name it IB2)
4. Calculate hfe
Calculate, using the formula: (delta IC / delta IB)
5. Compare to data sheet specifications:
Check that hfe is within the specified min/max limits
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