How do I test a bipolar transistor for Collector Cutoff Current on my curve tracer?
Collector Cutoff Current - ICEx or ICEv
What It Is:
Collector cutoff current is the IC that still flows when the specified VC and a specified reverse bias is applied. It is normally less than either ICEO or ICER (collector current with the base open, or with the base resistively connected to the emitter). This is because the reverse bias removes most thermal and avalanche carriers from the base.
Because of non-linearities and the effect of base drive source impedance, base drive is often specified as a current instead of a voltage. The symbol ICEX is commonly used for the current method, while ICEV is used for the voltage method. The current and voltage drive capabilities of the curve tracer permit either method to be used.
On the curve tracer, VC is applied by the Collector Supply. Reverse bias is applied to the base-emitter junction by the Step Generator, and IC is sensed at the collector terminal.
What The Display Shows:
The display shows the applied VC on the horizontal axis, and the resulting IC on the vertical axis. The specification is met when with the specified values of VC and reverse bias (current or voltage) applied, IC doesn't exceed the specified maximum.
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 the specified IC 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 (Icex) or Voltage (Icev)
H: Step Generator polarity to apply reverse bias (- for NPN), (+ forPNP)
I: Step/Offset amplitude to approx 50% of the specified base current (ICEx) or base voltage (ICEv)
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: Press the Step Gen Offset Oppose until Step Gen Offset reaches the specified value of reverse bias
C: Slowly increase the Variable Collector Supply % until the specified VC is reached
3. Compare to data sheet specifications:
Check that at the specified values of VC and reverse bias, IC is less than or equal to the specified maximum
Note 1: Collector cutoff current may be so low that it becomes necessary to use Mag mode to obtain a meaningful reading. Press the Mag X 10 button and use the Display Position buttons to center the display.
Note 2: If noise becomes a problem, select Avg as the Acquisition mode. Avg averages 16 samples to reduce noise.
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