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ADALM2000 activity: The BJT connected as a diode

1. Explain the circuit in Figure 8: Configuration to lower effective forward voltage drop of diode 

2. Decreasing Negative supply, increases current in other direction , or in other words, it starts conducting in negative direction. Explain ??

3. Decreasing R2 value in Figure 8, reduces slanting of current line, Explain ?

  • Normally when a BJT is connected as a diode the base and collector are shorted (wired) together. This can be thought of 100% negative feedback around the transistor (being considered as common emitter amplifier).  In this circuit a PNP emitter follower (approx. gain of one) is used in place of the wire to close the feedback path.

    The PNP emitter follower is biased by the 150 K resistor. The gain of the emitter follower is slightly less than one and if the emitter current is increased (by lowering the value of R2) the gain becomes closer to "one".

    If the circuit is operating in the proper conditions, Q1 base greater than ground, and not heavily saturated (collector to emitter voltage greater than around 0.2V), the voltage on the collector of Q2 should not matter as long as it is more negative than ground.

    For more detailed study of the circuit I would suggest simulating it in LTspice so that you can plot all the voltages and currents simultaneously.