# when a diode is forwards biased can current flow backwards through it?

when a diode is forwards biased can current flow backwards through it?

this question comes from me trying to understand class AB amplifier biasing, so any additional comments about the diodes' use here would be appreciated

• I think you need to show the relevant part of the schematic and explain where your confusion arises. While you're editing, please fix your capitalisation and punctuation. – Transistor Nov 26 '18 at 20:21
• In the context of audio amplifier biasing, the answer is "No." Current cannot flow backwards when the diode is forward biased. In the context of high speed signals, the answer might be "yes, but only for a very brief instant". Because a forward biased diode takes a bit of time to "turn off" and during that time, current can flow "backwards." – mkeith Nov 26 '18 at 21:15

If a diode is forward biased, by definition the potential across it is such that current is flowing in the forward direction. In this situation there's no current flowing in the reverse direction.

If you suddenly change the potential across the diode such that it's reverse biased, and it's NOT a Schottky rectifier, then some reverse current will flow during the reverse recovery period. The current stops when enough charge (Qrr) has been delivered.

There's also a junction capacitance in all diodes which can allow signals with high frequency components to couple across the junction. But with a DC forward bias, current flows in the forward direction only.

In relation to your diagram, As Vin moves higher, the forward biased diode maintains roughly the same forward voltage and the drop across the top resistor is smaller. Actual current is always flowing in the same direction in the diode, but for small-signal AC analysis the current can be considered to be bi-directional around a DC operating point.

• In addition to capacitance, an AC signal where the amplitude of the non-DC components never sums negatively to the point where the diode ceases to be forward biased could have effectively reverse AC currents. – Chris Stratton Nov 26 '18 at 21:01
• @ChrisStratton good point, agreed. – John D Nov 26 '18 at 21:10
• So with reference to the schematic i've added to the question, if the bias is large enough to always keep the diode forward biased the AC signals from the input can move through the diode in "reverse" ? – Andrew Davis Nov 28 '18 at 8:19
• @AndrewDavis As Vin moves higher, the forward biased diode maintains roughly the same forward voltage and the drop across the top resistor is smaller. Actual current is always flowing in the same direction in the diode, but for small-signal AC analysis the current can be considered to be bi-directional around a DC operating point. – John D Nov 28 '18 at 15:48
• can you right that comment as an answer and i'll accept it as the answer to my question? – Andrew Davis Nov 28 '18 at 15:52