Help with a Diode Circuit?
Find Vout for the following circuit when Vin = 12V and when Vin= -12V. http://imgur.com/MLLmd.jpg . To me, it seems like in both cases, the circuit would act as a voltage divider with Vout = (Equivalent Resistance of Grounded Resistors)*Vin / Req but with the negative voltage, I think that the node with the diode is an open circuit so its only Vout = 100 ohms*-12V / 100 ohms. The potential difference across the diode is to be taken at 0.6V but I can’t see how that would matter. I just see the diode blocking any current through R2 if the voltage is negative. Am I oversimplifying the problem?
Yes, for the negative case Vin = -12 V it is a simple voltage divider giving Vout = -6 V
But for the positive case where the diode is in the forward mode, things get a bit more involved. You solve your circuit in two steps:
a) the circuit without the diode and the associated resistor R2. This would give a voltage of +6 V with a source resistance of 50 Ohms (R1 parallel R2)
b) as a second step you add the resistor R2 connected to a fixed +0.6 V point. The equivalent circuit has a resistor of 50 Ohms connected to +6 V and a 100 Ohm resistor connected to +0.6 V
This voltage divider of the equivalent circuit is having a current of
5.4 V / 150 Ohms = 36 mA
Voltage across R2 is 36 mA * 100 Ohms = 3.6 V
Vout = 3.6 V + 0.6 V = +4.2 V
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