1 ELE1110D Basic Circuit Theory Tutorial 6 Diode Circuits By Xu Ceng SHB 832

2 Diode Circuits – Basic Concept Diode: –Simply a P-N junction Due to diffusion and recombination, Potential Barrier formed in Depletion Region. –Forward Bias (V D > 0): Apply a positive voltage V D across PN junction. Decrease the potential barrier  force diffusion Current I D can flow in the direction from P to N. –Reverse Bias (V D < 0): Apply a negative voltage V D across PN junction. Increase the potential barrier  further stop diffusion Current I D cannot flow in the direction from P to N. –Reverse Saturation Current (I S ): Minority Carriers created by thermal energy. A tiny current due to minority carriers flow

3 Diode Circuits – Current Model An equation that approximates the diode current: – –v D is the voltage across the diode –I 0 is the reverse saturation current –k is Boltzmann constant –T is temperature in unit of Kelvin –q is charge

4 Diode Circuits – Current Model Example : Consider the diode equation with I 0 =10 -9 A. If V D is varied between 0.4V to 0.8V, what is the corresponding diode current?

5 Diode Circuits – Simple Circuits Simplified model: –In practice, Diode conducts only when the potential difference from P to N is greater than 0.6V. –We say it is in Forward Bias. –Voltage drop across diode is fixed at 0.6V. –Diode is open circuit in Reverse Bias.

6 Diode Circuits - Simple Circuits Diode conducts only when _________. In Forward Bias, voltage across diode is _______. Therefore, voltage at B is ________. When _______, the diode is in Reverse Bias. The diode is open-circuit. Therefore, voltage at B is ________.

7 Diode Circuits - Simple Circuits Diode conducts only when _________. When in Reverse Bias, diode is ________________. Therefore, V OUT = ______________. When in Forward Bias, voltage across diode is always ________. Therefore, V OUT = ___________.

8 Diode Circuits – Simple Circuits Diode X conducts only when ________. When in Forward Bias, voltage across Diode X is ________. Therefore V A = V OUT = ________. Diode Y conducts only when ________. When in Forward Bias, voltage across Diode Y is ________. Therefore, V B = V OUT = ___________.

9 Diode Circuits – Simple Circuits Example (Midterm 2003): In the circuit shown below, the input is a sinusoid with peak voltage equal to +/- 10V. Assume that the diode starts to conduct when the forward biased voltage is greater than 0.6V, sketch the output voltage 0V 5V V OUT V IN

10 Diode Circuits – Application Full-wave bridge rectifier