**Created: 2023-07-08** ## At a glance - Used to restrict flow of current - They make good rectifiers (half-wave, full-wave, peak rectifiers) - We have devised useful models to analyze their working (small signal model, exponential model, iteration method, constant voltage drop model, ideal diode model) ## Content ### Models #### Exponential Model (modeling diode forward characteristic) Most accurate model for the diode. However, non-linear nature makes it hard to work with it. The diode current is given by:`` $I_{D} = I_{S}e^{\frac{V_D}{V_T}}$ Where $I_S$ is the saturation current, $V_D$ is the diode voltage and $V_T$ is the thermal voltage (25 mV at room temperature). The diode current $I_D$ can also be found by applying KVL. A combination of these equations is used in iterative and graphical analysis. #### Constant Voltage Drop Model Extremely simple model. Assumes that the voltage drop $V_D$ in a forward conducting diode remains constant within a range of 0.6-0.8V. Most commonly, the value is assumed to be 0.7V. #### Ideal Diode Model Extremely simple. If diode is forward biased, it conducts (ON state). If not, it is open (OFF state). Doesn't yield accurate answers, it is good for quick back of the envelope calculations #### Small Signal Model You will use one of the models above to first find $V_D$ and $I_D$. The diode is then modeled by a resistance equal to the inverse of the slope of the tangent to the exponential i-v characteristic at a point known as the 'bias point' This sounds complex if you are not comfortable with "reading" math. Here's a graphical representation: ![[diode_small_signal.png]] The first question in the Questions tab should show you how to use this in practice. We have more models that are concerned with the operation of diodes in the breakdown region. (Zener diodes) ## Questions ### When do we use the small signal model for a diode? Well, most of the times, the question will explicitly tell you which model to use. I was stuck on the exact same approach as this guy right here: https://www.physicsforums.com/threads/design-the-diode-circuit.796569/ I found their discussion quite interesting. The "mentor" mentions that in the small signal model the load current is the signal and that the DC operating point does not make its way into the final model. I still do not have an intuitive understanding of diode related models. I will note down any and all questions here.