5CCS2ELC - Electronic Circuits Quick LTspice Guide This document is a short guide to LTspice for Electronic Circuits module. SPICE Model Since in this module, we do not have a specific model of MOSFET or BJT, we will need to create a separate model for them. Used ’nmos’ or ’pmos’ component for MOSFET and ’npn’or ’pnp’ for the BJT as shown below. Then using the SPICE directive, insert the following command: .MODEL
NMOS( kp=?, vt0=?, lambda=?, W=?, L=?) Replace with the name of the transistor you are referring to. Also replace the value of each parameter according to the question or what you have obtained from analysis. Infinite Capacitor Value It is not possible to set the capacitor value to infinity and the software are unable to simulated like how we analysis it. Hence, when this happens, it is better to analysis the circuit part by part. Construct a DC Analysis Circuit and then a small signal circuit. Small Signal Circuit In small signal circuit, you will need a controlled current source. You can found this as ’g’ in the component menu. You will need to change the G value to be the gain of the voltage you put in. For example, to set it to gmvgs, you will need to connect the ’+’ and ’-’ to the vgs terminal and set G to the gm value you have calculated. © 2021 1 Department of Engineering King’s College London 5CCS2ELC - Electronic Circuits Simulation Command There are many useful simulation you can do in LTspice. It is useful to get used to how each one works and which suit each situation best. • Transient: Perform a non-linear, time domain simulation. • AC Analysis: Compute the small signal AC behaviour of the circuit linearised about its DC operating point. • DC Sweep: Compute DC operating point of a circuit while steeping independent sources and treating capacitances as open circuits and inductances as short circuits. • DC Transfer: Find the DC small-signal transfer function. • DC op pnt: Compute the DC operating point treating capacitances as open circuits and inductances as short circuits. Probing Option/ Graph In a simulation, you can always set your probing option to suit your analysis. For example, if you obtain a Vout-Vin graph, then the gradient of that graph is the gain. Credit. William Alsagoff (4th year Electronic Engineering) © 2021 2 Department of Engineering King’s College London 欢迎咨询51作业君
