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State variables Overview Diode Mosfet BJT

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Adjusting state variables

Overview of how the state variables affect a circuit

To properly understand how the state variables will affect the operation of your circuit, let me first explain to you how each element's state is calculated. Foremost, changes in these state variables don't affect how the elements interact with each other; they just change how the voltages and currents in the circuit are interpreted. With the data from PSpice, MoHAT knows the voltage of each node that an element is connected to. Using a simple model for each element, the voltages are plugged into an equation with a couple of other state variables, depending on the element. Those state variable defaults are either obtained from the PSpice .MODEL statement or by common approximations. By manually adjusting the state variables, not the data, you can change when and under what conditions the elements will be in each state. Continue reading below to learn what equations are used for each of the three simulated elements and how to change the state variables.

Diode

The diode is the simplest of the three simulated elements and is either in the "On," "Off," or "Breakdown" state. The diode defaults to the "Off" position unless the specified conditions are met to put it in either the "On" or "Breakdown" state. To be in the "On" state, the voltage and/or current through the diode must be above certain values. For the default to be "On" requires a voltage above 0.7 volts and a current greater than 0 amps. To be in the "Breakdown" state, the voltage and/or current must be below certain values. By default, to be in "Breakdown," a voltage less than -3.0 volts or a current less than -0.00001 amps is needed.

To adjust these values, click on the Set state variables: Diode link in the controls menu. A dialog box will open with text boxes that allow you to enter new values. You may also choose whether both the voltage and current conditions must be met or if only one of the two is required to put the diode in the appropriate state. To apply the new values, you may either choose a single diode or apply the values to all the diodes in the current circuit. To apply the values to a single diode, type the name of the diode in the text box with the text "<name>" in it, and click the Instance button. If the name you entered doesn't match any diodes, then an error will appear and ask you to retype the name. To apply the new values to all the diodes in the circuit, just click the button labeled All Instances.

Mosfet

The mosfet has no state variables that you can change from within MoHAT. The state variables it uses directly affect how it's simulated in PSpice. To make sure the changes you make are accurate, you would need to rerun the simulation in PSpice. The only variable used from the PSpice .MODEL statement is the turn-on voltage, which is 'VTO.' The table below lists how the calculations are made to determine the operation for mosfets.

VGS: VGATE - VSOURCE
VSG: VSOURCE - VGATE
VDS: VDRAIN - VSOURCE
VSD: VSOURCE - VDRAIN
VTN: Voltage turn-on for a NMOS mosfet
VTP: Voltage turn-on for a PMOS mosfet

BJT

The BJT has two state variables that help determine which of the four modes of operation it is in. They are the turn-on values for the base-emitter junction and the base-collector junction. To adjust these values, click on the Set state variables: BJT link in the controls menu. A dialog box will open with text boxes that allow you to enter new values. To apply the new values, you may either choose a single BJT or apply the values to all the BJTs in the current circuit. To apply the values to a single BJT, type the name of the BJT in the proper text box, and click the Instance button. To apply the new values to all the BJTs in the circuit, just click the button labeled All Instances. The table below shows the formula used to calculate the operating mode of BJTs. NOTE: The state variables that you can change are highlighted in purple.

VBE: VBASE - VEMITTER
VEB: VEMITTER - VBASE
VBC: VBASE - VCOLLECTOR
VCB: VCOLLECTOR - VBASE
VBE(ON): Voltage at which the base-emitter junction is considered ON
VBC(ON): Voltage at which the base-collector junction is considered ON