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Using the P key

During a DC analysis run, the value of the expressions for each curve can be seen by pressing the 'P' key. This key toggles the printing of the numeric values on the analysis plot adjacent to the expressions. This is a convenient way to check the course of a new, lengthy simulation when the initial plot scales are unknown. This feature may significantly slow the simulation, so only use it to "peek" at the numeric results, then toggle it off with the 'P' key.

Troubleshooting tips

Here are some useful things to remember for DC analysis:

IV curves:

To generate IV curves for a device, place a voltage source across the output leads and sweep its voltage using the Variable 1 source. Place a voltage or current source at the base or gate and step its voltage or current with the Variable 2 source. Look at the IVBJT sample circuit for an example of how to do this.

No convergence during a sweep:

DC analysis is the most difficult of all the analysis modes, because it does not benefit from the converging effects of capacitors and inductors, as transient analysis does. If convergence fails, especially during a sweep, try changing the starting and/or step value to avoid the problem area.

147

148 Chapter 8: DC Analysis

Chapter 9

Dynamic DC Analysis

What's in this chapter

This chapter describes the features of Dynamic DC analysis. This analysis mode is designed to dynamically display the DC voltage, current, power, and device conditions, as the circuit responds to user changes.

149

What happens in Dynamic DC analysis

Dynamic DC is an interactive process in which the user modifies the circuit and the program calculates the DC response immediately and displays one or more measures of the DC state. The process looks like this:

User modifies the circuit

MC7 finds the DC solution

The schematic display is updated

The schematic has four optional display buttons for displaying the circuit's time domain quantities. Each can be individually enabled.

Voltages/states

Device pin currents

Device power

Device condition (ON, OFF, SAT, LIN, etc.)

When the Dynamic DC mode is invoked, the Voltages/states button is enabled, so that, at a minimum, the schematic shows these values. To display current, power, or condition you must click these buttons separately.

You can make any kind of change to the schematic and the program will respond by calculating the new DC state. You can rewire, add or delete components, change parameter values or any edit you wish and the display will show the updated values.

The battery, V source, I source, and resistor values can be adjusted in one of two ways:

By selecting a device and dragging the slider which appears next to it. The presence of the slider can be enabled or disabled from the Preferences dialog box (SHIFT + CTRL + P). By default it is off. The attributes SLIDER_MIN and SLIDER_MAX control the slider range for the part. When a part is first placed in a schematic or edited, its SLIDER_MIN attribute is set to zero and its SLIDER_MAX attribute is set equal to the VALUE attribute.

150 Chapter 9: Dynamic DC Analysis

By selecting a device (clicking on it) you can use the UP ARROW and DOWN ARROW keys to increase or decrease the source value. One or more parts may be selected for simultaneous manipulation in this way. Each key press changes the VALUE attribute by a convenient portion of the difference between SLIDER_MAX and SLIDER_MIN.

Note that the display buttons show the indicated quantities for all analysis modes, not just Dynamic DC. After a transient analysis, the buttons display the ending conditions of the transient analysis, which typically is not the DC operating point but the last transient analysis time point. It would be the result of the transient analysis operating point if the Operating Point Only option has been selected.

After an AC analysis, the buttons show the results of the last DC operating point performed during the analysis, if one was performed. The DC operating point is recommended but optional in AC.

After a DC analysis, the buttons show the results of the last DC sweep point.

When the Dynamic DC mode is exited, either by deselecting the Dynamic DC button on the Analysis menu, or by selecting another analysis type from that menu, the program lets you optionally restore the circuit edits made during the Dynamic DC.

Note that if you drag a component away from a circuit and drop it in an empty space, MC7 will normally object during the analysis setup since this creates one or more nodes with no DC path to ground. Since this is likely to temporarily occur during Dynamic DC,

MC7 enables the Add DC Path to Ground option during Dynamic DC.

After the Dynamic DC mode is exited, the option's original status is restored.

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A sample of Dynamic DC analysis

To illustrate Dynamic DC analysis, load the file TTLINV. Select Dynamic DC from the Analysis menu. The display should look like this:

Figure 9-1 Display of initial node voltages

MC7 solved for the DC voltages in the circuit and displayed the voltages and digital states on the screen. Select the battery by clicking on it, and press the DOWN ARROW key. Each time you press the key, the battery voltage goes down by .1 volts so that by the seventeenth key press, the display looks like this:

Figure 9-2 Display of voltages at the switching point

152 Chapter 9: Dynamic DC Analysis

The battery voltage has declined to about 3.3 volts and the output of the analog stage and has fallen to about 2 volts. The digital states have all changed to X. The dynamic control can be used to adjust DC voltages precisely to show the switching point of the circuit.

Now disable the voltage button, and enable the button. This produces a displaylikethis:

Figure 9-3 Display of device currents

The display now shows each of the device pin currents. Redundant currents have been removed for clarity.

The display shows voltages, currents, and power values using a numeric format specified inPreferences / Format / Schematic Voltages/Current/Power. You can change this format to display more digits if you like. The numeric format is explained in Chapter 2. More digits produce more numbers on the screen and often less clarity, so there is a trade-off.

You can also display power terms. Depending upon the device, there may be generated power, dissipated power, or stored power. In general, active devices have both stored and dissipated power terms. In a DC operating point calculation, however, the stored power term will be zero. Sources generally have only generated power terms.

153

To see what the power displays look like, disable the button, and enable the button. This produces a display like this:

Figure 9-4 Display of device power terms

Finally, to see what the condition display looks like, disable the button, and enable the button. This produces a display like this:

Figure 9-5 Display of device conditions

154 Chapter 9: Dynamic DC Analysis

Chapter 10 Transfer Function Analysis

What's in this chapter

This chapter describes the features of Transfer Function analysis. This analysis mode is designed to calculate the DC transfer function from a specified input source to a specified output expression.

The principal topics described in this chapter include:

What happens in Transfer Function analysis

The Transfer Function analysis dialog box

A Sample of Transfer Function analysis

155

What happens in transfer function analysis

Transfer function analysis calculates the small-signal DC transfer function from a specified input source to a specified output expression. Depending upon the input source and the output expression the transfer function calculated can be:

Voltage gain: Input voltage source and output expression = V(OUT)

Current gain: Input current source and output expression = I(RL)

Transconductance: Input voltage source and output expression = I(RL)

Transadmittance: Input current source and output expression = V(OUT) This analysis mode also calculates the small-signal input and output impedances.

To measure the transfer function, the program makes a very small change in the input source DC value and measures the resulting change in the specified output expression value. The ratio of these two quantities produces the transfer function.

To measure the input impedance, the program makes a very small change in the input source DC value and measures the resulting change in the input current or voltage value. The ratio of these changes produces the input impedance.

To measure the output impedance, the program first adds a test voltage source across the node set implicit in the output expression. For example, an output expression such as "V(10,20)" would result in a voltage source between the nodes 10 and 20. If the output expression has no implicit output node set specified, as would be true with an expression like "IB(Q1)", the output impedance will not be calculated and the result N/A (meaning not available) supplied for the answer. If the output nodes fall across a battery, inductor, or other voltage-defined device, the answer 0.0 will be returned, since the DC resistances of these are all zero. Finally a small DC change is made in the output test source and the resulting change in its current noted. The ratio of these two quantities produces the output impedance result.

156 Chapter 10: Transfer Function Analysis

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