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Industrial Strength Simulation




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Creating Wingspread Plots


The wingspread plot is a type of plot used to display the large signal linearity of an output stage. It derives its name from the fact that the waveform plotted looks similar to the look of a simple drawing of a bird. It is typically used in audio amplifier design and is quite useful in simulating the crossover distortion. The wingspread plot helps in determining the optimal quiescent conditions that should be used in order to minimize the gain deviations around the crossover point for a specific load resistance. The following example uses a technique to simulate wingspread plots that is described in Bob Cordell's book " Designing Audio Power Amplifiers".

The circuit below is a class AB output stage. It uses a complementary pair of transistors from ON Semiconductor: the NJW3281G NPN transistor and the NJW1302G PNP transistor. The Vin battery defines the input voltage to the stage. The Vbiasp and Vbiasn batteries bias the output stage and set the quiescent bias current for the circuit. Both of the batteries have their VALUE attribute set as Vbias whose value is specified by the following define statement:

.define Vbias .7

The class AB output stage is powered by +/- 12V power supplies and is driving an 8 ohm load resistance.

Class AB output stage

The wingspread plots are created in DC analysis. The input voltage is swept over a range of -10V to 10V in steps of 10mV. In the DC Analysis Limits, the Variable 1 fields are set to the following:

Method: Linear
Name: Vin
Range: 10,-10,.01

Sweeping the input voltage will create a single wingspread waveform. In most cases, it is useful to view a range of these waveforms by stepping the bias conditions of the circuit in order to determine the appropriate bias condition. For this circuit, stepping the Vbias parameter from .6V to .7V in steps of 10mV creates a nice set of waveforms. In the DC Analysis Limits, the Vbias parameter is stepped by setting the Variable 2 fields to the following:

Method: Linear
Name: Param VBias
Range: .7,.6,.01

The gain of the output stage can be plotted by using an expression that takes the derivative of the output voltage with respect to the input voltage. The expression:


will produce the desired output. The dd( ) operator calculates the numerical derivative of the specified expression with respect to the parameter being swept in the Variable 1 field which in this case is the input voltage. When the simulation is run, the following wingspread plot is created.

Wingspread plot

As can be seen in the plot, small changes in the bias can create quite a bit of crossover distortion in the output gain. The optimal bias for this particular circuit appears to be at around .62V

1) Designing Audio Power Amplifiers, Bob Cordell, McGraw Hill, ISBN 978-0-07-164024-3

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