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Sweeping a Resistor in DC Analysis
The premise of DC analysis is to sweep either a voltage or current source to view the DC transfer functions of a schematic. However, in some types of circuits, the component that may need to be swept for meaningful results is a resistor. Sweeping a resistor can be accomplished by linking its value to a voltage source and then sweeping the voltage source in DC analysis.
The circuit below is a basic current mirror configuration. The components V1, R1, and Q1 define a reference current. An equivalent current is subsequently produced in the collector of Q2. There are two things to note in this circuit. The first is the battery floating to the right of the circuit. This battery has been given the PART attribute of Vsweep and is the source that will be swept in DC analysis. Since it is floating, it will have no direct effect on the current mirror operation. The second thing to note is the definition of the R1 resistor. The VALUE attribute for the resistor has been defined as V(Vsweep). Therefore, the resistance of R1 will be swept with the same values that are defined for Vsweep in the DC analysis limits.
In the DC analysis limits dialog box, Input 1 is defined as Vsweep and Input 1 Range is defined as 60K,10K,5. Two waveforms will be plotted. The Y expressions for these waveforms are I(R1) and I(R2). The X expressions have both been defined as R(R1). These settings will sweep the Vsweep battery from 10K to 60K which will in turn sweep the R1 resistor with the same values. The plot produced will compare the reference current with the output current to confirm that the output current tracks the reference current throughout the resistance range. The simulation below displays the results of the DC analysis.
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As can be seen below, the output current closely tracks the reference current for the specified values of the R1 resistor. The slight difference occurs due to the effect of the finite beta of the transistors and the effect that the output voltage has on the output current.
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