APPLICATION NOTE
1
Introduction
The ANL_MISC.LIB library file contains subcircuit models for voltage-controlled reactances and admittances.
These can be used to make voltage-controlled resistors and capacitors. In this application note, we will illustrate
the usage of voltage controlled impedance for controlling Q of a series RLC filter network and changing the
frequency of a Wien bridge oscillator.
Note: This modeling technique is not applicable to capacitances whose values change slowly. It applies to cases
where the capacitance changes very quickly between constant values.
Controlling Q of a Series RLC Filter Network using
Voltage-Controlled Resistor
In most circuits the value of a resistor is fixed during simulation. While the value can be made to change through a
fixed sequence of values, for a set of simulations using parametric sweep, a voltage-controlled resistor can be
made to change its value dynamically during a simulation. This is illustrated by the circuit shown in Figure 1. The
circuit uses a voltage- controlled resistor, X_VCRes. This special resistor is defined using the ZX subcircuit
from ANL_MISC.LIB. This subcircuit consists of two controlled sources and employs an external reference
component that is sensed. The output impedance equals the value of the control voltage times the reference.
Here, we will use Rref, a 50 ohm resistor as our reference. As a result, the output impedance is seen by the
circuit as a floating resistor equal to the value of Vcontrol times the resistance value of Rref. In our circuit, the
control voltage value is stepped from 0.5 volt to 2 volts in 0.5 volt steps. Therefore, the resistance between
nodes 3 and 0 varies from 25 ohms to 100 ohms in 25 ohm-steps.
Figure 1 Variable Q RLC Circuit
