Column: Circuit drill
Resistor multiplier circuit performance at various power supply voltages
By Dr. Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions R
esistor multiplier circuits are widely used in circuit design as they amplify the actual resistance through a negative-loop operational amplifi er
combination. Figure 1 shows one such circuit that can provide high resistance. It uses the LT1012 op-amp, which off ers a broad voltage supply range. To calculate the input resistance of such = R1 * (1
a circuit, there’s an equation, Rin
+ R3/R2). For the circuit in Figure 1, Rin is the input resistance seen by the power
supply, V1. T e values of resistors R1-R3 can be chosen based on the application needs.
Resistor multiplier circuits Resistor multiplier circuits are primarily used to divide a voltage into a fraction of its original value. T ey are frequently used in power supplies and sensor circuits to establish a stable reference voltage. T ey are also used in sensor circuits
to scale or attenuate the sensor’s output voltage, which aligns with the input range of a microcontroller or an analogue-to- digital converter. Additionally, resistor multiplier circuits are useful in monitoring battery voltage levels in systems that run on batteries; they trigger alerts or actions when the voltage falls below a certain threshold. In communication systems and audio circuits, resistor multipliers are used
to attenuate signals to desired levels with minimal distortion. In one of my projects, I considered
using this particular circuit, but I had concerns about the equation’s accuracy when the power supply voltage decreases during circuit operation. Additionally, I also wanted to determine the impact of declining power supply voltage on the circuit’s performance. Would the circuit maintain the same resistance multiplication ratio at various power supply voltages? T at’s why this experiment was set up.
The experiment I used the circuit shown in Figure 1 for the experiment due to its simplicity. Although there are various other circuits available, they mainly revolve around the same design and principle. During the experiment, I applied various
power supply voltages, calculating the circuit’s resistance at each step using the input voltage and current. T e applied voltage ranged from 1-20Vdc, which is the operating range of the LT 1012 op-amp. All other components I kept unchanged. I then plotted the measured circuit
input resistance; see Figure 2. T e curve reveals that the relationship between the power supply voltage and the circuit input resistance is not constant, departing from the Rin
= R1 * (1 + R3/R2) equation. T is
Figure 1: A simple resistance multiplier circuit
equation only holds true at a single point on the curve, when the supply voltage is 13.4Vdc. When the voltage falls below or rises above this 13.4Vdc point, the circuit
08 April 2024
www.electronicsworld.co.uk
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