A voltage-to-current converter (Figure 1) transforms voltage into a corresponding current. The input voltage (Vinput) triggers the circuit to produce a proportional output current via the load resistor (Rload).
These type circuits have a wide range of applications in consumer and industrial electronics, one of the most common being as a signal transmitter. There, signals like pressure, temperature and others are transmitted to central control rooms, used to eliminate any voltage drops that might occur across the wires from the measuring instruments. In control applications, voltage-to-current converter circuits may be used to provide a compensation loop-back. One example is in active rectifiers, where the output current is adjusted to be proportional to the voltage being checked. This compensation loop contributes to the active rectifier’s ability to provide a steady output.
However, there’s a question: Given that the voltage-to-current converter circuit is intended to manage an alternating input signal, does it maintain a stable and satisfactory performance at low input frequencies? It is essential for any electronics designer to know this before using this circuit into an application.
Figure 1: A typical voltage-to-current converter circuit
Experiment methodology
To get an answer to our question, we set up an experiment, supplying the circuit of Figure 1 with 1Vac. We’ll vary its frequency between 1Hz and 200Hz, in 1Hz increments, all the while measuring the peak-to-peak magnitude of the output signal, and chart the results.
For the experiment we assumed the following:
- The input signal is kept fixed at 1V (peak value) throughout the experiment.
- All circuit component values were kept unchanged.
- The temperature is kept at an ambient 25oC.
The relationship between the output peak-to-peak amplitude and the input signal’s frequency is seen in Figure 2. The circuit achieves frequency stability at input frequencies over 75Hz. Another stable zone is noticed between 24Hz and 50Hz; however, the output’s amplitude drops in this zone, to roughly half the input signal’s magnitude. In addition, the performance of the circuit is poor when the input frequency range is between 50Hz and 75Hz, as well as below 24Hz.
Figure 2: Input vs output
To prevent any distortion to the produced signals, engineers must consider the voltage-to-current circuit’s stable performance zone for the application.
By Dr Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions
