Column: Circuit drill
configuration. The gain of the circuit is set for unity (gain = 1). This was obtained by selecting identical values for R1 and R2, both 1kOhms.
• The DC offset voltage injected with the AC input signal ranged from 0 to 20VDC, quite enough for such a circuit.
• Resistors R3 and R4 were identical per the manufacturer’s recommendation, both 4.9kOhms. The purpose of this is to maintain maximum stability of the AC coupler circuit.
• The AC signal magnitude was assumed to be 1V (i.e., 2Vp-p
).
• The input signal frequency was set at 100kHz, which is within the bandwidth of OPA376. The output signal was then monitored,
and the observations logged for both conditions mentioned here, i.e., changing DC offset voltage with AC signal and changes in the output load resistance. Each change was examined separately.
DC offset variations Changing the magnitude of the DC offset (at the input with AC signal) from 0VDC to 20VDC, no change was observed in the output AC signal; the circuit is very stable, and it perfectly filters out the DC offset voltage, which is good news.
Load resistance variations In this part of the experiment, various load resistances were used to determine the limits that the AC coupler circuit can handle. I started using high output load resistors where the circuit can stabilise, then gradually decreased the load resistance and monitored the output AC signals. The relation between the load
resistance and the output AC signal was plotted; see Figure 4. It was observed the output AC signal amplitude started to drop when the load resistance goes below 500Ohms.
Figure 4: Relation between load resistance and output AC signal magnitude From the experiment, two findings The OPA376 AC
coupler circuit is very stable against various DC off set voltages injected with the input AC signal
were examined and illustrated. First, the OPA376 AC coupler circuit is very stable against various DC offset voltages injected with the input AC signal. Second, the circuit does not support an output load less than 500Ohms. When this output resistance is reached, the AC coupler output AC signal will attenuate severely. So, it is recommended to keep that in mind when designing such a circuit.
www.electronicsworld.co.uk May 2021 15 Figure 3: Stable output signal when AC input signal is joined with DC offset voltage (0-20VDC)
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