Hi...
I am trying to construct a DC electronic load. The circuit diagram is attached. The opamp in the circuit is configured as a voltage follower.
In this post, the resistor (HSA50R10J) is denoted as RM, whilst the voltage across the resistor is VM. Ideally, VM should track and be identical to Vset. Hence, the load current (ILoad) is given by Vset/RM.
When TL081 is used, the current increases linearly with Vset (see first graph in "EEV Blog 2.png"). However, when Vset is greater than 0.45V, the load current starts to saturate.
When the opamp in the circuit is replaced with TL071, the current also increases linearly with Vset (see 2nd graph in "EEV Blog 2.png"). In this case, the current starts to saturate at a higher Vset, i.e. 0.6V.
Ideally, the voltage at the non-inverting terminal of a negative feedback opamp circuit should be identical to that at the inverting terminal. However, as can be deduced from the experimental results ("EEV Blog 2.png"), the voltage at the inverting terminal can no longer follow up the increasing voltage at the non-inverting terminal after Vset is greater than 0.45V (in the case of TL081) or 0.6V (in the case of TL071). Consequently, the load current starts to saturate after Vset = 0.45V or Vset = 0.6V. Kindly advise why these anomalous behavior (e.g. voltage at opamp input terminals becomes different, current starts to saturate, etc.) happened.