If the input signal is a voltage source. And it needs to be transmitted to a distant load. The load current depends on the series resistance between the input signal and the load. Even a slight decrease in the voltage drop on the series resistor will greatly alter the percentage error of the load voltage. Any change in load resistance caused by loss or temperature will result in errors. The simplest type of voltage current converter is shown in Figure 3.40 (a). The current flowing through resistor R1 is
In this way, the output current i0 flowing through the load resistor R only depends on vs and R1, not on R itself. For a certain value of R1, i0 is directly proportional to vs. Note that there is no port connected to ground in Figure 3.40 (a), indicating that the load is floating. The advantage of this structure is that no common mode signals (such as noise) appear on the load.
Operational amplifiers are fundamentally voltage amplifiers, and their current carrying capacity is limited. Many applications, such as indicators or actuators, require variable current regulation that exceeds the capabilities of operational amplifiers. The circuit in Figure 3.40 (b) can provide a load current i1 proportional to the input voltage vs. The output of the operational amplifier drives the base current to flow through transistor Q1, resulting in a proportional collector current flowing through Q1, load RL, and R1. The load current i1 can be controlled by changing the input voltage or R1 value. The value of the base resistance R should be large enough to protect the base emitter junction of Q1 and limit the output current of the operational amplifier. Moreover, the DC power supply voltage VCC ≥ RLiL. The load resistor R1 is floating. In this way, the circuit cannot be used simultaneously with the grounded load.
