There are times when it is necessary to generate a precise current. It may be for some sensor that specifies a current stimulus. It may be a constant current source or it may change as a function of some input voltage. A variable current source controlled by a voltage is sometimes referred to as a trans-conductance amplifier. Several different types of current sources are described in this article.

Instrumentation Amplifier Current Source

A current source can easily be constructed with an Instrumentation Amplifier (Ins Amp). Just such a topology is shown in figure 1.

Figure 1: Ins Amp Current Source

An instrumentation amplifier is a device with two inputs and usually a single output. The output is defined as the difference of its inputs times some gain. It uses the same symbol as an op amp. To distinguish it from an op amp, its gain value is displayed. Its transfer function is defined in equation 1.

Equation 1

With its gain set to one, the output is the difference of the inputs. The output voltage is always Vset below the load voltage, Vset. The load current is defined in equation 2.

Equation 2

The resistor is connected from the output to the positive input. This is another example of how positive feedback isn't always necessarily bad and sometimes can actually be good. Although there is positive feedback, with a unity gain and the attenuation of the feedback resistor, the loop gain is guaranteed to be less than one. The amount of current is limit to what the Ins Amp can supply. This topology is not widely known or used primarily because of the high cost of instrumentation amplifiers. Still it is a good technique to store away in your collection of "techniques I may one day have a need for".

Op Amp Amplifier Current Source

Op Amps are a lot more affordable but any topology the uses one must be able to generate a differential x1 gain. Using a diode as a reference voltage can do this. Just such a topology is shown in figure 2.

Figure 2: Op Amp Current Source With a Diode Reference Voltage

The bias current through the diode is causing the negative input to be a diode drop below the output. Negative feedback causes the input the match for the load voltage must also be a diode drop below the output voltage as shown in the equation 3.

Equation 3

Negative feedback causes the op amp to adjust its output to make the inputs match. This happens when the output voltage is a diode drop larger than the load voltage. The resistor is connected from the output to the positive input. Again the voltage across it is a diode drop. This resistance sets the current value as shown in the equation 4.

Equation 4

Now you may wonder how the circuit is still working if the resistor is providing positive and negative feedback. Well, it is true that the resistor provides positive feedback, but the load's resistance will reduce it to less than unity. However the negative feedback is not attenuated (only knocked down a bit) so there is more negative feedback than positive feedback for a net negative feedback. The diode only conducts current when the bias voltage is less than the load voltage. This is an easy circuit to build. It primary limitation is that diodes make pretty crummy voltage references. Their voltage drop is a function of temperature. Still there are many applications where this is good enough. This current source only allows for a single set voltage value. But there are applications that require a voltage-controlled current source.