14.5

The Howland Current Source uses an operational amplifier employing both negative and positive feedback. The resistor network at the negative terminal determines the ratio between the voltages at the output and noninverting input terminals. The feedback resistor to the noninverting input creates positive feedback which causes a negative resistance to be seen from the noninverting node. The negative resistance effectively cancels the resistance from the Norton equivalent current source of Vin and Rset. In effect the load resistance (Rload) appears to be directly feed by the equivalent input current source (Vin/Rset). The output current is simply Iout = Vin / Rset This means that the output current is independent from the resistance of the load. In the circuit shown Iout = 5 V / 5 kOhms Iout = 1 mA While running in interactive mode the value of Rload can be altered, the output current should be essentially the same.