and the passive current source. The input voltage source, the resistor, and the op-amp constitutes an "ideal" current source with value, . The transimpedance amplifier and an op-amp inverting amplifier are typical implementations of this idea. The floating load is a serious disadvantage of this circuit solution. Current compensation implementation A typical example are Howland current source and its derivative Deboo integrator. In the last example (Fig. 1), the Howland current source consists of an input voltage source, , a positive resistor, R, a load (the capacitor, C, acting as impedance ) and a negative impedance converter INIC ( and the op-amp). The input voltage source and the resistor R constitute an imperfect current source passing current, through the load (Fig. 3 in the source). The INIC acts as a second current source passing "helping" current, , through the load. As a result, the total current flowing through the load is constant and the circuit impedance seen by the input source is increased. However the Howland current source isn't widely used because it requires the four resistors to be perfectly matched, and its impedance drops at high frequencies. The grounded load is an advantage of this circuit solution. Current sources with negative feedback They are implemented as a voltage follower with series negative feedback driven by a constant input voltage source (i.e., a negative feedback voltage stabilizer). The voltage follower is loaded by a constant (current sensing) resistor acting as a simple current-to-voltage converter connected in the feedback loop. The external load of this current source is connected somewhere in the path of the current supplying the current sensing resistor but out of the feedback loop. The voltage follower adjusts its output current flowing through the load so that to make the voltage drop across the current sensing resistor R equal to the constant input voltage . Thus the voltage stabilizer keeps up a constant voltage drop across a constant resistor; so, a constant current flows through the resistor and respectively through the load. If the input voltage varies, this arrangement will act as a voltage-to-current converter (voltage-controlled current source, VCCS); it can be thought as a reversed (by means of negative feedback) current-to-voltage converter. The resistance R determines the transfer ratio (transconductance). Current sources implemented as circuits with series negative feedback have the disadvantage that the voltage drop across the current sensing resistor decreases the maximal voltage across the