It passes dc without limitation, but it has infinite impedance at infinite frequency.įor the circuit in figure 4.4, if a DC voltage step is suddenly applied to the inverting input through resistor R L, the op amp ignores the sudden step because the change is also coupled directly to the non-inverting input via C 1.

The current in R 1 will thus be slightly larger than I IN and the voltage V OUT will not be exactly equal to I IN*R 1.Īn inductor passes low frequencies more readily than high frequencies, the opposite of a capacitor. In the MOSFET all the current in the drain also flows in the source (no current in gate) whereas in the case of a BJT the emitter current is increased due to the base current. There is an advantage to using a MOSFET, M 1, over a bipolar transistor. Unlike version (a) which accepts both sourcing and sinking currents, version (b) will only operate for currents flowing into the virtual ground at the - input terminal. Version (b) produces an output voltage V OUT from input current I IN as well but now it is produced with respect to some negative node potential, - V. The output voltage is produced with respect to ground due to the virtual ground at the - input terminal of the amplifier and is negative ( V OUT = -I INR 1) for currents flowing into the virtual ground (from the + V as shown) and positive for currents flowing out of the virtual ground (flowing toward - V). Version (a) produces an output voltage V OUT from input current I IN. Two configurations are shown in figure 4.2. More details on the Howland current source can be found in A large current source with highĪccuracy and fast settling and Choose resistors to minimize errors in grounded-load current source.Ĥ.2 Precision Current to Voltage Converterįigure 4.2 Precision Current to Voltage converter For example if R 3 were slightly larger than R 1 the bottom term would be negative. To the extent that the resistors do not match the output impedance could be either positive of negative. Looking at equation 2, the output impedance, Z OUT will be infinity if R 1=R 3, R 2=R 4 and R S=R S' because the bottom term in the ratio will be exactly zero. You can check the result by substituting in the general formula for the output current.

If R S changes from 50 Ω to 200 Ω, the feedback changes fourfold, and you would expect that the output current would change fourfold, to 0 to 2.5 mA. With R 1 = 150 kΩ, R 2 = 15 kΩ, R S = 50 Ω, and V IN can vary from 0 to 5 V, I LOAD will vary from 0 mA to 10 mA, and the circuit has a very high output impedance. In equation 1, any four of the terms can be arbitrarily selected and then the fifth term is determined by solving the resulting equation. Therefore R 1 = R 3, R 2 = R 4, and R S = R S'.įigure 4.1.1 non-inverting voltage to current converterįor a true current output, I LOAD, as a function of the input voltage, V IN, you must satisfy the following equations: Usually in order to improve stability, the circuit is made symmetrical. These formulas also provide a general method for calculating the output current, I LOAD, for any selection of resistor values, not just the constant-current selection. In this chapter we explore simple design formulas that can be used to determine the component values that produce a near ideal current source. Published literature describing the circuit provides design methods that are for special cases or are for approximate designs. The circuit has its limitations due in part to the requirement that the resistor ratios must be quite accurate to obtain a near ideal current source. The first is a high output impedance and the second is the ability to provide bipolar (both sourcing and sinking) output currents. Also often referred to as a Howland current pump this configuration has two advantages over the MOSFET based circuit shown in figure 4.1. The circuit is widely used in industrial instruments for supplying a 4 to 20 mA signal for example.

The resistor values can be selected such that the output current in the load, varies only with the input voltage, V IN, and is independent of the load. Figure 4.1.1 shows a classic voltage to current ( V-to-I) converter.

Differential op amp