A current to voltage converter turns an input current into a proportional output voltage. In signal-conditioning circuits, it is usually called a transimpedance amplifier, or TIA. The most common design uses an operational amplifier and a feedback resistor so that even a very small sensor current can become a voltage that an analog-to-digital converter, meter, controller, or oscilloscope can read.
The basic relationship is simple: the input current flows through the feedback impedance, and the amplifier adjusts its output to hold the input node at an almost constant reference voltage. For the standard inverting circuit, the ideal result is Vra = −Itrong × Rf. The rest of the design is about selecting gain, bandwidth, noise, stability, and voltage range correctly.
A current to voltage converter is a signal-conditioning circuit whose output voltage represents the magnitude and direction of an input current. The ratio between output voltage and input current has units of ohms and is called transimpedance. A feedback resistance of 100 kΩ, for example, gives an ideal sensitivity of 100,000 volts per ampere, or 0.1 volt per microampere.
This electronic meaning should not be confused with a mains voltage transformer. A TIA processes low-level signals from sensors and measurement circuits. A power transformer changes AC voltage for equipment and provides power at a useful voltage level. Both involve electrical conversion, but they solve different engineering problems and are specified in different ways.
The source may be a photodiode, photomultiplier, electrochemical sensor, current transformer interface, or another transducer. Instead of producing a convenient voltage, it produces a current related to light, radiation, chemical concentration, magnetic field, or another measured quantity.
In a typical inverting TIA, the non-inverting input is connected to ground or another reference voltage. Negative feedback makes the op-amp drive its output until the inverting input sits very close to that same voltage. Engineers call this point a virtual ground when the reference is 0V. It is not physically connected to ground, but feedback keeps it near ground potential during normal linear operation.
An ideal op-amp input draws no current. Therefore, almost all current from the source must flow through the feedback resistor Rf. The op-amp produces whatever output voltage is required to support that current through the resistor. If current flows into the inverting input, the output moves negative; if the current direction reverses, the output moves positive.
Ideal transfer equation: Vra = Vref − ItrongRf
With a 10 µA input, a 100 kΩ feedback resistor, and a 0V reference, the ideal output is −1V. Changing Rf to 1 MΩ raises the sensitivity to −10V for the same current, but it also affects noise, bandwidth, and output range.
Real sensors and op-amp inputs have capacitance. Combined with a large feedback resistance, that capacitance can create excessive gain peaking or oscillation. Designers often place a small capacitor Cf in parallel with Rf. This limits high-frequency gain and stabilizes the circuit, but too much capacitance slows the response. The correct value depends on source capacitance, op-amp gain-bandwidth product, noise gain, and the required signal bandwidth.
Ohm’s law means a resistor alone can convert current into voltage: V = I × R. However, the voltage developed across a passive sense resistor can change the source operating point. That loading error is especially important with photodiodes and other high-impedance or low-current sources.
| Tính năng | Sense resistor only | Op-amp transimpedance amplifier |
|---|---|---|
| Complexity | Rất thấp | Requires op-amp, supply, and feedback parts |
| Input node voltage | Changes with current | Held near a reference voltage |
| Sensor loading | Can be significant | Usually much lower |
| Output drive | Limited by the source | Driven by the op-amp |
| Precision and gain | Good for simple current sensing | Better for small signals and controlled gain |
A shunt resistor is often suitable for measuring moderate power-supply or motor current when a small voltage drop is acceptable. A TIA is usually better when the source current is tiny, source loading must remain low, or the measurement needs a buffered output.
Begin with the largest expected input current and the usable output swing of the op-amp. The first-pass value is:
Rf = desired output change ÷ maximum input current
Suppose a sensor produces 0–20 µA and the desired output change is 0–4V. The ideal feedback resistance is 200 kΩ. Choose a standard precision value, then confirm that the op-amp can reach the required output voltage under the intended supply and load. Leave margin from the supply rails and account for current direction.
A larger resistor increases volts-per-amp sensitivity, but it also increases resistor thermal noise and usually reduces bandwidth. For a very wide current range, selectable feedback resistors or multiple gain channels can give better resolution than one extremely large resistor.
Photodiodes generate current proportional to incident light. A TIA keeps the photodiode terminal voltage controlled while converting photocurrent into a voltage for measurement. Applications include optical receivers, light meters, spectroscopy, smoke detection, and industrial optical sensors.
A receiver can pass a 4–20mA loop signal through a precision resistor to produce a voltage such as 1–5V. Depending on isolation, grounding, accuracy, and protection requirements, the design may also use buffering, differential amplification, surge protection, or an isolated signal conditioner.
A current transformer produces a secondary current proportional to primary AC current. A burden resistor or active current-to-voltage front end converts that current into a voltage for metering or protection electronics. Current transformers must be used according to their manufacturer’s safety instructions; an energized CT secondary should not be left open when the design requires a burden.
Some digital-to-analog converters provide a current output. An op-amp I/V stage converts it into the voltage range required by the next circuit. Similar front ends are used with ionization detectors, electrochemical cells, and laboratory instruments.
A signal-level current to voltage converter is chosen by current range, transimpedance gain, bandwidth, noise, stability, and supply rails. A power transformer is chosen by input/output voltage, VA capacity, frequency, insulation, protection, connectors, and duty cycle. Do not use the two terms interchangeably.
For equipment-level AC voltage conversion, Johsun Tec offers a dedicated step-up and step-down transformer category. Related examples include the DT-1000VA transformer, DT-2000VA transformer, DT-3000VA transformer, và DT-5000VA voltage converter transformer. These products change AC voltage for loads; they are not substitutes for a PCB-level transimpedance amplifier.
A current to voltage converter works by directing input current through a known feedback impedance and using negative feedback to maintain a controlled input voltage. The basic equation is simple, but accurate performance depends on op-amp selection, feedback resistance, compensation capacitance, noise, bandwidth, and output headroom.
Explore Johsun Tec’s power solutions, learn about the company, or review its ODM and OEM capabilities. For a custom power transformer specification, contact Johsun Tec with your input voltage, required output, VA load, frequency, and application.
Its transfer ratio is output volts divided by input amps. Volts per ampere is equivalent to ohms, so the gain is an impedance rather than a dimensionless voltage gain.
In the standard inverting configuration, a current entering the inverting node requires the op-amp output to move in the negative direction to drive the same current through Rf. Reversing current direction or using a suitable reference can change the output polarity and range.
Yes. A resistor produces V = I × R, but the resulting voltage may load the current source. An op-amp TIA keeps the input node closer to a constant voltage and provides a buffered output.
Input capacitance interacting with the feedback network can reduce phase margin and create gain peaking or oscillation. A properly selected feedback capacitor and suitable op-amp restore stability.
Not by itself. A current transformer produces a secondary current. A burden resistor or active front end is then used to convert that secondary current into a measurable voltage.
