Transimpedance amplifier circuit diagram design

Impedance Amplifier/ Current to Voltage Converter

A resistor amplifier circuit converts current into a proportional voltage. It is also known as current to voltage converter or simple I to V conversion. Transimpedance amplifier with ideal zero input resistance and low output resistance. Transconductance amplifiers have many applications in everyday electronics. For example, photodiodes that convert current into proportional voltage, digital-to-analog converter (DAC) applications, etc. Current-to-voltage converters are of two types, passive and active sensing.

Passive current to voltage converter

As the name suggests, only passive components are used to execute the circuit. The circuit diagram of a passive current-to-voltage converter is shown below. Here, the current source resistor R converts the current I into a voltage proportional to the voltage across R itself. This mechanism follows Ohm's law (V = IR).

Passive current to voltage converter.

But in any case, passive current-to-voltage converters have significant limitations. For correct operation, the input current source and output load must be ideally selected. This means that the output load resistance is close to infinite, the current from the current source must be independent of the entire load resistance (the current source must have infinite capability) and the current source must also have infinite internal resistance to produce the voltage. These conditions are almost impossible to achieve so passive current-to-voltage converters are rarely used in practical applications.

Active current-to-voltage converter (impedance amplifier)

Active current-to-voltage converters (impedance amplifiers) are based on active elements such as bipolar junction transistors, field-effect transistors or op amps. Impedance amplifier using op amp is a commonly used one. The circuit diagram of an op amp based current to voltage converter is shown below.

Active impedance amplifier circuit

V+ and V- in the circuit diagram are the voltage sources of the op amp. RF is the feedback resistor and RL is the load resistor.

Expression in output voltage

The output voltage calculation formula of the operational amplifier based on the current-to -voltage converter can be obtained as follows. Here the op amp inverting mode closed loop feedback resistor = RF wired. Let IIN be the input current of the circuit that needs to be converted and Vout be the output voltage.

The calculation formula for the voltage gain (AV) of an op amp inverting amplifier can be written as

AV = - Rf/R1.

Therefore the output voltage Vout = - (Rf/R1)(VIN)............(1)

Since the non-inverting input is connected to ground and an ideal op amp has infinite input resistance, we can assume V1 = 0 and V1 = V2.

Me too = Vin/R1 .............................(2)

Substituting into equation (2)(1) we get

VOUT = - (I)(RF)

In simple terms, a current to voltage converter using an op amp consists of an inverting op amp replacing R1 and Vin with a current source i.