What are Transimpedance Amplifiers?

A transimpedance amplifier is an active current-to-voltage converter that converts the input current to a proportional output voltage. It uses active electronic components like Operational Amplifier (Op-Amp), Bipolar Junction Transistor (BJT), MOSFET, etc. for this conversion and ensures proportionally amplified voltage conversion of various input current levels.

The input current can be easily converted to voltage using a simple resistor circuit as the current flow through it creates a proportional voltage drop across the resistor depending on its resistance. But this circuit will fail to convert input current to a proportional output voltage if the current input is from certain components like the photodiode which contains inherent capacitance. The presence of the capacitance in combination with the resistance will make the circuit an RC oscillator. For a small load resistance, the gain produced by the amplifier will be smaller as the time constant (= resistance x capacitance) will be small. Similarly, if the load resistance is increased, it will produce an uneven gain. This will lead to insufficient SNR and linear proportionality across a wide range of input currents. So, a Transimpedance amplifier is preferred to perform the current-to-voltage conversion or input current measurement in various circuits with advanced electronic and photonic components.

A transimpedance amplifier circuit usually contains an Op-Amp in the inverting amplifier configuration without the input resistor. In this circuit, the feedback resistor is connected between output terminal (V_out) and the inverting terminal (V₂) while the non-inverting terminal (V₁) is grounded. For simple analysis of an OpAmp, two rules are important:

1. There is no current flow into the input terminals due to high input impedance.
2. The voltage difference between the input terminals is null or V_DIFF = V₁ - V₂ = 0.

Since the non-inverting terminal is grounded, the potentials at both inputs are set to zero. i.e., V₁ = V₂ = 0. This is known as virtual earth. When an input current ‘i’ is applied to the inverting terminal through the input resistance, the output voltage can be calculated using an equivalent circuit as shown below:

Here, the current reaching virtual earth can’t flow into the OpAmp and hence passes through the feedback resistor. So, the output put voltage can be calculated as 

V₂ – V_out = i*R_F

i.e.,  V_out = - i*R_F

Here gain in the current to output voltage conversion depends on the feedback resistor and is stabilized for a wide range of current input using the OpAmp used. The negative sign indicates inversion of the output signal. i.e., the input and output signals will be out of phase by π.

If the current output of a photon detecting component is taken as the input at the inverting terminal, 180 degree out of phase amplified voltage, V_out = - I_s * R_f, is obtained at the output.

Transimpedance amplifiers for sensitive light detecting instruments

Transimpedance amplifiers are widely used in systems designed for light measurements. Scientific instruments like spectrometers use photodiodes to accurately measure light at each wavelength and hence identify the constituent components or molecules in the samples. These samples will have different ranges of absorption, reflection, transmission, etc. while the incident light interacts with them and hence the photodiodes will produce current outputs at various current levels. To have a linear dependence for the measured voltage output to the current produced by these detectors, transimpedance amplifiers are essential. Also, multiple stages of these amplifiers can be employed.

Transimpedance amplifiers can be used to process the current output of various components like pressure transducers, photodiodes, accelerometers, etc. to a useful output voltage signal. They are also useful for optical communication, advanced driver assistance systems and in autonomous vehicles.

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