What is Transimpedance Amplifier : Working & Its Applications

The Transimpedance amplifier is a current to voltage converter that is designed with an active component like an operational amplifier to change the input current to a proportional output voltage.  It is also achievable to design an active current to voltage converter with active components like  IGBTs, BJTs, MOSFETs, etc. But, the most frequently used current to voltage converter is the TIA which is called as “Transimpedance Amplifier”. So this article discusses an overview of a Transimpedance Amplifier or TIA with applications.

Definition of Transimpedance Amplifier

A converter that is is used to change the current into voltage by using single or multiple operational amplifiers is known as a transimpedance amplifier or TIA. These amplifiers are mainly used to change the output current of photomultiplier tubes, Geiger–Müller tubes, photodetectors to a functional voltage. These converters use sensors that include a current response that is more linear as compared to the voltage response.

The simple trans-impedance amplifier circuit mainly includes a feedback resistor like Rf with a large value. This Rf resistor is used to set the gain of the transimpedance amplifier because the amplifier is connected in an inverting configuration.

There are different transimpedance amplifiers configurations available where each configuration is used for a specific application but there is one common factor in all configurations is, it converts the sensor’s low-level current to a voltage. The bandwidth, gain and voltage & current offsets will change through different types of sensors that require different transimpedance amplifiers configurations.

Transimpedance Amplifier Circuit & Working

The Transimpedance amplifier circuit is a very simple Inverting amplifier including negative feedback. A feedback resistor like ‘R1’ is connected to the inverting terminal (-) of the amplifier that is shown in the below circuit.

The input current of an operational amplifier will be zero because of its high input impedance, thus the flow of current (Is) from the current source should pass throughout the R1 resistor. The output voltage of the operational amplifier at this point can be calculated by using the following transimpedance amplifier formula.

Vout = -Is * R1

The above output formula will have an accurate value in an ideal circuit. However, in a real circuit, the amplifier will have some input and stray capacitance values across its input pins which causes ringing oscillation & output drift to make the whole amplifier circuit unstable.

To conquer this problem, two passive components are necessary instead of a single component like resistor and capacitor for the proper Transimpedance circuit working. These two components are simply connected in parallel in between the non-inverting terminal & the output of an amplifier which is shown below.

In the above circuit, again the op-amp is connected in negative feedback using the resistor & the capacitor as the feedback.

Once the flow of current like ‘Is’ is applied to the Inverting pin of the Transimpedance amplifier then the current will be converted into voltage like Vout. The output voltage of this amplifier can be determined through the resistor & input current values.

Here the output voltage depends on the feedback resistor ‘R1’ and also has a main relationship with the feedback capacitor ‘C1’ value.  The bandwidth of this amplifier circuit mainly depends on the value of the feedback capacitor like C1, so the value of this capacitor simply changes the overall circuit bandwidth.

For the circuit stable operation in the whole bandwidth, the capacitor value for the required bandwidth can be calculated as.

C1 ≤ 1 / 2π x R1 x fp

Where,

‘ R1’ is the feedback resistor

‘fp’ is the necessary bandwidth frequency.

Practically, both the input capacitance and parasitic capacitance of the amplifier play an essential role in amplifier stability. The circuit’s noise gain response will create instability because of the phase shift margin of the amplifier circuit & cause overshoot in step response behavior.

The working of trans-impedance amplifier circuit configuration is to change an input current source to an output voltage. Here, the flow of current to voltage gain mainly depends on the feedback resistance. So, this amplifier circuit is capable of maintaining a stable bias voltage across the source of input when the input current changes.

Transimpedance Amplifier Design

The following steps need to follow to design a trans-impedance amplifier.

• Use a CMOS or JFET input op-amp through low bias current to decrease DC errors.
• A bias voltage is provided to the non-inverting terminal of the op-amp to set the o/p voltage for input currents.
• Operate in the linear output voltage swing to reduce non-linearity errors.

Design Steps

The first design step is to calculate the feedback resistor value ‘R1’.

R1 = VoMax-VoMin/IiMax-IiMin

Inputs Iimin = 0A, IiMax= 50uA

Outputs VoMin=0V,VoMax=5V

Bandwidth fp = 10KHz

Supply Vcc = 15V & Vee = -15V

Substitute these values in the above equation

R1 = 5V-0V/50uA-0uA = 100 Kilo Ohms

The second step is to calculate the feedback capacitor. Choose the feedback capacitor to meet the bandwidth of the circuit.

C1 ≤ 1 / 2π x R1 x fp  ≤ 1 / 2π x 100 kΩ x 10kHz ≤ 150pF

Calculate the required op amp GBW (gain bandwidth) for the amplifier circuit to be constant.

GBW > Ci+C1/2π x R1 x C1^2

Ci = Cs+Cd+Ccm = 0pF+3pF+3pF = 6pF

GBW > 6pF+150pF/2π x 100 kΩ x (150pF)^2>11.03kHz

Cs: Input source capacitance.

Cd: Differential input capacitance of the amplifier.

Ccm: Common-mode input capacitance of the inverting input.

Advantages & Disadvantages

The advantages & disadvantages of a transimpedance amplifier include the following.

• The circuit designing is simple with Op-amp, resistor, etc.
• Similar to a resistor, this amplifier changes current into voltage, but not like a resistor, it includes low input & output impedances even with extremely high gain.
• A capacitor is connected with the feedback resistor in parallel to ensure stability within photodiode-based applications.
• These amplifiers operate in both active and passive I to V converters, so passive I to V converter uses passive components whereas active I to V converter uses active components. But most commonly, an op-amp is used in the trans-impedance amplifier to attain I to V conversion.
• The design stability is the most critical element in the Transimpedance circuit due to the noise-related and parasitic issues. So the circuit designer should be very careful while choosing a suitable amplifier.

Applications

The applications of a transimpedance amplifier include the following.

• Transimpedance amplifiers are mainly used for processing the current output of pressure transducers, photodiodes, accelerometers to a voltage like a useable signal output.
• Transimpedance amplifiers provide simple linear signal processing with an op-amp & a resistor to dissipate current.
• It is used in Optical equipment, Low-power analog sensors, RF equipment, Geiger–Müller tubes, Other kinds of sensors, Photomultiplier tubes, Photodetectors, Accelerometers
• TIAs are used in receivers of optical communication.
• There are different types of TIA configurations where every configuration is used for a particular application.

Thus, this is all about a description of a trans-impedance amplifier. Some significant transimpedance amplifier specifications mainly include linear range, compensation, transfer impedance, referred RMS current noise & bandwidth of trans-impedance. Here is a question for you, what is an operational amplifier?