Amplifiers are one of the classic components used in analog electronics, and they are used for more than just turning up or down a voltage. Operational amplifiers (op-amps) are very common components used for waveform manipulation, filtering, amplification/attenuation, and buffering. One version of an amplifier that is equally as important for certain sources in circuits is a transimpedance amplifier.
In a transimpedance amplifier, the function of the component is to provide conversion from low-level currents to a voltage that can be easily measured with a downstream amplifier circuit. Signals from certain sensors or regulated current sources can only be accurately sampled with this type of amplifier. There are limitations on the capabilities of these components as a function of accessible gain and frequency, so we will examine some of these points in this article.
Transimpedance Amplifier Circuits
Transimpedance amplifiers (TIAs) are electronic circuits that convert signals from a current source to a voltage. The conversion factor is given by Ohm’s law, where the modifying factor is known as the transimpedance. This value is typically chosen based on an external resistor or network of passive components. In effect, the output voltage would be on the order of:
TIAs are often used in applications where the signal being measured is coming from a controlled current source, such as in optical communication systems, biosensors, and some precision test and measurement equipment. A TIA circuit is usually designed using an op-amp.
These circuits consist of a straightforward inverting amplifier with negative feedback through a single feedback resistor. Op-amps have high input impedance, which means that all of the current from the current source will pass through the feedback resistor and will be converted to a proportional voltage signal. To counter the effect of oscillations in the feedback loop caused by stray capacitances/inductances, a capacitor is typically placed in parallel with the feedback resistor.
One of the most common instances where these components can be used is to collect current from a photodiode. These components act like current sources when illuminated, and the current can be so small that it will be very difficult to measure with an ADC. Instead of measuring the output directly, the output can be converted to a voltage using a TIA circuit with a large feedback resistor.
Op-amp or IC?
It is not required to design a TIA using a traditional op-amp, a TIA circuit can be designed around an IC with common packaging. Here is a list of important specs that need to be considered when selecting a transimpedance amplifier:
- Transfer impedance/gain: The transfer impedance (or transimpedance) determines the output voltage as mentioned above. For a TIA IC, this may be referred to as gain.
- Bandwidth: All amplifiers have a bandwidth limitation; TIA should have sufficient bandwidth as a function of gain, but remember that increasing the bandwidth will reduce the gain.
- Linearity: Saturation occurs when the TIA attempts to output a voltage beyond its rail voltages. In such a case, decrease the input signal, increase the rail voltages, or decrease the transimpedance.
- DC or AC coupling: If the input has a steady level or very slow modulation, a DC-coupled amplifier will likely be required. If the signal is an AC pulse, coupling can be used to eliminate the DC offset if needed.
- Input capacitance: Low input capacitance is desired; this is related to bandwidth, where low input capacitance corresponds to higher input bandwidth.
- Internal compensation: Sometimes, the current sensor that sends the input signal to the TIA has parasitic capacitance (e.g. a photodiode). This may cause resonance in the transfer function of the TIA and destabilize the circuit. A source component with a higher parasitic capacitance can be used with an amplifier that has internal compensation.
- Power Consumption: TIAs should have low power consumption, which is proportional to the amplifier’s quiescent current in the unloaded state. Low values of quiescent current are preferred so that the device will not leak excessive power.
TIA Circuits in a PCB
Finally, if you are going to use a TIA built from a discrete op-amp, or you want to use a TIA IC, the circuit will need to be assembled on a PCB. The layout of the circuit will appear somewhere in an analog signal chain, typically before being provided to an ADC for measurement. Therefore, it is typical to follow mixed-signal layout guidelines with TIA circuits on a PCB.
Because the input to the TIA is typically very low level, that source should be placed close to the TIA so that the coupling region where it could receive noise is minimized. There are two other simple rules that will help prevent or reduce noise from external sources and other circuits:
- Always use ground planes: TIAs should be placed over a ground plane to help minimize crosstalk and reception of external noise.
- Decoupling: Decoupling capacitors should be placed on the power rails close to the TIA’s input power pins as this will prevent voltage fluctuations from appearing on the output.
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