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The Basics
Published in Douglas Self, Small Signal Audio Design, 2020
A transimpedance amplifier accepts a current in (usually single-ended) and gives a voltage out. It is sometimes called an I-V converter. It has a transfer ratio A = VOUT/IIN, which has dimensions of V/I, or resistance. That is why it is referred to as a transimpedance or transresistance amplifier. Transimpedance amplifiers are usually made by applying shunt voltage feedback to a high-gain voltage amplifier. An important use is as virtual-earth summing amplifiers in mixing consoles; see Chapter 22. The voltage amplifier stage (VAS) in most power amplifiers is a transimpedance amplifier. They are used for I-V conversion when interfacing to DACs with current outputs; see Chapter 26. Transimpedance amplifiers are sometimes incorrectly described as “current amplifiers”.
Microflow Cytometer Electronics
Published in Frances S. Ligler, Jason S. Kim, The Microflow Cytometer, 2019
Jeffrey S. Erickson, Dustin J. Kreft, Matthew D. Kniller
In his review article, Snow4 has laid out a very general description of the tasks required in any flow cytometry data collection system. The components required to accomplish these tasks, along with power conditioning, communications, and data storage, make up the PCB as required for a microflow cytometer. The functional diagram that we use to describe these tasks is slightly different [Fig. 13.1], but still in line with Snow's basic concept for an analog detector. Briefly, objects flow through one or more illumination regions in the cytometer. Fluorescence and scattered light are emitted, filtered, and collected. A photodetector converts photons to an electrical current. A transimpedance amplifier converts this current into a voltage, usually in conjunction with electronics that provides negative feedback to minimize baseline drift. In an analog detector, the signal then moves through a low-pass filter to prevent aliasing effects. Custom analog circuits may be used to measure desired quantities, such as peak area. Analog-to-digital conversion takes place. The signal is then further processed by a microcontroller, and finally either stored in memory or transferred to an external device (such as a computer) for software manipulation and display. In a digital detector, many or all of the components labeled as optional in Fig. 13.1 are removed, and their corresponding operations are performed in software by a microcontroller, digital signal processor (DSP) or field-programmable gate array (FPGA).
Fundamental Light–Tissue Interactions: Light Scattering and Absorption
Published in Vadim Backman, Adam Wax, Hao F. Zhang, A Laboratory Manual in Biophotonics, 2018
Vadim Backman, Adam Wax, Hao F. Zhang
The advantage of using the transimpedance amplifier is that the low-output impedance of the operational amplifier (op-amp) will ensure that the voltage-sensing mechanism, such as the oscilloscope, will not draw too much current and affect the photodiode output. A resistor value (R) of 1–10 kΩ is usually sufficient. A further improvement to this circuit is to reduce high-frequency noise by including a small capacitor in parallel with the resistor. This will roll off the frequency response of the circuit according to the RC time constant of the combination. For most biophotonics applications, a response of <1–10 KHz is needed, suggesting a capacitance on the order of 50–500 pF. An additional improvement is to add a resistor between the noninverting input of the op-amp and ground with an equivalent value as the feedback resistor. This will serve to null out any offsets due to nonideal op-amp behavior.
Voltage-controlled oscillators using CFOAs with correction terminal Z
Published in International Journal of Electronics, 2023
As a result, in many practical cases, these amplifiers are called transimpedance amplifiers. The idealised model of the CFОA is given in Figure 2. It shows the input buffer between terminals y and x, as well as the current-to-voltage converter (or transimpedance stage), forming the output stage. The transmission coefficient αxz of the first stage has a value of unity, i.e. the current ix is approximately equal to iz.