# Transimpedance Amplifier for Noise Measurements in Low-Resistance IR Photodetectors  
[PERSON]  
1 Institute of Optoelectronics, Military University of Technology, 00908 Warsaw, Poland;  
[EMAIL_ADDRESS] (K.A.); [EMAIL_ADDRESS] (J.M.);  
[EMAIL_ADDRESS] (Z.B.)1  
[PERSON]  
2  
[PERSON]  
1 Institute of Optoelectronics, Military University of Technology, 00908 Warsaw, Poland;  
[EMAIL_ADDRESS] (K.A.); [EMAIL_ADDRESS] (J.M.);  
[EMAIL_ADDRESS] (Z.B.)1  
[PERSON]  
1 Institute of Optoelectronics, Military University of Technology, 00908 Warsaw, Poland;  
[EMAIL_ADDRESS] (K.A.); [EMAIL_ADDRESS] (J.M.);  
[EMAIL_ADDRESS] (Z.B.)1  
[PERSON]  
2 Department of Engineering, University of Messina, 98166 Messina, Italy;  
Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
Footnote 2: [[https://www.mdpi.com/journals/pdf/1](https://www.mdpi.com/journals/pdf/1)]([https://www.mdpi.com/journals/pdf/1](https://www.mdpi.com/journals/pdf/1))  
tion, as long as the current-voltage characteristic \(I_{B}\) (\(V_{B}\)) of the DUT and its small signal impedance \(Z_{D}\) at the selected bias point and vs. frequency are known, since [15; 16]:  
\[S_{VD}=S_{ID}|Z_{D}|^{2} \tag{1}\]  
It may be worth mentioning that throughout this paper, whenever quantities representing the PSD of voltage or current fluctuations appear in an equation, it is intended that these values be expressed in the appropriate units, \(\mathrm{V^{2}/Hz}\) and \(\mathrm{A^{2}/Hz}\). However, as is common practice in the field of noise measurements and instrumentation, when referring to specific values in the text, we will often express these quantities in terms of their square roots (i.e., \(\mathrm{V/\surd Hz}\) and \(\mathrm{A/\surd Hz}\)).  
From an experimental point of view, it is easier to perform voltage noise measurements in the case of low-impedance DUTs, while current noise measurements are restricted to cases of high-impedance DUTs [17; 18]. The advanced photodetectors we plan to investigate are characterized by impedances that can be on the order of or even below 1 k\(\Omega\), and thus belong to the class of devices for which voltage noise measurements are more easily performed. The problem, however, is that some interesting device parameters are more directly related to the current noise [19]. Using Equation (1) to obtain \(S_{ID}\) from \(S_{VD}\), however, requires detailed knowledge of the device impedance, which therefore needs to be accurately measured under exactly the same environmental and bias conditions as those under which the noise voltage spectrum was obtained. This procedure can become extremely time consuming and prone to error, since two different measurement steps and setups (for noise measurement and for impedance measurements) are involved.  
For this reason, we decided to explore the feasibility of the realization of an ultra-low-noise transimpedance amplifier dedicated to the reliable measurement of low-frequency current noise in low-impedance devices. In Section 2, the proposed approach for the design of the TIA is presented; in Section 3, details on the circuit implementation are given; in Section 4, the results of measurements performed using photodetectors as DUTs are presented; and in Section 5, conclusions are drawn.