Low-Noise Amplifier for Photovoltaic Devices
Abstract Generated abstract
This paper describes the design basis and implementation of a low-noise selective amplifier for signals from visible and infrared photodetectors, especially photoresistors with specified dark resistance, intrinsic noise, and time constant ranges. The authors relate photodetector detectivity, threshold signal power, receiver sensitivity, amplifier noise, gain, passband, and modulation frequency, deriving expressions used to choose amplifier parameters. A three-stage negative-feedback tube amplifier with a stabilized and filtered power supply is presented, providing 120 dB gain in a 4 to 60 Hz passband at 600 Hz modulation with intrinsic noise not exceeding 10^-6 V. The design includes shielding recommendations for feedback filters to prevent oscillation and allows smooth and stepwise gain adjustment.
Full Text
Vol. 228 1974
LOW-NOISE AMPLIFIER
FOR PHOTOELECTRIC DEVICES
M. S. ALEINIKOV, S. A. GRACHEV, E. P. KOLTSOV, K. L. PESTEREV,
V. S. TITOV
(Presented by the scientific and technical seminar of the Department
of Instruments and Devices of Automation Systems)
A low-noise electronic amplifier is intended for amplifying signals from photodetectors in the visible and infrared frequency ranges of radiation.
It is known [1] that the principal characteristic of a photodetector is the value of the specific detectivity
\[ D^*=\frac{\sqrt{A\cdot \Delta f}}{P_n}, \tag{1} \]
where:
\(A\) is the illuminated area of the photosensitive surface of the receiver,
\(\Delta f\) is the passband in which the noise is measured,
\(P_n\) is the minimum detectable signal power (threshold power).
In addition, the detectivity of a receiver depends on the noise voltage at the amplifier output and on the sensitivity of the receiver; this dependence is described by the expression
\[ D^*=\frac{\sqrt{A}}{N_f}\cdot R_{\omega}, \tag{2} \]
where \(N_f\) is the rms noise voltage at the frequency in a unit passband.
\(R_{\omega}\) is the integral sensitivity of the receiver.
The noise voltage is related to the gain and the magnitude of the amplifier output voltage. This relation is described by formula (3)
\[ N_f=\frac{1}{g_{\max}}\cdot \sqrt{\frac{U^2}{\Delta f}}, \tag{3} \]
where
\(g_{\max}\) is the maximum gain of the selective amplifier,
\(U\) is the magnitude of the amplifier output voltage.
The dependence of the integral sensitivity of the receiver on the measurement frequency in the simplest case has the form:
\[ R_{\omega}=\frac{R_0}{\sqrt{1+(2\pi f_c\tau)^2}}, \tag{4} \]
where
\(R_0\) is the integral sensitivity of the receiver at zero frequency,
\(f_c\) is the measurement (modulation) frequency,
\(\tau\) is the time constant.
Substituting (3) and (4) into (2), we obtain
\[ D^*=\frac{g_{\max}\sqrt{A\Delta f}\cdot R_0}{\sqrt{\overline{U^2}}\cdot \sqrt{1+(2\pi f_c\tau)^2}}. \tag{5} \]
Fig. 1. Schematic diagram of the amplifier
Equating the right-hand sides of expressions (1) and (5), we have
\[ P_n=\frac{U\sqrt{1+(2\pi f_c\tau)^2}}{g_{\max}\cdot R_0}. \tag{6} \]
\(R_{\omega}\) is usually taken in relative units; i.e., in (6) \(R_0=1\). Then
\[ P_n=\frac{U\cdot \sqrt{1+(\omega\tau)^2}}{g_{\max}}. \tag{7} \]
From (1), (3), and (7), by specifying the voltage at the amplifier output and the minimum power of the signal to be measured, and knowing the detectivity \(D^*\), the rms noise voltage \(N_f\), and the receiver time constant (from the data sheet), one can determine the measurement (modulation) frequency, the gain, and the passband of the selective amplifier.
We have developed an amplifier for amplifying signals from photoresistors whose dark resistance lies in the range of \(30 \div 150\ \text{k}\Omega\), whose rms intrinsic noise value does not exceed \(5 \cdot 10^{-6}\ \text{V}\), and whose time constant lies in the range of \(3 \div 5\ \mu\text{s}\).
The schematic circuit of the amplifier is shown in Fig. 1. The gain of the amplifier is \(120\ \text{dB}\) in a passband from 4 to \(60\ \text{Hz}\) at a measurement (modulation) frequency of \(600\ \text{Hz}\). The intrinsic noise of the amplifier does not exceed \(10^{-6}\ \text{V}\). To reduce noise, a highly stabilized power supply is used. The dependence of the output voltage of the power supply on the input is shown in Fig. 2. The amplifier circuit is built as a three-stage circuit with negative feedback, using 6N17B tubes. The use of tubes of this type makes it possible to reduce the dimensions in comparison with existing amplifiers of similar type built on 6N2P and 6N1P tubes [2].
Fig. 2. Dependence of the output voltage of the stabilized power supply on the input
In addition, to reduce noise and the influence of rectifier instability, filtering of the anode voltage has been introduced in the first amplifier stage, which makes it possible to reduce the requirements on the stabilization coefficient of the power supply while preserving the sensitivity and gain characteristics of the amplifier. The frequency response of the amplifier is shown in Fig. 3.
Fig. 3. Frequency response of this amplifier
Amplifiers of this type can easily enter an oscillation mode. To eliminate this phenomenon, the T-shaped bridges located in the negative-feedback circuit must be shielded.
Requirements for shielding the filters are recommended in [3]. At a frequency of \(600\ \text{Hz}\) and a bandwidth of \(5\ \text{Hz}\), a gain of \(5 \times 10^{7}\) was obtained with unchanged amplifier sensitivity. The amplifier provides both smooth and stepwise adjustment of the gain.
References
-
J. Sholl, I. Marfan, et al. Receivers of Infrared Radiation. Mir Publishers, 1969.
-
N. A. Soboleva, A. G. Berkovsky, et al. Photoelectronic Devices. Nauka Publishers, 1962.
-
E. V. Kuchis. Narrow-band electronic amplifier. PTE, No. 2, 74, 1960.