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Description
In this work a preamplifier for photodiodes is presented, with a very peculiar feature: the circuit provides two physically separated gain channels for the DC and the high-frequency components of the spectrum. This is achieved with an active transconductor as feedback network of the first gain stage. Even if this one is DC coupled to the photodiode, the low-frequency component of the signal is collected from the active transconductor and separately amplified. In this way the preamplifier behaves like if it was AC coupled to the photodiode. This enables to adopt different gains for the DC and the high-frequency channels.
This circuit was originally designed to be inserted inside a Pound-Drever-Hall (PDH) stabilization loop for a laser optical cavity. The mechanical vibrations of an optical cavity (from DC to some KHz) continuously bring it out of tune respect to the laser frequency. For this reason, an active loop is required to correct the position of the mirrors in order to keep the cavity in tune. In order to do so an “error function” is required. The reflectivity of a cavity is not a good parameter to be used as error function since it is “even” around the resonance. The PDH technique is used to produce an error signal which is an “odd” function around the resonance frequency. An electro-optic modulator is used to produce two sidebands in the laser’s spectrum with 10-100 MHz frequency difference with the carrier. Due to these sidebands the reflected beam from the cavity shows an amplitude modulation that is an “odd” error function respect to the difference between the laser’s frequency and the resonance of the cavity. This signal can be de-modulated with a signal mixer, sent to a PID (Proportional, Integrative, Derivative) module and used to make the cavity track the laser’s frequency.
The circuit was designed for a modulation frequency between 80 MHz and 130 MHz. The high-frequency power fluctuations of the reflected beam are generally two order of magnitude lower than the average power. Thus if the photodiode DC current in typical experimental conditions is around 200 μA, the 100MHz signal is only 1-2 μA. The problem of a small interesting signal surrounded by a huge low-frequency background is a common problem in very different applications involving photosensors [1,2]. The possibility to have two different gains enables to amplify the 100 MHz signal without saturations due to an extremely high DC voltage level.
The circuit characterization was performed with an Agilent 4395A network-spectrum-impedance analyzer, a Tektronix AFG3252 function generator and an Agilent 54832D oscilloscope. The circuit is based on a three-stage gain circuitry involving only discrete components to ensure high gain and wide bandwidth. The circuit was designed with wide-availability components. The chosen gain transistors are BFR82 while for active loads and buffers MMBTH81 and MMBTH10 were chosen. The gain of the circuit at 100 MHz is 3.79 mV/uA with 15 pF of photodiode capacitance. The test-bench characterization was performed injecting an AC current to the input node through a test capacitor of 1 pF. The circuit shows significant improvements respect to previous adopted solutions both in terms of S/N ratio and absolute signal amplitude.
[1] A. Pullia, T. Sanvito, A. Potenza e F. Zocca. «A low-noise large dynamic-range readout suitable for laser spectroscopy with photodiodes». In: Review of Scientific Instruments 83.104704 (2012).
[2] N. A. Lockerbie e K. V. Tokmakov. «A low-noise transimpedance amplifier for the detection of "Violin-Mode" resonances in advanced Laser Interferometer Gravitational wave Observatory suspensions». In: Review of Scientific Instruments 85.104705 (2014).
