Low-Noise Preamplifiers

Choose the input architecture from the detector output and source impedance before comparing gain or bandwidth. Voltage models suit sensors that already generate a measurable voltage, while current preamplifiers convert photodiode, ionisation or other detector currents into a voltage through a stated transimpedance. The SR551 provides an input impedance above 1 TΩ for sources that must not be resistively loaded, whereas the SR554 is intended for source impedances from 0.05 Ω to 1 kΩ and provides transformer isolation. For fast photomultiplier or photodiode pulses, the 350 MHz SR445A and 400 MHz SR446 provide 50 Ω signal paths. This first architectural choice prevents unsuitable loading and avoids selecting a noise figure that is irrelevant to the actual source.

The voltage-noise figure should be considered together with input impedance and input-current noise. The SR550 and SIM910 use FET or JFET inputs with 100 MΩ input impedance, making them suitable for higher-impedance voltage sources than the BJT alternatives. The SR552 and SIM911 reduce voltage noise to 1.4 and 1.8 nV/√Hz respectively, but their 100 kΩ input impedance can load a high-resistance source. The SR551 extends input impedance beyond 1 TΩ when resistive loading must be kept very low, although its voltage noise is 12 nV/√Hz at 1 kHz. For low-impedance sources between 0.05 Ω and 1 kΩ, the SR554 provides a different approach with 0.1 nV/√Hz transformer input noise.

The SR445A and SR446 are the primary RF choices for fast detector pulses. The SR445A provides four DC-coupled channels with 350 MHz bandwidth, 1 ns rise and fall time, and gain of ×5 per channel; cascading all four produces a nominal gain of ×625. The SR446 is a single-channel 400 MHz instrument with 21 programmable gains from ×1 to ×100, selectable bandwidth limits and complementary outputs. Both are designed around 50 Ω signal paths, although each provides a 500 Ω input option for selected current- or charge-output arrangements. Correct coaxial termination remains important because an unterminated RF cable can alter amplitude, introduce reflections or affect amplifier stability.

The SR560 is suited to a stand-alone bench setup requiring wide gain adjustment, configurable analogue filtering and battery operation. It provides gain from ×1 to ×50,000, two configurable filters with cutoff settings from 0.03 Hz to 1 MHz, differential or single-ended inputs and up to 15 hours of battery operation. The SIM910 and SIM911 are modular alternatives with gain from ×1 to ×100, DC-to-1 MHz operation and serial control through a compatible mainframe. The SIM910 provides a 100 MΩ JFET input, while the SIM911 provides lower voltage noise with a 100 kΩ BJT input. Selection therefore depends on required gain range, source impedance, filtering, physical integration and whether line-independent operation is necessary.

The SR570 is the flexible choice when one experiment must cover a wide range of current levels and bandwidths. Its sensitivity extends from 1 pA/V to 1 mA/V, with configurable filters, adjustable input offset current, ±5 V bias and operating modes that trade noise, bandwidth and drift. The SR555 and SR556 are compact fixed-gain alternatives: the SR555 provides 10⁷ V/A with 120 kHz bandwidth, while the SR556 provides 10⁹ V/A with 3 kHz bandwidth and 5 fA/√Hz input noise. The SIM918 provides selectable gains from 10⁶ to 10⁸ V/A, gain-dependent bandwidth from 22 to 4 kHz and an autozero function that holds input offset below 10 µV. The appropriate choice depends on whether the design requires broad adjustability, fixed conversion gain, low offset or modular remote control.

Place the preamplifier close to the detector when practical, particularly for current sources whose cable capacitance can increase noise and restrict bandwidth. RF models should use correctly terminated 50 Ω coaxial paths, while the 500 Ω settings should only be used when their higher input impedance suits the detector and cabling arrangement. The SR555 and SR556 provide balanced differential outputs, so equal-length output cables and a differential receiving input are preferable. Floating inputs and shields on the SR560, SIM910 and SIM911 can help control ground loops, provided common-mode limits are respected. Where galvanic separation is required, the SR554 provides greater than 40 dB isolation from DC to 500 MHz and allows its transformer primary to float to ±100 VDC.

The SR560 and SR570 provide receive-only RS-232 control at 9600 baud, allowing instrument settings to be changed remotely while keeping the interface isolated from the analogue circuitry. The SR446 uses USB with serial-port emulation and supports remote configuration of input coupling, impedance, gain and output filtering. The SIM910, SIM911 and SIM918 use a serial interface through the modular instrumentation platform and permit settings to be queried as well as changed. The SR445A, SR550, SR551, SR552, SR554, SR555 and SR556 are primarily configured through hardware controls or fixed operating arrangements. Automated-system designers should therefore confirm whether they need remote setting changes, status readback or only a fixed analogue front end.

Higher current-to-voltage conversion generally reduces available bandwidth, so both values must be selected together. On the SR570, a sensitivity of 10⁻³ A/V supports bandwidth up to 1 MHz, while the 10⁻¹² A/V setting is limited to 10 Hz. The SIM918 provides 22 kHz bandwidth at 10⁶ V/A, 12 kHz at 10⁷ V/A and 4 kHz at 10⁸ V/A. The fixed-gain SR555 and SR556 illustrate the same trade-off, offering 120 kHz at 10⁷ V/A and 3 kHz at 10⁹ V/A respectively. In practice, choose enough transimpedance to place the signal above downstream noise, then limit the measurement bandwidth to the part of the spectrum containing useful information.