Model 312B Current Amplifier
Application Note

AMI’s Model 312B is a low noise high gain (1MV/A) transimpedance amplifier.  It is designed to amplify small current signals, typically produced by a photodiode, to a voltage signal employing AMI’s proprietary compensation scheme to accommodate a significant variance in input capacitance rang.

Input capacitance, the intrinsic capacitance of a photodiode plus the capacitance of any external cabling or connectors, introduces boosting potentially causing the amplifier to oscillate as well as limiting the amplifier’s bandwidth if not compensated properly.

The Model 312B is available with AC or DC coupling.  For AC coupling the output is capacitively coupled.  The Model 312B is non-inverting to create a positive output.  When DC coupled more output drive is available in the positive direction.

The Model 312B uses a single power supply and proprietary circuitry is included to maintain the input at 0 V to accommodate detectors with low bias voltage requirements.

Input Terminals:

The three input side terminals on the Model 312B consist of input, bias, and ground.

Current to be amplified is fed to the input terminal, which is low impedance and maintained at 0 V. When amplifying current signals from a positively reversed biased photodiode, the anode end of the diode is connected to this terminal.

The bias output terminal is a voltage source generated by an on-board voltage regulator intended to reverse bias a photo-diode. The voltage regulator is programmed to output ~9.5 V*. In instances where higher bias voltage is required, the voltage regulator can be overridden externally by applying a higher voltage to the external bias voltage terminal.

*(Internal bias voltage can be set as low as a few millivolts on a custom basis, contact factory.)

The ground terminal may be used to ground the photodiode’s case.

Ideally, connections to these terminals should be made as short as possible and shielded as to minimize any EMI.

External Bias Terminal:

This terminal is used to supply a positive bias to a photodetector.  External bias voltage is filtered with a 10 kΩ resistor and 0.01 μF cap in-route to the bias terminal at the input side of the amplifier.

 Output:

SMB coax connector, SMB to BNC coax cable is supplied.  Output must be terminated into 50Ω.

Power Supply and Ground Return:

Apply a 12 to 15 V (100 mA) to the power terminal.

Ring Lug at the base of the power terminal is the power ground return.

Please contact AMI today for price and delivery information or if you have any questions.