Michael J Gambuzza, General Electric Measurement and Control Solutions, Billerica, MA
Use a transimpedance amplifier to drive an LED
You can test an IR (infrared) link with a circuit that converts an IR-generated photocurrent to an amplified current that drives a standard LED (Figure 1). This approach provides a visual feedback to indicate that the transmitter is working. The circuit can be enclosed in a small plastic or metal box and requires just a 9V transistor battery for operation. Diode D1 is a basic Everlight PD333-3C/H0/L2 or equivalent IR photodiode in a T1¾ package.
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| Figure 1. | This circuit detects the transmitter in an IR communication link and provides a visual output from the LED. |
You can configure amplifier IC1A as a photovoltaic amplifier. When the IR-light energy impinges on photodiode D1, it generates a small photocurrent that tries to pull the inverting input negative. Meanwhile, the output of IC1A goes positive, maintaining the virtual-ground node on Pin 2 of the amplifier at 0V. The transfer function for the circuit is VOUT = I×R1. If you set the gain high, IC1A goes to the power-supply rail when the circuit detects light. Analog Devices’ AD823AR JFET-input amplifier directly drives the LED through a 750Ω current-limiting resistor. C1 compensates the amplifier, preventing it from oscillating due to capacitive load from D1 and the input parasitic capacitance.
If the output of IC1 oscillates, you may need to increase the value of C1. You can determine the value of C1 by using the following equation for a 45° phase margin:
where FC is the unity-gain-crossover frequency of IC1A – typically, 16 MHz for the AD823 – and CD is D1’s 0V junction capacitance, including any parasitic capacitance on that node. Adjust R1 for optimum gain. For testing, the remote-control transmitter window should be as close as possible to photodiode D1 for maximum signal transfer.
