Transimpedence Amplifier With Roll-off

This instructable will show you how to build a simple transimpedence amplifier with roll off at high frequencies. This circuit has a photodiode which can be used to vary the output voltage. Applications of this circuit will be described in Step 3.

Parts:

• Op amp
• Photodiode
• 100 kΩ resistor
• 1.5 nF capacitor
• Breadboard
• ±15 V AC power supply (ours is part of our breadboard)
• Connecting wires

Step 1: Building the Circuit

Construct the circuit as shown in the above diagram.

Our breadboard is integrated with a power supply, with a row of pinholes already powered with +/-15V and ground, but you may need to power your breadboard manually. Place the op-amp on the breadboard, inserting it between two sets of the 5-pin rows. Connect pins 7 and 4 to +15V and -15V respectively. Continue to follow the diagram to build the rest of the circuit.

Remember to always switch your power supply off while building or changing the wiring of your circuit- some components are easy to fry.

Step 2: How It Works

Current is generated as light hits the photodiode. This current goes through the resistor, resulting in a voltage drop. Since the op-amp wants to keep the voltage at its positive and negative inputs equal, it will put out a voltage V-out such that the inputs will be equal for a large range in currents from the photodiode. The value of the voltage output depends on the size of the resistor. Because voltage equals current times resistance, a large resistor (like the 100 kΩ one we used) will let a very small current be converted into a voltage that is much easier to detect. That is to say, a larger resistor will result in the circuit having a larger gain.

The capacitor causes roll-off at high frequencies, because as the frequency increases, the capacitor charges less completely, and behaves more and more like a wire. This has the same effect as reducing the size of the resistor at high frequencies, and lowers the gain of the circuit.

Step 3: Applications

Because this circuit has a high gain, it is useful in situations where you want to measure small currents. Examples include photo detectors, accelerometers, and photo multiplier tubes. All of these may at one time or another be measuring very small movements or amounts of light, and in order to be very accurate, they need to be able to register small changes. Currents in these circuits can be on the order of microamps or less, which is difficult to measure without the right equipment, but a transimpedance amplifier converts these into voltages on the order of a few millivolts, which is much easier to measure and use.

Having the capacitor reduce the gain at high frequencies is very important for noise control. Noise almost always increases with frequency, so having a high gain is not as desirable for high frequencies as it is for low ones. By building roll-off into the circuit, the measurable voltage will decrease but the noise will not be as amplified either, so readings will be more accurate.