# Op-amps, noise and high frequency/low current input signal

• GuitarOfWar
In summary, if you are amplifying a current with an order of magnitude and you are trying to reduce noise, you can use an op-amp to amplify the voltage across a resistor. You can also use a high-pass filter to reduce the amount of noise.
GuitarOfWar
Hello everyone,

In my lab class we've been given task of using op-amps to amplify a current of 1nA and frequency of order of madnitude 1MHz to a discernable level. We've been given breadboards, a selection of different op amps, resistors, an oscilloscope etc to achieve this task. We've made good progress so far as we've gotten down to 10nA. But at 1nA we get to much noise to make a discernible waveform on the oscilloscope. Does anyone have any tips on how we may go about reducing noise in this setup?

The level of the current is not very relevant as long as the source of that current can drive a relatively large load.

Since your desired output signal is a voltage, all you need to do is to pass the current through a large resistor and then use the op-amp to amplify the voltage across that resistor;. If you can use a resistor with a value of say 10 MOhm or so you will get a voltage any "normal" op-amp can handle.
That said, at 1nA you might need to use a few more "tricks". Since you know that the signal is at 1 MHz you can high-pass (or even better band-pass) filter the signal from say 300kHz or so, which will dramatically reduce the amount of noise

Also, this should probably be in the EE forum.

f95toli said:
The level of the current is not very relevant as long as the source of that current can drive a relatively large load.

Since your desired output signal is a voltage, all you need to do is to pass the current through a large resistor and then use the op-amp to amplify the voltage across that resistor;. If you can use a resistor with a value of say 10 MOhm or so you will get a voltage any "normal" op-amp can handle.
That said, at 1nA you might need to use a few more "tricks". Since you know that the signal is at 1 MHz you can high-pass (or even better band-pass) filter the signal from say 300kHz or so, which will dramatically reduce the amount of noise

Also, this should probably be in the EE forum.
f95toli said:
The level of the current is not very relevant as long as the source of that current can drive a relatively large load.

Since your desired output signal is a voltage, all you need to do is to pass the current through a large resistor and then use the op-amp to amplify the voltage across that resistor;. If you can use a resistor with a value of say 10 MOhm or so you will get a voltage any "normal" op-amp can handle.
That said, at 1nA you might need to use a few more "tricks". Since you know that the signal is at `1 MHz you can high-pass (or even better band-pass) filter the signal from say 300kHz or so, which will dramatically reduce the amount of noise

Also, this should probably be in the EE forum.
I can't bandpass the signal because the input is meant to be simulating particles hitting a plate at a frequency of say 10000kHz to 1MHz. I've put decoupling capacitors on the opamp so far. I was thinking of putting multiple opamps in stages in series. Would it be better to do it in paralell? What ever little tricks I can do to reduce the noise as much as possible. I usually put the input current through some voltage dividers to reduce the nouse the function generator makes at low ampltidues.

GuitarOfWar said:
the input is meant to be simulating particles hitting a plate

You have misstated the requirement. You do not have an AC signal. You have events arriving at certain rates. If you want to further reduce noise beyond a bandpass, think of an event detector, then count events per second.

What distinguishes a valid event from noise? If you can't answer that, you can't filter noise.

## 1. What is an op-amp and how does it work?

An op-amp, short for operational amplifier, is a type of electronic amplifier that is commonly used in many circuits. It consists of a differential input stage, followed by a gain stage and an output stage. The differential input stage allows for high input impedance, while the gain stage amplifies the input signal. The output stage provides a low impedance output.

## 2. Why is noise a concern in op-amps?

Noise is a concern in op-amps because it can affect the accuracy and stability of the output signal. Op-amps have a high gain, meaning even small amounts of noise can be amplified and affect the output. This can lead to errors in measurements and can impact the overall performance of the circuit.

## 3. How does high frequency affect op-amps?

High frequency signals can cause instability in op-amps due to the limited bandwidth of the amplifier. This can result in distortion of the output signal or even oscillation. It is important to consider the frequency range of the input signal when selecting an op-amp for a specific application.

## 4. Can op-amps handle low current input signals?

Op-amps are designed to have a high input impedance, meaning they can handle low current input signals without drawing much current from the source. However, at very low current levels, the noise in the circuit can become more significant and may need to be taken into consideration.

## 5. How can noise in op-amps be reduced?

Noise in op-amps can be reduced by using low noise op-amps, minimizing the input and output impedance, and properly shielding the circuit. Additionally, using filtering techniques such as low-pass filters can help to reduce noise in the circuit.

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