# How to measure rise time/ fall time of transimpedance amplifier?

#### guru@123

Joined Oct 23, 2020
10
Hello, I'm using a trans-impedance amplifier and I need to measure the rise time at the output, but I'm getting a AC sinusoidal waveform at the output. In order to measure the rise time do I have to use any modulation circuits?

#### crutschow

Joined Mar 14, 2008
28,513
What is you input for the test?
It needs to be a current pulse.

#### OBW0549

Joined Mar 2, 2015
3,566
Hello, I'm using a trans-impedance amplifier and I need to measure the rise time at the output, but I'm getting a AC sinusoidal waveform at the output.
Before you do anything else, figure out why you're getting a sine wave at the output. My first guess is that your amplifier is probably oscillating. Read the attached.

In order to measure the rise time do I have to use any modulation circuits?
No.

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#### guru@123

Joined Oct 23, 2020
10
What is you input for the test?
It needs to be a current pulse.
It's a current pulse.

#### tindel

Joined Sep 16, 2012
847
as @OBW0549 points out - if you're getting a sine wave out of your amplifier you likely are unstable (assuming your input isn't also a sine wave). This is usually a result of either too much load capacitance or too large of a feedback resistance. The corrective action for this is either to add resistance to the output of the amplifier or to reduce the resistance, respectively. The first issue can also be solved by reducing your load capacitance.

#### crutschow

Joined Mar 14, 2008
28,513
Set current pulse to zero.
Do you still see a sinewave out?

#### guru@123

Joined Oct 23, 2020
10
Set current pulse to zero.
Do you still see a sinewave out?
Actually I'm using UV source light that passes through optical chopper which falls on photodetector and this is connected to a transimpedance amplifier. At this amplifier output I'm trying to measure rise time on oscilloscope, instead getting a sine wave for which I have no idea how to calculate. Thanks for your reply.

#### OBW0549

Joined Mar 2, 2015
3,566
One of the most frustrating things about hanging out here trying to assist people is the occasional person who asks for help, but cannot or will not provide enough information to enable us to help. So far, the information you've provided is useless.

If you expect to get any help here, there are at least three things you absolutely MUST do so we can get started:

1) Post a schematic diagram of your circuit, EXACTLY as built, COMPLETE with ALL components and connections shown, and all components identified. DO NOT OMIT ANYTHING IN THE CIRCUIT.

2) We need to know about this sine wave you're seeing: what is its frequency? Is it the same frequency as your chopped light source? What is the amplitude of the sine wave? What is the light source chopping frequency?

3) Do as @crutschow suggested, and remove the input signal (i.e., kill your UV light source and turn off the chopper). Do you still see the sine wave? If so, what are its frequency and amplitude?

Without doing these three things, you are simply asking us to waste our time playing senseless guessing games because there are a gazillion different reasons you might be seeing what you're seeing.