Amplify a frequency (phototransistor - Op Amp) - Page 1
Forum section: electronics / beginners

author: Tricka90
author_rank: Regular Contributor
timestamp: October 27, 2013, 12:13:08 pm
content: I'm trying to amplify a 1kHz square wave signal sent from a IR LED with a IR phototransistor and an Op-Amp (LM358).I made up with this scheme but it doesn't work.On point 1 I read a DC component plus the frequency which becomes bigger in amplitude as I approach the IR LED that transmits it.On point 2 I only have the frequency.It's all good for now. So I try to amplify this frequency by using tha Op Amp in Inverting Voltage Amplifing mode.So Vout should be: ( -R2/R1 ) x Vin.But it just doesn't work. The Vout is just too little, even less than Vin.The all circuit is powered with 15V (not dual).What am I doing wrong?

author: Hideki
author_rank: Frequent Contributor
timestamp: October 27, 2013, 12:41:29 pm
content: You have grounded the positive opamp input, and the capacitor blocks DC, so the signal is forced to swing around ground.The opamp output can't go negative, so that part of the signal will be lost. It can go positive, but you can't expect the output to behave nicely when it's close to the negative rail.Try connecting the + input to a higher DC voltage.

author: Tricka90
author_rank: Regular Contributor
timestamp: October 27, 2013, 02:16:01 pm
content: Thank you. I've tried connecting the plus terminal to a Vcc/2 DC voltage with a resistor voltage divider but still no good result.Maybe I'm doing all wrong from the beginning.Can I please ask you what is the best way to achieve what I'm trying to do?I just want to amplify the IR frequency with the phototransistor and the Op Amp and get the amplified frequency on Out, which becomes bigger or smaller in amplitude when I approach or distance the IR transmitter LED.

author: Stefan
author_rank: Contributor
timestamp: October 27, 2013, 04:11:59 pm
content: I've never done this as well, I'll probably won't be any help, but I'm wondering why you need to amplify the signal by a factor 1000 with the opamp at all. I'd expected it to be a voltage follower only.You are using a phototransistor which is already amplifying. What is the 'on' collector current of the phototransistor? The resistor below it should be 15V/current, I guess. 15k would be the choice for a 1mA phototransistor. So, with your IR transmitter close, you should get a full 0 to 15V AC signal at position 1.Also the capacitor forms a lowpass filter together with the 100Ohm resistor behind it. But I guess 1kHz is well below the cutoff frequency.cheersStefan

author: Zero999
author_rank: Super Contributor
timestamp: October 27, 2013, 05:23:25 pm
content: The op-amp will be practically operating in its open loop region as there's hardly any negative feedback so the gain will roll-off at a low frequency, between 10Hz and 100Hz.The input impedance to the op-amp circuit is too low.ZI= SQRT(R2+XC2)XC= 1/(2pi*FC)Plug the numbers in to the above and you get 188R which is far too low compared to the 15k in series with the IR transistor.C = 1*10-6F = 1000R = 100

author: Hideki
author_rank: Frequent Contributor
timestamp: October 27, 2013, 06:04:14 pm
content: Oh, I read it as 100k and 100k, which seemed a bit on the high side, but I see now that one of them is 100 ohms. As Hero999 points out, that sure won't work very well.

author: Tricka90
author_rank: Regular Contributor
timestamp: October 28, 2013, 09:59:40 am
content: Thank you for all your help!I managed to solve the problem using this configuration:I'm using a TL081CN, and instead of the 100kohm resistor between inverting terminal and output I'm using an 1Mohm one to get even more gain, and all works fine!There is only a little last problem. I found that the amplitude of the frequency on output varies a lot with ambient light.After some tests I found that this problem is caused by the polarization of the phototransistor.When the ambient is well lighted all works fine, but when the ambient is dark the phototransistor can't get enough polarization from ambient light so when the IR LED transmitter is far it cannot output enough collector current. If I use a photodiode instead of the phototransistor the problem disappears (but of course the output frequency is too low in amplitude). Do you think there is any way to solve this problem?

author: Tricka90
author_rank: Regular Contributor
timestamp: October 28, 2013, 10:12:46 am
content: I found there is also a more serious problem. When there is some light from lamps in the room, which lights at 100Hz (I live in Europe), the op amp amplifies also this wave. Is there any way to filter this out?

author: Tricka90
author_rank: Regular Contributor
timestamp: October 28, 2013, 02:44:35 pm
content: Could you please tell me what is the function of the three 100kohm R and the 1uF C connected to the non-inverting input?I can say that two R are used as a voltage divider to give the non-inverting input a Vcc/2 voltage, but I can't say what the last 100k resistor and the 1uF C should do. Is it some kind of filter?