Current amplifier using OpAmp

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,744
For testing purposes I would like to build a simple current amplifier using maybe only resistors, and OpAmp, probably a U741 and probably a LED to see if the current is actually being amplified or not!

I have a +5V (DC) voltage supply (Arduino) but if needed I can use a voltage source I have here (but it's not connected) to get +12V DC and -12V DC.

So, let's say I have like 20uA and I need to light a LED on. So I need a current amplifier to make the 20uA something like 10mA or so!

How can I do this?
 

Jony130

Joined Feb 17, 2009
5,487
And this current (input current) is coming from where? In other words, what is the source of the input current?
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,744
And this current (input current) is coming from where? In other words, what is the source of the input current?
Well, as I don't have a current source, I was thinking about an input resistor with a voltage across it to make a small current and get it amplified!
 

Alec_t

Joined Sep 17, 2013
14,263
You don't need a current amplifier to light a LED. Use a voltage source V and a resistor R in series with the LED.
Assuming the LED is a red one, R = (V-2)/10 kΩ.
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,744
You don't need a current amplifier to light a LED. Use a voltage source V and a resistor R in series with the LED.
Assuming the LED is a red one, R = (V-2)/10 kΩ.
The goal is exactly to amplify a current... The LED was just to check that actually the current is amplified! Probably this will be used in another application! For now I just want to test and check how things works!

So maybe this circuit will do the job
View attachment 119903
You can use a BJT instead of a MOSFET.
Ok, I'm going to try that! Thanks
 

crutschow

Joined Mar 14, 2008
34,201
I don't want a voltage, I want a current!
So do you want a precise amplification of the current, i.e. 10μA in gives 10ma out, 20μa in gives 20mA out, etc.?

How much input voltage can this input current generate.
In other words, can a resistor be put in series with the current to measure it's voltage and then amplify the voltage as @Jony130's circuit does?

Can the current load be floating or does it need to be grounded?
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,744
So do you want a precise amplification of the current, i.e. 10μA in gives 10ma out, 20μa in gives 20mA out, etc.?

How much input voltage can this input current generate.
In other words, can a resistor be put in series with the current to measure it's voltage and then amplify the voltage as @Jony130's circuit does?

Can the current load be floating or does it need to be grounded?
Well, there are some question I'm not sure I would give a reasonable answer because I'm just a noob in electronics as you can see!

So, my goal was to check that whatever current amplifier I build, is actually amplifying the input current. And as I don't have a current source, I was asking if I could use a voltage source (Arduino +5V DC) and a series resistor of like 50kΩ to get something like 1μA and then amplify it back to 5mA for instance, to light up the LED.

20 uA times 100,000 ohms gives you 2 V to drive a red, orange or green LED. The op amp can drive 10 mA or so as needed. White, UV or blue LEDs need about 3.5 V. Increase the resistor as needed.
Yes, but I need to get those 20μA out of somewhere. That must be a voltage source with a series resistor!
I'm using Arduino's +5V DC and for example a 50kΩ resistor to get a 1μA of input current and then amplify it as much as needed to light up the LED (green one).



Edited;
I have a new problem. My multmeter is not working properly! It is not measuring in the correct scale! So it has a 10x scale factor error, not sure why!
The only measurement tool I have now is a scope. It's a simple Rigol MSO1104Z.
 
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hp1729

Joined Nov 23, 2015
2,304
Design 1103 current amplifier.PNG
Well, there are some question I'm not sure I would give a reasonable answer because I'm just a noob in electronics as you can see!

So, my goal was to check that whatever current amplifier I build, is actually amplifying the input current. And as I don't have a current source, I was asking if I could use a voltage source (Arduino +5V DC) and a series resistor of like 50kΩ to get something like 1μA and then amplify it back to 5mA for instance, to light up the LED.



Yes, but I need to get those 20μA out of somewhere. That must be a voltage source with a series resistor!
I'm using Arduino's +5V DC and for example a 50kΩ resistor to get a 1μA of input current and then amplify it as much as needed to light up the LED (green one).



Edited;
I have a new problem. My multmeter is not working properly! It is not measuring in the correct scale! So it has a 10x scale factor error, not sure why!
The only measurement tool I have now is a scope. It's a simple Rigol MSO1104Z.
Yes, use the resistor you mentioned. The op amp circuit with the N-MOSFET would be better for that.

(edited to add ...)
See attachment. I haven't tried it yet but it should be right. the transistor should work as an NPN or an N-MOSFET.
 
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Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,744
Ok, I think I have built that circuit of yours @hp1729 ... I just got a bit lost on about where to place the LED! I've used 220kΩ for R6 and a combo of 10kΩ + 10kΩ + 2k2Ω + 2k2Ω for R5 and I used an U741 insstead of the LM358 because I don't know where I have kept the LM358 I have!
 

Jony130

Joined Feb 17, 2009
5,487
Ok, I think I have built that circuit of yours @hp1729 ... I just got a bit lost on about where to place the LED! I've used 220kΩ for R6 and a combo of 10kΩ + 10kΩ + 2k2Ω + 2k2Ω for R5 and I used an U741 insstead of the LM358 because I don't know where I have kept the LM358 I have!
From your description I concludes that you do not understand how this circuit work.
R5 resistor represent the load (any load). And the Op amp together with BJT and R3 resistor is a Voltage controlled current source (this current source can only sink current).
The input voltage is Vp = 5V*R4/(R6+R4) = 0.2V and this voltage op amp will "force" across R3 resistor (Vp = Vn).
So, the emitter current is Ie = Vp/R3 = 0.2V/1kΩ = 0.2mA = 200μA. And the output current ( collector current) is equal to:
Iout = Ic = Ie*β/(β+1) ≈ 200μA
 

hp1729

Joined Nov 23, 2015
2,304
Ok, I think I have built that circuit of yours @hp1729 ... I just got a bit lost on about where to place the LED! I've used 220kΩ for R6 and a combo of 10kΩ + 10kΩ + 2k2Ω + 2k2Ω for R5 and I used an U741 insstead of the LM358 because I don't know where I have kept the LM358 I have!
The LM741 will not work too well with an input voltage close to either rail voltage. Signals close to ground are okay if you are using +V and -V for power. But if you look at the data sheet it will not work well within about 2 V of either rail. Running at 5 V this only gives you a small window between 2 V and 3 V for a signal.
same for the output. Small window for an output voltage.
This pretty much kills the LM741 for operation at 5 V.

Why is the M358 so different? Look at the data sheet that shows the internal structure of the op amp. The LM348 differential amp is made of PNP Darlingtons Ground referenced inputs fit nicely.
 
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hp1729

Joined Nov 23, 2015
2,304
For testing purposes I would like to build a simple current amplifier using maybe only resistors, and OpAm, probably a U741 and probably a LED to see if the current is actually being amplified or not!

I have a +5V (DC) voltage supply (Arduino) but if needed I can use a voltage source I have here (but it's not connected) to get +12V DC and -12V DC.

So, let's say I have like 20uA and I need to light a LED on. So I need a current amplifier to make the 20uA something like 10mA or so!

How can I do this?
LM741 limitation running at +5 V.
A simple voltage follower. The input can vary from -12 to +12, starting at +2.5 V and seeing how far positive and negative the op amp will recognize.
The LM741 would not recognize an input voltage lower than 1.8 V or higher than 4.4 V.
The LM741 is used in many schools because it is a good example of the problems with op amps.
In contrast the LM358 went from 0 V to 3.8 V. So if you are working with low input voltages and an output not more than 3.7 V the LM358 is a better choice.
Newer devices are Rail-to-Rail input and output.


Design 1103 LM741 limitations.PNG [
 
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Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,744
Ok, I'm going to study that circuit, but looks like to me that that is not exactly what I wanted. But I'm going to stick to it!

In the meantime I search my book for a circuit like that and I found this Fig. A. Is it the same thing??? The only difference I find is the load connected to ground instead of Vcc as the circuit posted by @hp1729.
Can I take this example to read and try to understand the circuit?


Edited;
By the way I already found an LM358N...
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,744
@hp1729 I tried to build the circuit you suggested with the LM358N, 220kΩ(R6), 22kΩ + LED (R5 + Green LED), 10kΩ (R4), 1kΩ (R3), 5.4V and +12.4V (Vcc). I did some math for Vin at +input of the OpAmp and I also got ~0.2V just like @Jony130, and also got the same result as he did for I_R3 = 200μA. But when I tried to check with the scope, I got 1.16V for +input of the OpAmp and consequently 1.16V / 1kΩ = 116μA for R3.

I'm not sure what was the Beta considered by @Jony130 in order to him to get roughly the same I_Load as I_R3. But I'm not getting the 0.2V at +input of the OpAmp as I was expecting with math!

I can also see that the OpAmp is being capable of keeping both inverting and non-inverting inputs at the same voltage difference!

Edited;
Not sure why, now I'm reading the expected values and I changed nothing in the circuit!
 
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