Trying to understand the µCurrent - Page 1
Forum section: electronics / beginners

author: thommy12
author_rank: Newbie
timestamp: July 18, 2023, 01:27:44 pm
content: Hello everyone!I hope this is the right place under the "Beginners" topic, as it seems to primarily involve questions for beginners. If not, feel free to move it...I'm currently trying to understand the circuit diagram and design principles of the µCurrent. I stumbled upon it by chance and found the project very interesting, so I'd like to build something similar myself.For this purpose, I'd like to gain a better understanding of the entire circuit and hope to receive appropriate assistance here.Here's the link to the circuit diagram:http://www.eevblog.com/files/uCurrentRev5schematic.pdfFirstly, I'm curious about why the amplification is split between two op-amps. A 100-fold amplification should not be a problem for a single operational amplifier, right?Furthermore, I don't fully grasp the purpose of capacitors C3 and C4. I'm familiar with using capacitors when you want to use the op-amp as an integrator, for example, but what is their purpose in this particular setup? Are they used for filtering?Lastly, I'd like to understand the purpose of R8 and R12. My assumption is that they serve as protection for the op-amps, but I'm not entirely sure about their role on the input side. What could happen at this point? Also, why isn't there a resistor between the output of U1 and the input of U4?I would greatly appreciate some explanatory answers and thank you in advance!Simon

author: mwb1100
author_rank: Frequent Contributor
timestamp: July 18, 2023, 05:23:00 pm
content: Quote from: thommy12 on July 18, 2023, 01:27:44 pmSomething to do with increased bandwidth (personally I have no idea how the staged opamp design increases bandwidth): 300KHz vs 8KHz on the original uCurrent.-"the GOLD is a two stage unit to get the extra bandwidth"

author: macboy
author_rank: Super Contributor
timestamp: July 20, 2023, 03:08:57 am
content: Quote from: mwb1100 on July 18, 2023, 05:23:00 pmAn important opamp specification is gain bandwidth product, GBW or GBWP. As implied this is bandwidth times gain. Let's say that an opamp has 10 MHz GBW. If you set gain to 100, then you can expect 10 MHz/100 or 100 kHz of bandwidth. If you instead use two stages, each with gain of 10 (total gain 100), you will get bandwidth of 1 MHz.

author: gnuarm
author_rank: Super Contributor
timestamp: July 20, 2023, 03:40:26 am
content: Quote from: mwb1100 on July 18, 2023, 05:23:00 pmOpamps have what is called gain/bandwidth product, meaning the gain times the bandwidth is a constant. If you crank up the gain of your circuit, the opamp will have a lower high frequency cutoff.Meanwhile, C3 and C4 limit the gain at higher frequencies. They won't work as a proper filter, with gain dropping very low as frequency rises. Because they are in the feedback leg, they can only ever cut the gain to 1, not lower.I assume R8 is to protect the output from a short on the output terminals. I would say R12 is to balance the offset voltage from the input bias current, but it should be closer to 900 ohms to match the inverting input. Then there is none on the non-inverting input to U4. The signal is ten times larger here, but why introduce any errors you don't need to?