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I'm looking for examples of how current mirror is used in real life.
Most tutorials cover the circuit itself but don't go into examples. Therefore, I'm still struggling to understand how it's used and what the benefits of it are.
Update: I'm looking for simple enough examples for a newbie to understand, please don't post complete op-amp schematics, etc. Thanks!
The two transistors in a current mirror should have equal characteristics; therefore, current mirrors are almost never built from discrete transistors, but are used inside chips where the transistors are next to each other on the same die. As a newbie, you would probably never design a circuit with a current mirror.
In practice, current mirrors are most often used as an active load in the input stages of op amps and comparators. A current mirror has a lower voltage drop than reasonable emitter resistors, and allows larger amplification. (Its primary purpose is not really to mirror the current.)
One of the simplest devices is the LM339 comparator:
To understand how it works, ignore the diodes, and first replace the current mirror with two resistors. Also note that in a comparator, the two inputs should not have equal voltages, which implies that the current mirror fails to have equal currents; you get a digital output that is either high or low.
LM339 datasheet
Application Design Guidelines for LM339, LM393, TL331 Family Comparators
Current mirrors are quite common in audio power amplifiers as well:
TR10-TR11 pair in the schematic above (bottom-left) form a current mirror to make the collector currents of TR2-TR3 differential pair equal.
Fun fact: A class-AB power amp is a bigger (more powerful) version of a transistor-operational-amplifier.
Current mirrors are frequently used in linear integrated circuits such as op amps. They are used to provide a stable current for biasing transistors and transistor pairs such as differential pairs.
I saw a real life use of a discrete current mirror a couple of months ago in this question.
Here's the circuit diagram from the question:
The circuit originally came from this thesis (see page 14.)
There's a current mirror in the upper left corner. It is used to convert the voltage at Vin to a current to control the charge current (and thus the frequency) of the 555 oscillator. The circuit as a whole is a voltage to frequency converter - which happens to work by converting the voltage to a current and then to a frequency.
Varying Vin causes a varying current to flow through the left transistor. Because of the way the current mirror works, the same current flows through the right transistor. This takes the place of the fixed resistor you would normally find connecting V+ and pin 7 in a normal 555 circuit.
The CA3096AE contains multiple matched transistors. They aren't as good as having identical transistors made on one chip, but they are closer to identical than you will get by trying to manually match transistors yourself.
For comparison, a regular 555 oscillator:
From the TI LM555 datasheet.
The current mirror takes the place of RA in the example from TI. RA (together with RB) controls the charge time of the capacitor, and thus the frequency.
Here is a trivial, but real life example.
I had an existing design of a product with a motherboard having a built in current pwm sink for controlling the backlight of a single screen on the front panel.
For a new product I needed to add a second screen without changing the motherboard PCB in any way.
The question was how to control the brightness of the second screen (I could switch it on and off over its interface, but the backlight was separate). Finding a way to switch the backlight was doable, but I only had that one current sinking control line for the brightness and the supply voltage was not sufficient to support just throwing the backlight LEDs in series.
A PNP current mirror in the anode circuit to make the current in the second screen match that in the first was the solution, simple, one dual PNP transistor in a SC-70 package and a couple of 0603 resistors, job done, took about a minute to cad up, and was obviously going to work.
There are hundreds of those boards out there at this point.
Question
Are there are real life examples of using a current mirror?
Answer
I am using the following current sensing amplifier chip, which is using a current mirror.
My real life project still in progress is to plot the I-V curve of a tunnel diode. Because the current I am measuring is very small, in the range of uV to a couple of mV, so I need a current sense amplifier to convert the tiny current to 0 ~ 5V, and I also use a 12-bit ADC to read the digital value which will be read and stored by a Raspberry Pi.
The photo below shows my calibration hardware setup.
Wiring the tiny MAX4172 is a bit tricky. There is no assembled module available, so I need to do the point to point wiring, luckily only 8 pins, to a SOC8 to DIP8 breakout board. I have about 100 hours of SMD hand soldering experience, so it took me only 15 minutes to do the first chip, and then 10 minutes in average for the later ones. The photo below shows my craftmanship. :)
The software part is not that difficult. I skimmed the datasheet, and found that there is actually no programming required. I only need to find out the a bit complicated amplification gain factor table, and choose the gain resistor to fit my current range. My datasheet summary is show below.
The picture below shows my working on the current sense and output resister values.
My quick and dirty conclusion is that the hardware soldering is a bit tricky, but there is no software need, just simple calculation.
So all in all, a damn cheap newbie friendly toy to learn new stuff.
Update 2021jan29hkt1711
MAX4172 is an example of using current mirror at the current sensing amplication application level. Another example of using current mirror at the circuit design level can be found in Refs 6, 7, where current mirror is used in CCS, Constant Current Source
Update 2021jan31hkt2053
I am making a constant current source, with reference to the Zener based one transistor circuit of the Wikipedia article on "Constant Current Source". The Wiki article points out that a "boost current mirror" is used this circuit. The definition of current mirror by the Wiki article is also very good.
References
(1) MAX4172 Low-Cost, Precision, High-Side Current-Sense Amplifier Product Sheet - Maxim
(2) MAX4172 Low-Cost, Precision, High-Side Current-Sense Amplifier Datasheet -Maxim
(3) Maxim MAX4172ESA High Side Current Sense Amplifier - TaoBao ¥1.25
(4) SOP8 SMD to DIP8 Breakout Board - TaoBao, ¥2/10pcs
(5) Current Mirror - Wikipedia
(6) The Basic MOSFET Constant-Current Source June 06, 2016 by Robert Keim - aac
