RE: The Wilson Current Mirror

jc2048 — Sat, 13 Feb 2021 22:12:31 GMT

I don't know what the original video, referred to in the blog, covered (sorry, I don't choose to sign up

to You Tube and things like that, and so apologies if I'm repeating things already covered), but I later

got curious as to what the response time of the Wilson one was like. The basic two-transistor mirror (is

that one due to Widlar? can't remember) is fast because it's just the internal feedback within the output

transistor that's regulating the current. The three-transistor one is a current regulator loop [the pair

of transistors at the top are a mirror feeding back the output current to form an error term], so we might

expect it to be a bit slower. The response would be of interest if the mirror were used to provide a load

for an amplifier stage to increase the gain, or something like that, so I thought I'd try measuring it.

I've wired it up on a plug-in breadboard and I'm going to try a) changing the input current and see how

quickly the output responds, and then b) try changing the output load and see how long the output takes to

settle.

Here's the circuit for a)

[View:/resized-image/__size/400x300/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-b111a046-ecbd-4e3b-8560-b93b910326eb/contentimage_5F00_207283.jpg:400:300]

The UNO output switches a second resistor (and diode drop) in parallel with the 330R resistor. I'm not too

concerned what the change in input current actually is, just that there's a reasonable difference that I

can look at.

Here's the response (yellow is the UNO pin, blue is the output load resistor)

[View:/resized-image/__size/480x234/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-b111a046-ecbd-4e3b-8560-b93b910326eb/contentimage_5F00_207284.png:480:234][View:/resized-image/__size/480x234/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-b111a046-ecbd-4e3b-8560-b93b910326eb/contentimage_5F00_207285.png:480:234]

Although it's somewhat messy whilst the switching occurs [down to the way I'm using the switching diode, I

think, and the fact it's on a plug-in breadboard] , we can see it's mostly done after a fairly respectable

10ns, though there is a bit of an extended settling time after, and the overshoot is a bit embarrassing.

Not sure I can explain the overshoot, though it might be the fast edge, combined with the diode reverse

capacitance, lifting the 330R momentarily higher than the level that it then settles back down to.

Here's the circuit for b).

[View:/resized-image/__size/400x300/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-b111a046-ecbd-4e3b-8560-b93b910326eb/contentimage_5F00_207286.jpg:400:300]

This time, I've got the UNO applying the diode drop as an addition to the load resistor, and we're looking

at how long it takes to settle at the diode on voltage, followed by it going back up to the voltage

dropped by the load resistor alone.

[View:/resized-image/__size/480x234/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-b111a046-ecbd-4e3b-8560-b93b910326eb/contentimage_5F00_207287.png:480:234][View:/resized-image/__size/480x234/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-b111a046-ecbd-4e3b-8560-b93b910326eb/contentimage_5F00_207288.png:480:234]

That's nicely done by the mirror, though there's a bit of a difference between increasing the load and

decreasing it again.

When you consider that this is a negative feedback loop, operating on a timescale of a few ns, it's

surprisingly good.

RE: The Wilson Current Mirror

Jan Cumps — Mon, 01 Feb 2021 20:34:19 GMT

I've been digging in that too. Also inspired by a video of w2aew: Measure JFET drain current with a current mirror .

Great when you can experiment with just a few components. 1$ well spent.

RE: The Wilson Current Mirror

DAB — Mon, 01 Feb 2021 19:17:19 GMT

Nice post.

It takes me back to my tech school days when they had us build all of those basic circuits so that we would know how they worked.

DAB

RE: The Wilson Current Mirror

genebren — Mon, 01 Feb 2021 17:39:21 GMT

Nice demo/experiment. This is something I should look into for future designs.

Thanks Frank!

RE: The Wilson Current Mirror

navadeepganeshu — Mon, 01 Feb 2021 13:53:20 GMT

That's a cool experiment. Remembered this Super Beta - AC Voltage detector circuit by learnelectronics. Didn't work very well for me too though :[

RE: The Wilson Current Mirror

jc2048 — Mon, 01 Feb 2021 13:13:37 GMT

Good experiment.

RE: The Wilson Current Mirror

neilk — Mon, 01 Feb 2021 12:01:55 GMT

Thanks Frank, I enjoyed that.

It's been many, many years since I even picked up a transistor!

Neil

RE: The Wilson Current Mirror

Andrew J — Mon, 01 Feb 2021 11:41:15 GMT

That led me down the rabbit hole of W2AEW videos checking out current sources, op amp set ups and so on. They are good.