Clever op-amp circuits, but are they practical?

If you have read my first blog, you may have noticed that I really like op amps. My primary expertise lies in designing them, but I’m also intrigued by clever op-amp application circuits. To promote one of Touchstone’s op amps, I recently posted a video describing some unusual applications. The last one in that video connects the op amp as a three-terminal V-to-I. I didn’t think of this one myself; I got it years ago from my college professor Johan Huijsing.

The circuit, shown at left, works with any rail-to-rail op amp; but it works best if the quiescent current is very small compared with the largest current the op amp can sink and if there is a good ratio between the maximum and minimum supply voltage. That’s why I picked it to tout the TS1001; that op amp meets those criteria.

To understand the functionality, consider that if the op amp works as an op amp is supposed to, then the voltage at the two input terminals will be identical. If that’s the case, then a current equal to V_in /1k will flow through the resistor; that current is then apparently equal to the current coming out of the negative supply pin of the op amp, since no current is supposed to come out of the input pins. With the output and the positive supply pin shorted, that V_in /1k current then also flows into these two pins, from the terminal shown on top of the schematic.

So what you have is a compact V-to-I converter that works for currents down to the quiescent current of the op amp and up to the maximum current the amplifier can sink.

Now, you may think: Wait a minute, there is no feedback from the output to the negative input, so this circuit will not work.

The trick is that the negative supply pin in this case acts as a second, negative “output” terminal. That’s where the “output” current appears. The polarity is opposite to that of the normal output, so the feedback needs to be to the positive, rather than the negative, input.

Think about it: If the positive input of the op amp is made more positive, the (real) output wants to go up, resulting in less current through the output stage of the op amp. That lower supply current then causes the voltage on the resistor to go down, so this is indeed negative feedback.

At right is another interesting circuit. I don’t remember the name of a configuration like this, but basically, it allows you to transfer an impedance, such as a load or a sensor output, to a higher or more convenient value.

Once again, understanding the functionality starts with the realization that if an op-amp circuit uses negative feedback, the input terminals have the same voltage.

In this case, all op-amp inputs see V_in , the voltage at the input terminal. That then means that a current of V_in /Z5 flows through the bottom impedance, Z5.

That same current has to also flow through Z4, so now we know the voltage on the node between Z3 and Z4.

Working your way up, you eventually get the expression for the impedance that the input sees when it is loaded with the string Z1-Z5:

Note that if the current through Z1, Z4, and Z5 flows from top to bottom, then the current through Z3 and Z3 goes in the other direction, from the bottom to the top, and vice versa.

So, for instance, if Z5 is a stiff 100W load, and Z2 and Z4 are also 100W resistors while Z1 and Z3 are 1kW, then the input sees a much lighter load of 10k—100× larger than Z5, but still proportional to it.

It gets even more interesting if you assume that either Z2 or Z4 is a complex impedance. Now you can make large inductors out of small capacitors, but I will let you fiddle with that yourself. And of course, as far as bandwidth is concerned, all of this is only as good as the op amps you use.

Circuits like these are really clever, but they are a bit, shall we say, academic. Do they have any real-life application? Does anybody actually use them? I haven’t seen it, but I’m interested to hear from you.

Jeroen Fonderie, Vice President of Engineering, Touchstone Semiconductor Inc

Related Technical Paper:
The TS1001: An Operational Amplifier