Configure a current-mode op amp

Article By : John Dunn

Learn the governing equations for the other kind of op amp.

A current-mode operational amplifier (op amp) is configured quite differently from the garden variety op amps we usually use. That configuration and its algebraic circuit analysis will be covered here. Yes, the algebra is complex, but you’ll find it worth the effort of going through it anyway.

Analysis begins with two quick calculations for Ex in terms of E1, E2, and Eo. From there, the two expressions for Ex are taken as equal to each other and the rest is algebraic derivation.

current mode op amp circuit analysisHere is how you begin the circuit analysis.

At this point, just as a sanity check, we can look at what the stage gain from E2 to Eo will be for E1 at ground.

check circuit analysis
Check the analysis done thus far.

This looks okay so we move on as follows.

current mode op amp circuit analysisThis is the next step in the circuit analysis.

At this point, we have a computable form for Eo versus the two inputs E1 and E2. Please note that we can vary our stage gains, inverting and non-inverting, by choosing our R3 and R4 values, but which we choose to make the variable resistance is not an arbitrary decision. If we make R4 our variable to set some specific gain value, we get non-inverting and inverting results as follows.

non-inverting gains
This graph shows the non-inverting gains versus the changes of R4.

inverting gains
This graph shows the inverting gains versus the changes of R4.

The gain bandwidth product tends to be constant, just like we get with ordinary voltage mode op amps. However, if we instead make R3 our variable, we get very different kinds of results.

R3 non-inverting gains
This graph shows the non-inverting gains versus the changes of R3.

R3 inverting gains
This graph shows the inverting gains versus the changes of R3.

Here, the gain bandwidth product is clearly not a constant, but the corner frequency at each gain setting tends to be constant. If bandwidth preservation is important at variable gains, these kind of frequency responses may be valuable.

Just for the sense of certainty of all of this, the forgoing algebraic results were confirmed in Multisim SPICE.

SPICE confirmation
SPICE confirms the analysis results.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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