# Unusual opamp topology: driving a dummy load and using supply currents to drive common-source

Instead of having the output of the op-amp drive some subsequent stages, it uses a dummy resistive load, supply currents to the opamp are cascoded and drive the subsequent stages in a push-pull fashion. The real load is the 4.7u cap. Is this op-amp topology common? Does it have a name? What are the advantages/disadvantages of doing this?

The other question was about the feedback path specifically. This one is about the topology of using supply currents to drive amplification stages. Yes it's the same circuit, but completely different questions.

• Possible duplicate of Understanding opamp feedback loop with parallel paths – Warren Hill Nov 8 '17 at 18:34
• Can you show as full circuit diagram? – G36 Nov 8 '17 at 18:35
• You have already asked this question today. Please don't ask the same question twice. – Warren Hill Nov 8 '17 at 18:35
• Really? It's not the same question at all. The other question was about the feedback. This one is about using the supply currents to drive next stages. – Tyler Lungrin Nov 8 '17 at 18:36
• @WarrenHill - He's asking about the same circuit, but it's certainly not the same question. – brhans Nov 8 '17 at 18:37

In this circuit, the Opamp output stage (push-pull) drives the rest of a circuit.

A Very old trick to increase the opamp output current capability and supply voltage can be much larger than typical +/-15V value.

Also, as you can see we have two feedback loops.

Inside loop (inner loop) $R_8$ and $R_9$ because of the MOSFET's work here as a common source amplifier. And this inner loop is a typical example of current feedback amplifier in use. https://en.wikipedia.org/wiki/Current-feedback_operational_amplifier

And the global feedback loop (outer loop) $R_{10}$, $R_1$. And this outer loop "set" the closed loop gain (around 40V/V). $R_7$ and $C_1$are here for frequency compensation. Limits the inner loop gain at HF.

And the simplified schematic (but not quite equivalent) will look something like this

Where U1 is AD8297 opamp and U2 is AD8297 output stage + the rest of transistors.

• I first saw this technique in the book "Op Amp Cookbook" by Walt Jung. I successfully used this technique in production many years ago. – Dwayne Reid Dec 10 '17 at 13:23
• When you talk about R8, R9, R10, R1, I'm not clear if you're talking about OP's schematic, your first schematic, or your second schematic. – The Photon Nov 1 '18 at 17:11
• @ThePhoton My second schematic and OP's schematic. – G36 Nov 1 '18 at 17:16

Sometimes called bootstrapping.