Electronic – Op-amp inverter followed by buffer. Why

analogbuffercircuit analysisoperational-amplifier

In a schematic I've been trying to understand I came across this sub-circuit:
inverter-followed-by-buffer

It's an op-amp inverter directly followed by a buffer. VIN comes from a DAC in a microcontroller and this circuit produces a VOUT which is negative VIN. The op-amp is supplied by positive and negative rails (not shown here). So far so good.

But I don't fully see the rationale of using OA2 in this circuit. The only reason I can see is this: Without the buffer (OA2) a sudden load at VOUT would draw a current from VIN until the op-amp OA1 feedback adjusts (about 1µs). With the buffer (OA2) this is not the case anymore. Am I getting this right? Or am I missing something?

Best Answer

You are right. In most cases this is silly, adds offset voltage, and uses another part. Most likely this is just someone's knee jerk reaction, or blindly following a rule of "always buffer the signal" without thinking about it too hard. Not all schematics out there are the result of good design.

There are some subtle advantages to the second buffer-only opamp:

  1. The feedback current thru R2 eats into the total output current capability of OA1. OA2 has all of its current capability available to drive the output.

    In this case with R2 being 10 kΩ, this is a weak argument since the feedback current is so small relative to the capability of most opamps. Sometimes a circuit like this happens because R2 was much lower before, and the second opamp wasn't removed after a design change that raised R2.

  2. OA2 protects the input signal from abuse of the output signal. Vin sees the fixed impedance of R1 only as long as OA1 is acting in closed loop operation. If something loads Vout so that OA1 can't drive it to the desired voltage, then the negative input of OA1 is no longer at 0 V, and the Thevenin equivalent that Vin is driving changes.

    In this circuit, the output of OA2 can be abused without affecting the output of OA1, which in turn won't affect Vin, maybe. The reason I say "maybe" is that some opamps have back to back diodes between their inputs. I didn't look up your opamp, so I don't know whether that is the case here. If so, then abuse of Vout will get back to the positive input of OA2, which will get back to Vin.

    This is again a weak argument since loading a opamp output to the point where it can't drive to the desired voltage is generally running the opamp out of spec.

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