Electronic – Does this circuit problem have a solution without an Op-Amp

circuit analysisoperational-amplifiervoltage divider

I was going through the definition of a voltage follower in this site

And a basic problem was presented, and an op-amp voltage follower as the solution.

Can this problem be solved without an op-amp ?

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Best Answer

Oh, geez. I see a diode/BJT solution and a MOSFET solution.

No one did the obvious BJT-only solutions.

So I might as well add those too, now:


simulate this circuit – Schematic created using CircuitLab

Here, I'm starting with a PNP follower and cascading it with an NPN follower (on the left.) Or, cascading an NPN follower by a PNP follower (on the right.) Either way, if you set things up so that \$R_1\approx R_2\$ then the collector currents will be similar and the \$V_{BE}\$ values therefore also similar. (It can be adjusted easily, of course, to tweak it in better.)

It's an okay way to cancel the \$V_{BE}\$ offset. And does the work of your opamp without the use of an opamp (which would be better to use because the opamp would have gigaohms of input impedance and active sink and source at the output.)

Put a resistor divider at the input, if you want.

How did this idea get missed? I don't know.