0
\$\begingroup\$

simple model of a diff opamp with an independent differential signal source

figure 1 enter image description here

how does current flow through the transistors since V1 and V2 and referenced to each other and not to vee? i understand how the circuit is supposed to work for example in a case where V1 and V2 are each referenced to vee, but need some help to understand figure 1 where there is no reference of the differential ac source to the opamp vee rail.

figure 2 enter image description here

\$\endgroup\$
0
\$\begingroup\$

It's an over-simplified model for the purposes of your question.

Yes, each transistor needs an input bias current, and there needs to be a path through the external circuit for that current. So the differential voltage source can't be truly floating with respect to the power supply.

Some opamps have extremely low input bias currents (a few nanoamps or even less), and a very large resistor (10s of megohms or more) is sufficient to supply the bias current.

\$\endgroup\$
0
\$\begingroup\$

Consider that Diff. Amps and Op Amps usually have constant current sources for 1st stage loads and thus some hFE requires a tiny input DC bias current and thus implied high input impedance.

Consider the ultra low input offset OP07 Op Amp, and read all the specs. Note that the typ. Common Mode (CM) Zin is 33 GOhm and DM Zin is 8 MOhm.

The CM defines the non-floating input currents that you were concerned about.

Also note the valid Vcm input range must be offset from rails. While other designs using PNP inputs can sense down to the bottom rail are good for single supply with inputs near 0V.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.