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Here is a more unconventional explanation in five steps:

We assemble a voltage divider...

Figuratively speaking, the voltage divider consists of two resistors "pulling" the midpoint (virtual ground) with equal force but in the opposite direction - R3 "pulls" it up and R4 "pulls" it down.

But the voltage divider is weak...

The problem is that the resistors "pull weakly" (because they have relatively high resistance) and when another device starts to "pull" on that point, it gives in and starts to "move".

The straightforward solution

is to decrease both R3 and R4 resistances... but then the voltage divider will consume a lot of current.

The ingenious solution

can be found if we begin looking for a more clever solution. Thus we arrive at the idea that it is not necessary for both resistors to be of low resistance; only the "opposite" resistor should have low resistance. For example, if some device is "pulling" the virtual ground up, R4 should have low resistance to "pull" hard it down and v.v., if the device is "pulling" the virtual ground down, R3 should have low resistance to "pull" hard it up.

That is how we get to the idea that the resistors should be dynamic.

We implement the idea

This idea is implemented by the op-amp output stage which is inherently such a "dynamic voltage divider" consisting of two transistors (the so-called "push-pull" or complementary stage). An NPN pull-up transistor acts as a "dynamic R3" and a PNP pull-down transistor - as a "dynamic R4". This transistor pair can be thought as a "low-resistance copy" of the R3-R4 voltage divider.

Here is a more unconventional explanation in five steps:

We assemble a voltage divider...

Figuratively speaking, the voltage divider consists of two resistors "pulling" the midpoint (virtual ground) with equal force but in the opposite direction - R3 "pulls" it up and R4 "pulls" it down.

But the voltage divider is weak...

The problem is that the resistors "pull weakly" (because they have relatively high resistance) and when another device starts to "pull" on that point, it gives in and starts to "move".

The straightforward solution

is to decrease both R3 and R4 resistances... but then the voltage divider will consume a lot of current.

The ingenious solution

can be found if we begin looking for a more clever solution. Thus we arrive at the idea that it is not necessary for both resistors to be of low resistance; only the "opposite" resistor should have low resistance. For example, if some device is "pulling" the virtual ground up, R4 should have low resistance to "pull" hard it down and v.v., if the device is "pulling" the virtual ground down, R3 should have low resistance to "pull" hard it up.

That is how we get to the idea that the resistors should be dynamic.

We implement the idea

This idea is implemented by the op-amp output stage which is inherently such a "dynamic voltage divider" consisting of two transistors (the so-called "push-pull" or complementary stage). An NPN pull-up transistor acts as a "dynamic R3" and a PNP pull-down transistor - as a "dynamic R4". This transistor pair can be thought as a "low-resistance copy" of the R3-R4 voltage divider.

Negative feedback point of view

The op-amp follower can be considered as a negative feedback system with constant input quantity (Vd/2 set by the R3-R4 voltage divider). Any device that injects/draws current into/from the op-amp output (virtual ground) is a disturbance that the op-amp tries to compensate for.

For example, if some device is "pulling" the virtual ground up, the lower PNP transistor "pulls" it down and v.v., if the device is "pulling" the virtual ground down, the upper NPN transistor "pulls" it up. As a result, the virtual ground does not "move".

Here is a more unconventional explanation in five steps:

We assemble a voltage divider...

Figuratively speaking, the voltage divider consists of two resistors "pulling" the midpoint (virtual ground) with equal force but in the opposite direction - R3 "pulls" it up and R4 "pulls" it down.

But the voltage divider is weak...

The problem is that the resistors "pull weakly" (because they have relatively high resistance) and when another device starts to "pull" on that point, it gives in and starts to "move".

The straightforward solution

is to decrease both R3 and R4 resistances... but then the voltage divider will consume a lot of current.

The ingenious solution

can be found if we begin looking for a more clever solution. Thus we arrive at the idea that it is not necessary for both resistors to be of low resistance; only the "opposite" resistor should have low resistance. For example, if some device is "pulling" the virtual ground up, R4 should have low resistance to "pull" hard it down and v.v., if the device is "pulling" the virtual ground down, R3 should have low resistance to "pull" hard it up.

That is how we get to the idea that the resistors should be dynamic.

We implement the idea

This idea is implemented by the op-amp output stage which is inherently such a "dynamic voltage divider" consisting of two transistors (the so-called "push-pull" or complementary stage). An NPN pull-up transistor acts as R3 and a PNP pull-down transistor - as R4. This transistor pair can be thought as a "low-resistance copy" of the R3-R4 voltage divider.

Here is a more unconventional explanation in five steps:

We assemble a voltage divider...

Figuratively speaking, the voltage divider consists of two resistors "pulling" the midpoint (virtual ground) with equal force but in the opposite direction - R3 "pulls" it up and R4 "pulls" it down.

But the voltage divider is weak...

The problem is that the resistors "pull weakly" (because they have relatively high resistance) and when another device starts to "pull" on that point, it gives in and starts to "move".

The straightforward solution

is to decrease both R3 and R4 resistances... but then the voltage divider will consume a lot of current.

The ingenious solution

can be found if we begin looking for a more clever solution. Thus we arrive at the idea that it is not necessary for both resistors to be of low resistance; only the "opposite" resistor should have low resistance. For example, if some device is "pulling" the virtual ground up, R4 should have low resistance to "pull" hard it down and v.v., if the device is "pulling" the virtual ground down, R3 should have low resistance to "pull" hard it up.

That is how we get to the idea that the resistors should be dynamic.

We implement the idea

This idea is implemented by the op-amp output stage which is inherently such a "dynamic voltage divider" consisting of two transistors (the so-called "push-pull" or complementary stage). An NPN pull-up transistor acts as a "dynamic R3" and a PNP pull-down transistor - as a "dynamic R4". This transistor pair can be thought as a "low-resistance copy" of the R3-R4 voltage divider.

Here is a more unconventional explanation in five steps:

We assemble a voltage divider...

Figuratively speaking, the voltage divider consists of two resistors "pulling" the midpoint (virtual ground) with equal force but in the opposite direction - R3 "pulls" it up and R4 "pulls" it down.

But the voltage divider is weak...

The problem is that the resistors "pull weakly" (because they have relatively high resistance) and when another device starts to "pull" on that point, it gives in and starts to "move".

The straightforward solution

is to decrease both R3 and R4 resistances... but then the voltage divider will consume a lot of current.

The ingenious solution

can be found if we begin looking for a more clever solution. Thus we arrive at the idea that it is not necessary for both resistors to be of low resistance; only the "opposite" resistor should have low resistance. For example, if some device is "pulling" the virtual ground up, R4 should have low resistance to "pull" hard it down and v.v., if the device is "pulling" the virtual ground down, R3 should have low resistance to "pull" hard it up.

That is how we get to the idea that the resistors should be dynamic.

We implement the idea

This idea is implemented by the op-amp output stage which is inherently such a "dynamic voltage divider" consisting of two transistors (the so-called "push-pull" or complementary stage). It can be thought as a "low-resistance copy" of the R3-R4 voltage divider.

Here is a more unconventional explanation in five steps:

We assemble a voltage divider...

Figuratively speaking, the voltage divider consists of two resistors "pulling" the midpoint (virtual ground) with equal force but in the opposite direction - R3 "pulls" it up and R4 "pulls" it down.

But the voltage divider is weak...

The problem is that the resistors "pull weakly" (because they have relatively high resistance) and when another device starts to "pull" on that point, it gives in and starts to "move".

The straightforward solution

is to decrease both R3 and R4 resistances... but then the voltage divider will consume a lot of current.

The ingenious solution

can be found if we begin looking for a more clever solution. Thus we arrive at the idea that it is not necessary for both resistors to be of low resistance; only the "opposite" resistor should have low resistance. For example, if some device is "pulling" the virtual ground up, R4 should have low resistance to "pull" hard it down and v.v., if the device is "pulling" the virtual ground down, R3 should have low resistance to "pull" hard it up.

That is how we get to the idea that the resistors should be dynamic.

We implement the idea

This idea is implemented by the op-amp output stage which is inherently such a "dynamic voltage divider" consisting of two transistors (the so-called "push-pull" or complementary stage). An NPN pull-up transistor acts as R3 and a PNP pull-down transistor - as R4. This transistor pair can be thought as a "low-resistance copy" of the R3-R4 voltage divider.