Talk:Circuit Idea/Building an Op-amp Follower

Here is the building "scenario" that my students (groups 65a, 66a and 67a) and I were using during laboratory excercises to consider the op-amp non-inverting amplifier. Circuit-fantasist (talk) 20:03, 19 April 2008 (UTC)

Lab 4: Op-amp circuits with series negative feedback
Tuesday, April 15, 2008, 13.45 h

What is op-amp?


Inputs. Grounded voltage source... What is single-ended input? Floating voltage source... What is differential input? Why do we need a differential input? Input resistance... Do input currents flow? How does a non-inverting input behave? How does an inverting input behave? Can we use an op-amp with a differential input as an op-amp with single-ended (inverting or non-inverting) input? What do we do in these cases?

Output. How the op-amp output is made? What is the idea of a complementary, push-pull emitter follower? Output resistance...

Supplying. The idea of bipolar (split) supply (+12 V and -12 V here)... Why do we need a bipolar supply? Where currents flow? We can imagine the output part of the op-amp consists of two parts - a positive and a negative. Each of them works when its output voltage has the according polarity. By the way, is it possible to supply the op-amp with only one (e.g., positive) voltage and, at the same time, to obtain the opposite (negative) output voltage? And can the op-amp output voltage exceed the supply voltage? Maybe, it would be possible, if there were capacitive or inductive differentiating circuits inside the op-amp... But (for now) an op-amp contains only resistive and active components:(

Gain. Typical value is A > 200000. It seems too high. Can we manage to utilize all this gain? What is the maximum input voltage to keep the op-amp in the active region (assume 10 V maximum output voltage)? Can we use a bare op-amp as an amplifier? Or we can use it as another useful device having analog inputs and descrete output (write its name here)? We may conclude: If we use the op-amp as an amplifier, the voltage difference between its inputs is zero. But if we use it as a comparator, the input voltage might be whatever.



Transfer characteristic. Microlab can help us to obtain this curve on the screen. There is no time in this graphical presentation; there is only input and output quantities. We might discern three regions on this curve: negative saturation (large), active region (narrow) and positive saturation (large).

Generalization. What does the op-amp actually do? A possible answer is: an op-amp converts the steady power supply into varying voltage source; it acts as a voltage-controlled voltage source.



Analogies


Driving: keeping up a constant speed, direction, temperature, volume... More negative feedback analogies: studying, teaching, keeping up a room temperature (body weight, mental equilibrium...) In all these cases we do all that is possible to realize our goals, to move toward the goal...

Structure (block diagram)
So, the simplest negative feedback follower consists of only three components: power (energy) source E, regulating element R and a subtractor (-).

Operation
This humble device operates according to a simple algorithm: if X > Y, then increase Y; if X < Y, decrease Y; if X = Y, do nothing. As a result, always X = Y. So, this is an "active copying" principle and this device is an "active follower". This "algorithm" is so simple that humble devices (tubes, transistors, op-amps...) having no any intelect can perform it.

Advantages
Why do we make followers in this odd way? What are the advantages of this "negative feedback" approach?



Passive electric follower


Now, we have only to convey this powerful idea from real life to electronics. Well, let's begin.

VIN will represent X in our electric follower; accordingly, VOUT will represent Y. But let's first persuade ourselves what is the use of building such an active follower instead a "passive" one.

What is the simplest voltage follower? Of course, a piece of wire can act as a humle voltage follower. It works perfectly, if there is no load (there is no current flowing). Only, is it still a follower, if we connect a big load at the end of the wire (especially, if it is a thin and long wire)? And imagine what happens, if the load and wire resistances vary... For example, the wire length can vary...

Another nasty problem of the passive voltage follower is that the load consumes energy from the input voltage source.; so, it droops when loaded.



Op-amp negative feedback follower


No, the bare wire is not a perfect follower in all the cases; so, it is worth the trouble to build an active negative feedback follower...

First, we connect a power supply - a voltage source V representing the power source E from the block diagram. We assume a bipolar input voltage; so, we get a bipolar (split) power supply. Then, we connect an op-amp OA acting as the regulating element R. Finally, we need a voltage subtractor to make the op-amp compare its output voltage VOUT with the input voltage VIN. What is the simplest voltage subtractor? Of course, this is the bare loop (a piece of wire) according to Kirchhoff's voltage law (KVL). In other words, we have to connect the two voltage sources in series and in opposite directions (+ -, - +) traversing the loop, in order to subtract their voltages. Then, we have to cut the loop and to feed the op-amp input by the voltage difference.



Operation


We are human beings, not computers... It seems strange but, in order to understand what the abstract op-amp really does in this circuit, we have to think of the op-amp as a "slow-thinking" device, not as a fast, non-inertial, instant device... From this human viewpoint, the op-amp compares continuously its output voltage VOUT with the input voltage VIN and changes VOUT in the right direction so that to keep almost zero difference VOUT - VIN = 0. It does exactly the same what we, human beings, were doing in all the analogies above.

Where do currents flow in this circuit? Draw them with whole loops in the two cases - at a positive and a negative input voltage. Remember: a current always arrives at the point from where it has gone. Another tip: in the two cases (at positive and negative input voltage) the load current will flow through different power supplies (accordingly, through the positive and negative voltage sources). Imagine also the complementary output emitter follower inside the op-amp: the upper n-p-n transistor will "blows" the output current when the op-amp "wants" to produce a positive voltage and the lower p-n-p transistor will "sucks" the output current when the op-amp "wants" to produce a negative voltage.



Resources
Labs: group 65a, group 66a and group 67a. Op-amp amplifiers with negative feedback

<<< contents - Group 64a - Group 65a - Group 67a - Group 68a - page stage >>>

<<< Group 64b - Group 65b - Group 66b - Group 67b - Group 68b >>>