Negative Feedback OpAmp Circuits

The purpose of this circuit is to provide a pushbutton-adjustable voltage. Pressing one button causes the output voltage to increase, while pressing the other button causes the output voltage to decrease. When neither button is pressed, the voltage remains stable:

After working just fine for quite a long while, the circuit suddenly fails: now it only outputs zero volts DC all the time.

An experienced technician first checks the power supply voltage to see if it is within normal limits, and it is. Then, the technician checks the voltage across the capacitor. Explain why this is a good test point to check, and what the results of that check would tell the technician about the nature of the fault.

A student builds the following regulated AC-DC power supply circuit, but is dissatisfied with its performance:

The voltage regulation is not as good as the student hoped. When loaded, the output voltage “sags” more than the student wants. When the zener diode’s voltage is measured under the same conditions (unloaded output, versus loaded output), its voltage is noted to sag a bit as well. The student realizes that part of the problem here is loading of the zener diode through the transistor. In an effort to improve the voltage regulation of this circuit, the student inserts an opamp “voltage follower” circuit between the zener diode and the transistor:

Now the zener diode is effectively isolated from the loading effects of the transistor, and by extension from the output load as well. The opamp simply takes the zener’s voltage and reproduces it at the transistor base, delivering as much current to the transistor as necessary without imposing any additional load on the zener diode.

This modification does indeed improve the circuit’s ability to hold a steady output voltage under changing load conditions, but there is still room for improvement. Another student looks at the modified circuit, and suggests one small change that dramatically improves the voltage regulation:

Now the output voltage holds steady at the zener diode’s voltage with almost no “sag” under load! The second student is pleased with the success, but the first student does not understand why this version of the circuit functions any better than previous version. How would you explain this circuit’s improved performance to the first student? How is an understanding of negative feedback essential to being able to comprehend the operation of this circuit?

Predict how the operation of this regulated power supply circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):

•

Transformer T₁ primary winding fails open:

•

Rectifying diode D₃ fails open:

•

Rectifying diode D₄ fails shorted:

•

Resistor R₁ fails open:

•

Zener diode D₅ fails open:

•

Operational amplifier U₁ fails with output saturated positive:

•

Transistor Q₁ fails open (collector-to-emitter):

For each of these conditions, explain why the resulting effects will occur.