Analog Integrated Circuits

Precise Diode Circuits

14 questions By Tony R. Kuphaldt

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Question 4 of 14

Determine the output voltage of this circuit for two different input voltage values: 5 volts, and -5 volts, assuming the use of ordinary silicon rectifying diodes:

Based on this data (and any other input conditions you wish to test this circuit under), describe what the function of this circuit is.

Reveal answer

When V_in = 5 volts, V_out = -5 volts

When V_in = -5 volts, V_out = 0 volts

This circuit is a precision rectifier.

Notes:
Work with your students to analyze the behavior of this circuit, using Ohm’s Law and the basic principle of negative feedback (zero differential input voltage). Ask your students whether or not it matters what types of diodes are used (silicon versus germanium versus light-emitting).
Question 5 of 14

This opamp circuit is called a precision rectifier. Analyze its output voltage as the input voltage smoothly increases from -5 volts to 5 volts, and explain why the circuit is worthy of its name:

Assume that both diodes in this circuit are silicon switching diodes, with a nominal forward voltage drop of 0.7 volts.

Reveal answer

Any positive input voltage, no matter how small, is “reflected” on the output as a negative voltage of equal (absolute) magnitude. The output of this circuit remains exactly at 0 volts for any negative input voltage.

Follow-up question: would it affect the output voltage if the forward voltage drop of either diode increased? Explain why or why not.

Notes:
Precision rectifier circuits tend to be more difficult for students to comprehend than non-rectifying inverting or non-inverting amplifier circuits. Spend time analyzing this circuit together in class with your students, asking them to determine the magnitudes of all voltages in the circuit (and directions of current) for given input voltage conditions.

Understanding whether or not changes in diode forward voltage drop affect a precision rectifier circuit’s function is fundamental. If students comprehend nothing else about this circuit, it is the relationship between diode voltage drop and input/output transfer characteristics.
Question 6 of 14

Explain why the following opamp circuit cannot be used as a rectifier in an AC-DC power supply circuit:

Reveal answer

Here’s a hint: where does the opamp get its power from?

Notes:
Believe it or not, I actually sat in an electronics class one time and listened to an instructor present the precision rectifier opamp circuit as a “precision rectifier for a power supply”. He was serious, too, claiming that this type of circuitry was used to provide split ( V/-V) voltage outputs for bench top power supplies. The saddest part of this ordeal is that none of his students recognized anything wrong with his statement (or at least did not feel comfortable in raising a question about it).