Op-amp function blocks: An exponential (anti-log) amplifier configuration uses which component placement relative to the operational amplifier?
-
AA diode in the feedback path (with input through a resistor)
-
BA diode at the input and a resistor in feedback
-
COnly resistors in a non-inverting configuration
-
DA zener diode in the feedback to clip the output
-
EA capacitor in feedback to integrate the input
Answer
Correct Answer: A diode at the input and a resistor in feedback
Explanation
Introduction / Context:Log and antilog (exponential) amplifier building blocks are classic op-amp applications that exploit the exponential I–V characteristic of a diode or transistor junction. Distinguishing between the two is frequently tested in analog electronics.
Given Data / Assumptions:
- Ideal op-amp behavior around its linear region.
- Use of a diode (or BJT junction) obeying i = I_s * exp(v/(nV_T)).
Concept / Approach:
Log amplifier: place the diode/junction in the feedback path and drive the input through a resistor; output is proportional to ln of the input magnitude. Antilog (exponential) amplifier: place the junction at the input (in series from source to the summing node) and keep a resistor in the feedback path; the output then becomes proportional to the exponential of the input voltage.
Step-by-Step Solution:
Recognize that the prompt asks about an exponential amplifier (antilog).For antilog behavior, the nonlinear element must translate an input voltage into an exponential current into the summing node.Use a linear feedback resistor to convert that current into a proportional output voltage.Verification / Alternative check:
Standard analog design texts show: log amplifier → diode in feedback; antilog amplifier → diode at input. This matches option (b).
Why Other Options Are Wrong:
- (a) describes a log amplifier, not exponential.
- (c), (d), and (e) do not produce exponential transfer characteristics.
Common Pitfalls:
- Confusing log and antilog configurations.
- Ignoring temperature compensation requirements for precision designs.
Final Answer:
A diode at the input and a resistor in feedback.