Question

Which of the following is correct with reference to the given diode limiter circuit?

• A. \[ -V_{R2}<v_i<V_{R1} \Rightarrow D_1 \text{ OFF},\; D_2 \text{ ON} \Rightarrow v_o=V_{R1} \]
• B. \[ v_i<V_{R1} \Rightarrow D_1 \text{ ON},\; D_2 \text{ OFF} \Rightarrow v_o=V_{R1} \]
• C. \[ v_i<V_{R2} \Rightarrow D_1 \text{ OFF},\; D_2 \text{ OFF} \Rightarrow v_o=v_i \]
• D. \[ -V_{R2}>v_i \Rightarrow D_1 \text{ OFF},\; D_2 \text{ ON} \Rightarrow v_o=-V_{R2} \]

Hint:

For a biased clipper: \[ v_i>V_R \Rightarrow \text{Positive clipping} \] \[ v_i<-V_R \Rightarrow \text{Negative clipping} \] The conducting diode clamps the output to the corresponding reference voltage.

Answer 1

The Correct Option is D

Concept: The given circuit is a two-level biased clipper (limiter).
• Diode \(D_1\) limits the positive excursion of the output.
• Diode \(D_2\) limits the negative excursion of the output. Whenever the input tries to exceed the preset clipping level, the corresponding diode conducts and clamps the output voltage.

Step 1: Examine the operation for moderate input voltages.
When \[ -V_{R2}<v_i<V_{R1}, \] both diodes remain reverse biased. Hence neither diode conducts. Therefore, \[ v_o=v_i. \] Thus option (A) cannot be correct because it states that \(D_2\) is ON and \(v_o=V_{R1}\).

Step 2: Consider large positive input voltage.
When \[ v_i>V_{R1}, \] diode \(D_1\) becomes forward biased. The output gets clamped at \[ v_o \approx V_{R1}. \] At this time \(D_2\) remains OFF.

Step 3: Consider large negative input voltage.
When \[ v_i<-V_{R2}, \] the output attempts to become more negative than the reference level. Diode \(D_2\) becomes forward biased and conducts. Hence the output is prevented from decreasing further and is clamped at \[ v_o=-V_{R2}. \] During this condition, \[ D_1 \text{ is OFF} \] and \[ D_2 \text{ is ON}. \]

Step 4: Match with the given options.
The correct operating condition is \[ -V_{R2}>v_i \] \[ D_1 \text{ OFF} \] \[ D_2 \text{ ON} \] \[ v_o=-V_{R2}. \] Therefore, \[ \boxed{ -V_{R2}>v_i \Rightarrow D_1 \text{ OFF}, D_2 \text{ ON} \Rightarrow v_o=-V_{R2} } \] which corresponds to option (D).