Clipping Circuit Using a PN Junction Diode

Experiment: Clipping Circuit Using a PN Junction Diode

1. Aim

To study the action of a diode clipping circuit — a circuit that removes (clips) the portion of an AC waveform that exceeds a set voltage level — and to observe how the clipping level shifts when the DC bias on the diode is varied using the programmable source PV1.

2. Apparatus / Components Required

• SEELab3 unit
• PN junction diode (1N4148)
• Resistor $R = 1\text{ k}\Omega$
• Connecting wires
• PC or Smartphone with SEELab3 software

3. Theory & Principle

3.1 Clipping Action

A clipper (or limiter) circuit passes the input signal unchanged as long as it stays within a defined voltage window, and flattens (clips) it to a fixed level once it crosses that threshold.

In the circuit here, $R$ and the diode form a series path from WG to PV1. Channel A2 monitors the voltage at the junction between $R$ and the diode anode — i.e., the output node.

When the input swings positive and the output node voltage tries to rise above $V_{clip}$, the diode becomes forward biased and clamps the output to:

Excess current is diverted through the diode into PV1 rather than appearing across the output. Below $V_{clip}$ the diode is reverse biased and the output follows the input (reduced slightly by the $R$–load divider).

Setting PV1 = 0 V gives $V_{clip} \approx +0.6\text{ V}$ (one diode drop above ground).

Raising PV1 raises the clipping level proportionally — the diode does not conduct until the output reaches $V_{PV1} + V_f$.

3.2 Negative Clipping

Reversing the diode direction flips the circuit so the diode conducts on the negative half-cycle instead. The clipping level becomes:

Applying a negative voltage to PV1 with the reversed diode clips the negative half at a programmable level below zero.

4. Circuit Diagram / Setup

1. Connect WG to one end of $R$ ($1\text{ k}\Omega$).
2. Connect the other end of $R$ to the anode of the diode. This junction is the output node — connect it to A2.
3. Connect the cathode of the diode to PV1.
4. Connect WG also to A1 to monitor the input.

For negative clipping: reverse the diode (cathode to $R$, anode to PV1) and set PV1 to a negative value.

5. Procedure

Part A — Positive clipping, variable level

1. Open the SEELab3 app. Set WG to a sinusoidal signal at $f = 1000\text{ Hz}$, amplitude $\approx 3\text{ V}$ peak.
2. Set PV1 = 0 V. Click “Start” and observe A1 (input) and A2 (output).
   • The positive peaks of A2 should be clipped flat at $\approx +0.6\text{ V}$; the negative half should follow the input.
3. Increase PV1 in steps: 0.5 V, 1.0 V, 1.5 V, 2.0 V. At each step note the clipping level on A2 and confirm it tracks $V_{PV1} + 0.6\text{ V}$.
4. Set PV1 high enough (e.g., $> 3\text{ V}$) that clipping disappears entirely — the output should now follow the full input waveform.

Part B — Negative clipping

1. Reverse the diode in the breadboard (cathode to $R$, anode to PV1).
2. Set PV1 to 0 V and observe — the negative peaks are now clipped at $\approx -0.6\text{ V}$.
3. Set PV1 to −0.5 V, −1.0 V, −1.5 V (negative values) and record the clipping level each time.

Mobile App

Desktop App

6. Observation Table

6a. Part A — Positive Clipping

6b. Part B — Negative Clipping (diode reversed)

7. Results and Discussion

• With PV1 = 0 V and the diode in the forward orientation, the positive peaks were clipped at ____ V, agreeing with the expected $V_f \approx 0.6\text{ V}$.
• As PV1 was increased, the clipping level rose by the same amount, confirming $V_{clip} = V_{PV1} + V_f$.
• With the diode reversed and PV1 set to negative values, the negative half was clipped at programmable levels below zero, verifying $V_{clip} = V_{PV1} - V_f$.
• The unclipped half of the waveform reproduced the input faithfully, confirming that the diode is fully off (reverse biased) during that half-cycle.

8. Precautions

1. PV1 polarity: SEELab3’s PV1 may not support negative voltages on all firmware versions. Check the specification before attempting Part B; if unavailable, Part B can be demonstrated with PV1 = 0 V and diode reversed (clips at $-V_f$).
2. Resistor is essential: The series resistor $R$ limits the current through the diode when it clamps. Never omit it — a direct WG-to-diode connection with no $R$ will draw excessive current from the WG output stage.
3. Input amplitude: Keep $V_{in,\text{peak}} \leq 3\text{ V}$ to stay within the safe input range of A1 and A2.
4. Diode orientation check: Before powering on, confirm the diode orientation with a multimeter in diode-test mode — it is easy to inadvertently reverse the diode when switching between Part A and Part B.

9. Troubleshooting