Clippers and Limiters: A Guide to Signal Shaping Circuits

Rectifier Diodes
- Used in low-frequency power supplies
- Optimized for 60 Hz
- Power rating: \(>\) 0.5 W
- Forward current rating: Amperes
- Limited use outside power supplies
Small-Signal Diodes
- Optimized for high frequencies
- Power rating: \(<\) 0.5 W
- Current rating: Milliamperes
- Smaller and lighter construction

The Positive Clipper
- Removes positive parts of waveform
- Useful for signal shaping, protection, communications
- Positive clipper circuit
- During positive half-cycle: Diode conducts, output voltage \(\approx\) 0 V
- During negative half-cycle: Diode is open, negative half-cycle appears at output
- Series resistor much smaller than load resistor
- Output peak: \(2V_p\) (Fig.(a))
- Diode voltage when conducting: 0.7 V
- Clipping level: 0.7 V, not zero
- Example: Input signal peak of 20 V results in output

The Negative Clipper
- Reversed Diode Polarity
- Removes negative parts of the signal
- Output waveform ideally has only positive half-cycles
- Imperfect Clipping: Due to diode offset voltage (barrier potential)
- Clipping level: \(20.7\) V
- For input signal peak of \(20\) V, output signal shown in Fig.(b)

SECTION 01

The Limiter or Diode Clamp

Normal Operation:
- Input: Peak at 15 mV
- Output: Same signal (Diodes off)
Purpose of the Circuit:
- Protection: Limits input signal to protect sensitive circuits
- Positive-Negative Limiter:
  - Limits output to 0.7 V if input exceeds 0.7 V
  - Limits output to -0.7 V if input drops below -0.7 V

Examples of Sensitive Circuits:
- Op Amp:
  - Normal input \(<\) 15 mV
  - Voltages \(>\) 0.7 V are abnormal
  - Limiter prevents excessive voltage
- Moving-Coil Meter:
  - Limiter protects against excessive voltage/current
Diode Clamp:
- Function: Clamps/limits voltage to a specified range
- Operation: Diodes conduct only during abnormal conditions (signal too large)

SECTION 02

Reference Level:
- Positive Clipper: Ideally 0 V or 0.7 V
Changing Reference Level:
- Biasing: Apply external voltage to shift reference level
- Positive Biased Clipper:
  - Add a DC voltage source in series with the diode
  - New reference voltage (\(V\)) must be less than peak voltage (\(V_p\))
  - Ideal Diode: Conduction starts when input \(> V\)
  - Second Approximation: Conduction starts when input \(> V + 0.7\) V
- Negative Biased Clipper:
  - Reverse diode and battery
  - Reference level changes to \(-V - 0.7\) V
  - Output waveform negatively clipped at the bias level

SECTION 03

Configuration:
- Diode \(D_1\): Clips positive parts above the positive bias level
- Diode \(D_2\): Clips negative parts below the negative bias level
Output Signal:
- When input voltage is much larger than bias levels
- Output is a square wave
Signal Shaping:
- Clippers enable shaping signals into specific waveforms

SECTION 04

Using Silicon Diodes:
- Each diode produces a bias of 0.7 V
- Example: Three diodes produce a clipping level of \(\approx 2.1\) V
- Application: Can act as a diode clamp (limiting) to protect sensitive circuits
Using a Voltage Divider:
- Configuration: \(R_1\) and \(R_2\) set the bias level
- Bias Level:
- Output is clipped when input \(> V_{\text{bias}} + 0.7\) V

\[V_{\text{bias}} = \frac{R_2}{R_1+R_2}V_{\text{dc}}\]

Biased Diode Clamp:
- Example: Bias level set to +5 V
- Protects against large input voltages (e.g., +100 V)
- Output limited to +5.7 V
Removing Offset:
- Variation: Bias diode \(D_2\) into slight forward conduction
- This puts diode \(D_1\) on the verge of conduction
- Result: Diode \(D_1\) conducts near 0 V, minimizing offset