Electronic Circuits and Oscillators

Posted on Jun 4, 2024 in Electronics

Crossover Distortion in Class B Amplifiers

In Class B push-pull amplifiers, one transistor turns on while the other turns off. However, a small delay occurs during the transition when both transistors are off. This delay causes crossover distortion, a flat spot in the output waveform around the zero-crossover point.

To minimize crossover distortion:

Pre-biasing: This ensures each transistor starts conducting when its base-to-emitter voltage rises slightly above zero.
Class AB Amplifiers: Both transistors are biased slightly above their cut-off points, ensuring one transistor always conducts a small amount, even when the input signal crosses zero.

Hartley Oscillator

The Hartley oscillator is a type of LC oscillator generating undamped sinusoidal oscillations. Its key component is the tank circuit, consisting of two inductors (L₁ and L₂) and a capacitor (C).

These inductors are connected in series and form a parallel combination with the capacitor. The tank circuit determines the oscillation frequency.

When power is applied:

The transistor allows current to flow.
The capacitor in the tank circuit starts charging.
Once fully charged, the capacitor discharges through inductors L₁ and L₂.

This continuous charging and discharging process creates sinusoidal oscillations at the output.

Transistor Biasing

Transistor biasing is crucial for proper transistor operation. It involves applying external DC voltages to the base-emitter and collector-base junctions.

The primary purpose of biasing is to maintain correct operating conditions for the transistor during signal passage. Proper biasing ensures the transistor operates in the desired region (cut-off, active, or saturation) and delivers the expected performance.

Miller’s Theorem

Miller’s theorem states that any linear voltage gain device with an impedance (Z) connected between input and output can be replaced by two separate impedances:

Impedance (Z₁) from input to ground.
Impedance (Z₂) from output to ground.

These impedances are calculated as follows:

Z₁ = Z / (1 - A_v)

Z₂ = Z / (1 - (1 / A_v))

Where (A_v) represents the device’s voltage gain from input to output.

Transformer-Coupled Class A Power Amplifier

A transformer-coupled Class A power amplifier uses a transformer for impedance matching and signal coupling between its output stage and the load. In Class A operation, the amplifier conducts over the entire 360 degrees of the input signal cycle, meaning the output device (transistor or tube) constantly conducts current. This results in high linearity and low distortion but low efficiency (~25-30%).

Transformer coupling connects the amplifier’s output to the load via a transformer. The primary winding connects to the amplifier’s output, and the secondary winding connects to the load. This setup provides impedance matching for efficient power transfer to the load.

Amplification Process

The input signal enters the amplifier’s input stage.
The active device in the power amplification stage amplifies the input signal, remaining in the active region throughout the entire input signal cycle.
The amplified signal goes to the primary winding of the output transformer.
The transformer converts the high-voltage, low-current signal in the primary winding to a low-voltage, high-current signal in the secondary winding. This impedance matching maximizes power transfer and minimizes power loss.
The secondary winding delivers the amplified signal to the load, with the transformer’s turns ratio determining the voltage and current levels delivered.

Advantages

Improved power transfer efficiency
Isolation protecting the amplifier from potential issues
Slightly improved overall efficiency through better impedance matching

Disadvantages

Large size and weight of transformers
High cost of high-quality transformers
Potential bandwidth limitations introduced by transformers

Summary

Transformer-coupled Class A power amplifiers use transformers to match impedance between the amplifier and the load, improving efficiency and power transfer. They are valued for their linearity and low distortion in high-fidelity audio applications.

Factors Affecting Output Voltage Variation in a Regulator

Several factors can cause output voltage variations in a voltage regulator:

Input Voltage Variations: Changes in input voltage can cause output variations. Line regulation measures this effect.
Load Variations: Changes in load current affect output voltage. Load regulation measures the regulator’s ability to maintain voltage with varying loads.
Temperature Changes: Temperature affects component performance, potentially altering output voltage. Thermal shutdown can also cause temporary drops in output.
Component Tolerances: Variations in resistor and capacitor values impact voltage precision and stability.
Power Supply Noise: Poor ripple rejection can introduce fluctuations from AC components in the input.
Regulator Design: The stability of the internal reference voltage and feedback loop design are critical for maintaining consistent output.
External Interference: EMI and RFI can induce noise, affecting output voltage.
Aging and Degradation: Component aging and wear over time can degrade performance and cause voltage variations.

Understanding and mitigating these factors can improve voltage regulator performance.

Crystal Oscillator Working Principle

A crystal oscillator leverages the piezoelectric effect in a quartz crystal to generate a stable and precise electrical signal.

The crystal, typically quartz, is placed within an oscillator circuit, forming a feedback loop with an amplifier.
When power is applied, the crystal resonates at its natural frequency due to the piezoelectric effect.
The amplifier amplifies this oscillation and feeds it back to the crystal.
The feedback loop ensures the crystal continues oscillating at its resonant frequency, resulting in a clean and stable output signal.

Key components include the quartz crystal, amplifier, and feedback network. Crystal oscillators offer high stability, precision, and low phase noise, making them essential in applications like clocks, communication devices, and microprocessors.

Wien Bridge Oscillator Working Principle

The Wien bridge oscillator uses a bridge network and an operational amplifier to generate sinusoidal oscillations. The Barkhausen criterion is satisfied by ensuring the loop gain is equal to or greater than unity. The oscillation frequency is determined by the conditions for oscillation in the bridge network, typically given by the expression f = 1 / (2 * pi * R1 * C1).

Components and Configuration

Bridge Network: Consists of two resistors (R1 and R2) and two capacitors (C1 and C2) in a bridge configuration. At a specific frequency, the voltage across the bridge’s mid-point becomes zero.
Operational Amplifier (Op-amp): Provides feedback by amplifying the voltage difference at its two input terminals. Positive feedback is achieved, meaning a fraction of the output voltage returns to the input in phase with the input signal.

Conditions for Oscillation

in the bridge network and is typically given by the expression f = 1 / (2 * pi * R1 * C1). The bridge network consists of two resistors (R1 and R2) and two capacitors (C1 and C2), configured in a bridge configuration. At a specific frequency, the voltage across the bridge’s mid-point becomes zero. An operational amplifier (op-amp) provides feedback by amplifying the difference between the voltages at its two input terminals. Positive feedback is achieved, meaning a fraction of the output voltage is fed back to the input in phase with the input signal. The loop gain (Aβ) in the Wien bridge oscillator, where A is the op-amp gain and β is the feedback fraction determined by resistor ratios, must be equal to or greater than unity for oscillation to occur. The frequency of oscillation can be derived from the bridge network conditions, where the reactance of the capacitors equals the resistance in the resistive branch.