Colpitts Oscillator Using Op-Amp

An oscillator in analog electronics is a circuit that generates a continuous and repetitive waveform output without the need for an external input signal. Oscillators are widely used in various applications, such as signal generation, frequency synthesis, clock generation, and modulation. The primary function of an oscillator is to convert direct current (DC) power from a power supply into an alternating current (AC) waveform at a specific frequency.

The generated waveform can be sinusoidal, square, triangular, or other types, depending on the oscillator’s configuration and design. Oscillators are crucial components in electronic systems, serving as timing references, frequency sources, and waveform generators. They play a significant role in communication systems, audio devices, control systems, and many other applications. Here are some key points to understand about oscillators:

Basic Components and Operation

An oscillator typically consists of an amplifying device (often a transistor or op-amp) and a feedback network. The feedback network provides a fraction of the output signal back to the input with the appropriate phase shift, resulting in sustained oscillations.

Frequency and Amplitude Stability

The frequency of an oscillator is determined by the characteristics of its feedback network and components. Stable and accurate frequency generation is crucial for applications requiring precise timing, such as communication systems and microcontrollers.

Start-Up and Sustainment

When power is applied to an oscillator, it needs to “start-up” and establish oscillations. The initial disturbance or noise is amplified by the circuit until the oscillator reaches a steady-state output. Some oscillators may require external circuitry or components to facilitate start-up.

Amplitude Control and Modulation

In some applications, the amplitude of the oscillator’s output can be controlled or modulated. Amplitude modulation (AM) and frequency modulation (FM) can be achieved by modifying the parameters of the feedback network or the components within the oscillator circuit.

Colpitts Oscillator

A Colpitts Oscillator was invented by Er. Edwin H. Colpitts, a Canadian-American Engineer. In Colpitts Oscillator, instead of the transistor, an amplifier is used. The op-amp provides the amplification and the feedback is used for stabilizing the oscillator frequency. The op-amp is connected as an inverting amplifier with a high gain as compared with a transistor circuit. The LC network is in the positive feedback of the operational amplifier.

Even though there is no signal, the small noise voltages are amplified by the op-amp when the power supply is given to the circuit. The capacitor starts charging and discharging.Colpitts oscillator uses positive feedback for oscillation. The feedback is provided through a capacitor voltage divider tank circuit to the amplifying element.

The overall phase shift provided by tank circuit is 180 degrees. Since the active element produces a phase shift of 180 degrees the tank circuit should create 180 degrees to obey Barkhausen’s criteria. Common emitter configuration is used for amplifying distorted oscillatory signal to a perfect oscillation. The transistor provides a 180° phase shift and tank circuit another 180°, a total of 0 phase shift.

The gain device in Colpitts oscillator act as a band pass filter and thus limits frequency of oscillation to a certain range. Colpitts oscillator provides more performance than Hartley oscillator because of its capacitive feedback elements. Due to less self and mutual inductance in the circuit, frequency stability of the oscillator is improved along with a more simple design.

Working of Colpitts Oscillator

The Colpitts oscillator is a type of LC oscillator that uses a capacitive voltage divider in its feedback network. It generates sinusoidal output signals and is commonly used in RF applications.

Colpitts Oscillator using Op-Amp

In this circuit:

• The NPN transistor (commonly used as an amplifier).
• C1 and C2 are capacitors.
• L1 is an inductor (coil).
• R1 provides the necessary biasing and is part of the transistor’s collector circuit.
• C3 is the output coupling capacitor.

Working Principle

• The transistor is used as an amplifier. It is biased in the active region, and its collector is coupled through the inductor L1.
• Capacitor C1 and the parallel combination of C2 and L1 form the feedback network. The capacitors C1 and C2 act as a capacitive voltage divider.
• The voltage across the capacitors C1 and C2 is out of phase, providing the necessary feedback for oscillation.
• As the circuit is powered on, noise or any initial signal starts the oscillation process.
• The feedback loop causes the transistor Q1 to amplify the oscillating signal, and the output is taken from the collector through coupling capacitor C3.

Resonant Frequency Formula

The resonant frequency (f_res) of the Colpitts oscillator can be derived from the formula for the resonant frequency of an LC tank circuit:

frequency = 1/√L₁(C₁ + C₂)

Where:

• f_res is the resonant frequency in Hertz (Hz).
• L1 is the inductance of the coil in Henrys (H).
• C1 and C2 are the capacitances of the capacitors in Farads (F).Advantages of Colpitts Oscillator.

Derivation

1. We’ll start by analyzing the feedback network formed by C1, C2, and L1. At resonance, the phase shift around the loop must be 0° or 360° for positive feedback to occur.

2. The impedance of L₁ at resonance (X_L = X_C) is given by:

⇒X_L = X_C

⇒ωL₁ = 1/ωC₁ + 1/ωC₂

3. Solving for ω (angular frequency) gives:

⇒ω² = 1/L₁(C₁ + C₂)

4. The resonant frequency f_res is:

⇒(f_res)² = 1/L₁(C₁ + C₂)

5. Solving for f_res gives:

⇒f_res = 1/√L₁(C₁ + C₂)

This is the derived resonant frequency formula for the Colpitts oscillator. It represents the frequency at which the oscillator will naturally oscillate when powered on, provided that the circuit components are appropriately chosen and the oscillator conditions are met. Please note that this derivation assumes ideal components and neglects parasitic effects. In practice, variations in component values and non-ideal characteristics can affect the actual oscillation frequency of the Colpitts oscillator circuit.

Advantages of Colpitts Oscillator

• Colpitts Oscillator can with extreme temperatures, i.e., high and low temperatures due to high machinery and friction in the electronic circuit.
• Colpitts Oscillator is less complex. It can be designed efficiently with less number of components.
• Colpitts Oscillator is more resistant to damping and can maintain a constant amplitude for a very long period of time..
• Colpitts Oscillator has high frequency stability.
• Colpitts Oscillator can produce sinusoidal signals of very high frequencies.

Applications of Colpitts Oscillator

• Colpitts Oscillator is used for high frequency range and high frequency stability.
• Colpitts Oscillator is used as a surface acoustical wave (S.A.W.) resonator.
• Colpitts Oscillator is used in microwave applications.
• Colpitts Oscillator is used in mobile and communication systems.
• Colpitts Oscillator is used in noisy circuits which can generate oscillations from audio frequency range to the optical band. These include areas in broadband communications, spectrum spreading, signal masking, etc.

FAQs on Colpitts Oscillator Using Op-Amp

Q.1: What is a Colpitts Oscillator?

Answer:

A Colpitts Oscillator was invented by Er. Edwin H. Colpitts, a Canadian-American Engineer. In Colpitts Oscillator, instead of transistor, an amplifier is used. The op-amp provides the amplification and the feedback is used for stabilizing the oscillator frequency.

Q.2: List an application of Colpitts Oscillator in daily life.

Answer:

Colpitts Oscillator is used in microwave applications, mobile and communication systems.

Q.3: The frequency of Colpitts oscillator is?

Answer:

1/(2ᴨ√LC), where L is the inductance and C is effective capacitance.

Q.4: Colpitts oscillator uses which type of feedback?

Answer:

Colpitts oscillator uses positive feedback for oscillation. The feedback is provided through a capacitor voltage divider tank circuit to the amplifying element.

Q.5: Which configuration of transistor amplifier is used for Colpitts oscillator?

Answer:

Answer: Common emitter configuration is used for amplifying distorted oscillatory signal to a perfect oscillation.