Oscillator Types: Wein Bridge, Colpitts, Clapp, Hartley, and Crystal

This page explores various oscillator types, including the Wein Bridge, Colpitts, Clapp, Hartley, and crystal oscillators. These oscillators are vital components in many electronic circuits, often serving as reference oscillators in Phase-Locked Loop (PLL) or synthesizer circuits.

Wein Bridge Oscillator

wein bridge oscillator

The image above illustrates a Wein Bridge oscillator circuit using an operational amplifier. As shown, the oscillation frequency is determined by the following equation:

$f = \frac{1}{2\pi\sqrt{(R_3 R_4 C_1 C_2)}}$

Where, if $R_3 = R_4 = R$ and $C_1 = C_2 = C$, the equation simplifies to:

$f = \frac{1}{2\pi RC}$

Colpitts Oscillator

colpitts oscillator

The figure above showcases a Colpitts oscillator circuit using a transistor. The oscillation frequency is dictated by the following formula:

$f = \frac{1}{2\pi\sqrt{L \left( \frac{C_1 C_2}{C_1 + C_2} \right)}}$

With the condition:

$g_m R_c \ge \frac{C_2}{C_1}$

Hartley Oscillator

Imagine taking the Colpitts oscillator from the previous section and making a slight alteration. If we replace $C_1$ and $C_2$ with inductors $L_1$ and $L_2$, and replace the inductor $L$ with a capacitor $C$, we’ve effectively created a Hartley Oscillator.

The oscillation frequency for a Hartley Oscillator is described by:

$f = \frac{1}{2\pi\sqrt{C(L_1 + L_2)}}$

Subject to the condition:

$g_m R_C \ge \frac{L_1}{L_2}$

Clapp Oscillator

Starting with the Colpitts oscillator circuit, if we swap the inductor with a variable capacitor (let’s call it $C_3$), the oscillation frequency changes. This adjusted circuit becomes a Clapp Oscillator, and its frequency is approximately:

$f = \frac{1}{2\pi\sqrt{L C_3}}$

This approximation holds true when:

($\frac{C_3}{C_1} >> 1$) and ($\frac{C_3}{C_1} >> \frac{C_1}{C_2}$)

Crystal Oscillator

crystal oscillator

The image above presents a typical crystal oscillator circuit. The series resonance frequency, $f_s$, is given by:

$f_s = \frac{1}{2\pi\sqrt{LC}}$

The parallel resonance frequency, $f_p$, is expressed as:

$f*p = \frac{1}{2\pi\sqrt{L C*{eq}}}$

Where, $C_{eq}$ is defined as:

$C\_{eq} = \frac{C C_o}{C + C_o}$

The oscillation frequency is typically chosen between $f_s$ and $f_p$.

For more in-depth information, refer to the Quartz Crystal page.