Colpitts oscillator and Hartley oscillator are commonly used oscillator used in both audio circuit and RF circuit. A basic Colpitts oscillator has two capacitor and one inductor as the feedback filter that introduces 180 phase shift to the input signal which is fed back to the amplifier. Hartley oscillator has two inductor and one capacitor as the feedback filter. The amplifier can be bipolar junction transistor, field effect transistor and even MOSFET transistor. In this blog post Colpiits and Hartley oscillator design with 2SC1815 NPN bipolar transistor are illustrated. The 2SC1815 NPN bipolar transistor is a general purpose transistor designed for amplification purpose. The power dissipation of this transistor is 400mW and the beta(hfe) is between 50 to 700.

The following shows Colpitts oscillator based on 2SC1815 NPN bipolar transistor.

In this circuit the 2SC1815 BJT is used as amplifier which is biased using voltage divider biasing method and the resistors R1,R2, RE and R3 are DC biasing resistor for the amplifier which can be calculated using the BJT amplifier calculator calculator.

The feedback network of this oscillator is formed by the two capacitors C1=1nF and C2=15nF and the inductor L1=1mH. The frequency of oscillation is given by the following equation.

\(f_{r} = \frac{1}{2 \pi \sqrt{L_1 C}}\)

where,

\(C = \frac{C_1 C_2}{C_1+C_2}\)

That is \(C= \frac{1nF \times 15nF}{1nF+15nF} = 0.937nF\)

And therefore, \(f_{r} = \frac{1}{2 \pi \sqrt{L_1 C}} = \frac{1}{2 \pi \sqrt{1mH \times 0.937nF}} = 164.37KHz\)

Using the values the frequency of oscillation is 164.37 KHz.

The feedback fraction is the fraction of the output signal of the oscillator which is fed back to the input of the amplifier.

\(\beta = \frac{C_1}{C_2}=\frac{1nF}{15nF}=0.07\)

This values can be directly calculated using the oscillator design calculator.

Colpitts oscillator can also be designed with operational amplifier as illustrated in the tutorial Colpitts oscillator with LM358 and TL072 Op-Amps.

Following is **Hartley oscillator** designed with 2SC1815 BJT.

In the above circuit, again the 2SC1815 NPN bipolar junction transistor is used as the amplifier. The capacitor C1=1nF and the two inductors L1=1mH, L2=1mH are used as the oscillating tank or the feedback filter of the oscillator. The frequency of the oscillation of the Hartley oscillator is given by,

\(f_{r} = \frac{1}{2 \pi \sqrt{L C_1}}\)

where, \(L = L_1+L_2\)

That is, \(L = 1mH+1mH=2mH\)

Therefore, \(f_{r} = \frac{1}{2 \pi \sqrt{2mH \times 1nF}} = 112.54 KHz\)

The feedback fraction is given by,

\(\beta = \frac{L_2}{L_1}=\frac{1mH}{1mH}=1\)

**References**:

[1] Different Topology of BJT Colpitts Oscillator