PRACTICAL DESIGN OF COLPITTS OSCILLATOR USING BJT

EDITOR: B. SOMANATHAN NAIR

I. CE RC-COUPLED COLPITTS OSCILLATOR

Colpitts oscillator was invented in 1918 by Edwin H. Colpitts. Just like Hartley oscillator, it is a radio-frequency oscillator. The feedback network consists of two capacitors in between which we have a inductor, as shown in Fig. 1.

1.  SPECIFICATIONS

·    Output swing                                  :           4.5 V (peak)

·        Frequency of oscillation                :           1 MHz

·         Current swing                               :           1 mA

2.  DESIGN PROCEDURE

Steps 1 to 7: Design of the Standard Amplifier

Follow the steps described in a previous blog to design the Standard Amplifier.

Step 8: Design of the B Network

We first choose C₁ = C₂ to satisfy the gain condition given by the expression

                                                            A_V = C₂/C₁  (1)

We are now left with the freedom of designing an amplifier whose gain need not be very high to produce oscillation (as in the case of Hartley, here also we may choose C₁ to be greater or less than C₂, and design the appropriate amplifier to suit the requirement on gain). The inductor can now be calculated using the expression

                                                            f_o = 1/2π√LC  (2)

where

                                                       C = C₁C₂/(C₁ + C₂)  (3)

With f_o = 1 MHz, and

  C₁ = C₂ = 200 pF

we get

L = 253 mH

The completely designed amplifier is shown in Fig. 1.

 

II. CE LC-COUPLED COLPITTS OSCILLATOR

Figure 2 shows the circuit of a CE LC-coupled Colpitts oscillator. This is similar to the CE LC coupled Hartley oscillator shown in an earlier blog with L₁, L₂, and C replaced by C₁, C₂, and L, respectively. An RF choke is used to prevent AC oscillations from reaching the power supply, and also ensure proper DC biasing for the transistor.

1. SPECIFICATIONS

·    Output swing                                  :           4.5 V (peak)

·        Frequency of oscillation                :           1 MHz

·         Current swing                               :           1 mA

2. DESIGN PROCEDURE

As this is similar to the CE LC Hartley oscillator, the specifications and design procedure follow the same pattern in this case also. So, no attempt is made here to repeat them. It can be seen that an RF choke is used in this case to complete the DC path from V_CC to the B network. It (RF choke) also blocks the ac oscillation signals from getting mixed with the DC supply, thus providing a steady DC supply to the system.

            The design of the circuit is left as an exercise to the reader. The completely designed oscillator is shown in Fig. 2.

 

III. CB COLPITTS OSCILLATOR

The designs of the CB and CC types also follow the procedures of the corresponding Hartley types. Design of the CB type is given below. However, no attempt is made here to design the CC type. This is left as an exercise to the reader.

1.  SPECIFICATIONS

·    Output swing                                  :           4.5 V (peak)

·        Frequency of oscillation                :           1 MHz

·         Current swing                               :           1 mA

2. DESIGN PROCEDURE

Steps 1 to 7: Design of the Standard Amplifier

Design the Standard Amplifier first as per the procedure given earlier.

Step 8: Design of the Tank Circuit

The tank circuit is designed according to Eqs. (1) and (2). This yields the values of L and C’s as

C₁ = C₂ = 200 pF

L = 253  μH

In practical situations, we choose a standard medium-wave coil and use an appropriate set of capacitors to give the desired frequency. The fully designed oscillator is shown in Fig. 3.