20 MHz Colpitts CB Oscillator

Nov 18, 2015

This article describes a buffered-output 20 MHz Colpitts CB oscillator. It is important to understand in this type of circuit that the oscillation frequency is determined by any capacitive loading effects of the entire output coupled circuit. The schematic diagram below shows an example of buffering the oscillator tank-circuit with a low capacitance unity-gain follower consisting of an LT1363 op-amp with an input capacitance of ~ 3pF:

The equivalent tank circuit which determines the oscillation frequency is shown below where Cv is a trimming capacitor (5 to 20pF) to allow adjustment of the oscillation frequency and Coa is the combination of the op-amp input capacitance (3pF) plus stray capacitance (estimated at 2pF). The transistor collector-emitter shunt capacitance is very low and is neglected. Cv and Coa are in parallel with Lc and therefore directly affect the oscillation frequency. Similarly, using a high-frequency scope probe at the transistor collector to measure the oscillation frequency will lower the oscillation frequency according to the scope probe capacitance (typically ~ 15pF) in the same way as Cv and Coa do. The low input capacitance of the LT1363 op-amp and the unity-gain buffer circuit helps isolate the tank circuit oscillation condition from capacitive loading at the op-amp output:

The oscillation frequency is given by:

An assembled protoboard circuit is shown below. It is quite possible to assemble such circuits using leaded components on protoboards which work well up to about 50MHz provided that the parasitic capacitances of the board are known and taken into account, and connections are kept tight and very short and proper power-supply decoupling is used:

The scope trace below at the output of the opamp, taken with a Digilent Analog Discovery oscilloscope near the limits of its resolution shows a stable oscillation frequency of 20.0 MHz for Cv~ 10pF. With Coa ~ 5pF, Ceq ~ 65pF and the predicted oscillation frequency is 19.7 MHz in close agreement with the measured frequency. With a 50Ω load connected (total output load 89Ω), the output amplitude is about half and the frequency drops slightly to 19.8MHz demonstrating good buffering of the oscillating tank circuit:

Colpitts Oscillator


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