Simple MF Colpitts Oscillator

Reference Circuit

The following Colpitts Oscillator is built around a 2N3904 transistor.  I verified its operation for various frequencies from 250 kHz to 1.5 MHz.  The inductor is 20 turns of insulated 22 AWG wire on a 1.5 inch diameter cardboard core (the turns are as closely wound as practical, taking about 1.0 inches axially along the core).  I measure the inductance as approximately L \approx 11 \mu H .  Capacitor values for the LC tank (lower left of schematic) are presented in a table.

Practical Colpitts Oscillator
Colpitts Oscillator for 250 kHz – 1.5 MHz



Capacitor Value Observed Frequency
1.5\, nF 1.6\, MHz
10\, nF 670\, kHz
33\, nF 370\, kHz
68\, nF 250\, kHz

The oscillator gave best performance at capacitor values 33 nF and 22 nF.

Measuring Inductance


Measuring inductance is a complex task.  Methods proposed elsewhere require precise measurements with an oscilloscope in its usual (voltage vs. time) display mode.  Methods based on building a filter with the inductor and finding the cutoff or natural frequency assume a sufficiently high-Q that peak/null detection is obvious.  In practice, I find these methods hard to apply.

Is there an easier method to measure inductance using a hobby-grade oscilloscope?

Continue reading Measuring Inductance

AC Coupling using an Emitter Follower


Input signals often have a DC component that is not of interest and inconvenient to work with.  The DC component might be unknown or of a value that doesn’t match the circuit the input is driving.  Under standard linear circuit  assumptions, the response to the time-varying component of the input is independent of the DC component, so we can strip away the DC component and replace it with a more convenient value.  A blocking capacitor generally provides the “AC Coupling” of the input signal.

A problem with blocking capacitors arises when I try to select one for my audio circuits.  To the signal source, my circuit appears to have some input impedance, R, and the equivalent circuit becomes a single-pole high-pass filter:


The commonly-defined “cut-off” frequency for such a circuit is f_{c}=\frac{1}{2\pi RC}, and I often try to have a cut-off frequency one decade below my lowest frequency of interest.  To accomplish this I need a very large RC product, but the input impedance of my circuit is generally too low. Continue reading AC Coupling using an Emitter Follower