Colpitts Oscillator - did it work ?

[lordvader88]

Yesterday I learned of and looked up Colpitts Oscillator and so threw it together from here
http://www.electronics-tutorials.ws/oscillator/colpitts.html

I made this 1 with pretty much the same Rs and Cs


And my DMM said it was oscillating at about 1.8kHz or maybe 2.4kHz. So I wanted to hear it and spent 1-2 hours trying to get a little mini speaker hooked up to it, and I tried a 1 BJT driver, and none of it worked, and then I didn't know if I had a peizo buzzer or a speaker or what was wrong or if I just was doing it all wrong for setting up the trannie

So I was like this


LOL, gotta love those cats

So I built a little 555 circuit to make sure I could work the speaker, and it worked fine. So I checked the Colpitts Osc with my DSO and it wasn't showing any noticeable oscillation, just a fixed 2V output from the 741 opamp, and hence why the speaker did nothing.

I never bothered with any equations to even see what freq. it was supposed to work at, or the amplitude, but it was fixed on my scope.

 So what am I missing here ? I'll play with it again now soon, but my DMM does work fine for Hz in my limited experience, and the scope confirms it usually.

[Andy Watson]

So what am I missing here ?

Negative power supply?

[lordvader88]

 IDK, I used 0-12V. The article says "Note that being an inverting amplifier configuration, the ratio of R2/R1 sets the amplifiers gain. A minimum gain of 2.9 is required to start oscillations.", I used 3.9k/1k so I got that part.

I haven't touched yet today

On DC and AC coupling the only variation looked like background noise it was so small, I'm not sure what range the sine wave is supposed to be over.

A few months back I knew the basics of calculating op-amp circuits, time to learn again.

[Andy Watson]

Your op-amp circuit will (try to) output a signal that goes positive and then negative. The negative portion requires a negative power supply. In the case of the 741 you will need a negative supply that exceeds the output voltage by approximately 3V (IIRC). What/where have you connected pin 4 of the op-amp?
Alternatively, you could make a potential divider to generate 6V from your 12V supply and use this to drive the positive input of the op-amp (pin 3).

[Audioguru]

The lousy old 741 opamp is 48 years old. Get a better newer opamp and bias its (+) input at half the supply voltage with two resistors making a voltage divider..

[lordvader88]

I have an old printer SMPS that puts out/in -12/12V, and some other SMPS with -5V rails, must try later

[LvW]

The lousy old 741 opamp is 48 years old. Get a better newer opamp and bias its (+) input at half the supply voltage with two resistors making a voltage divider..

Audioguru - don`t you think that biasing with "half the supply voltage" applies to an amplifier with unity DC gain only? Here, we have a DC gain of (1+3/1.2)

[Zero999]

The lousy old 741 opamp is 48 years old. Get a better newer opamp and bias its (+) input at half the supply voltage with two resistors making a voltage divider..

Audioguru - don`t you think that biasing with "half the supply voltage" applies to an amplifier with unity DC gain only? Here, we have a DC gain of (1+3/1.2)

The DC gain needs to be changed to 1, whist keeping the AC gain the same.

[Andy Watson]

Audioguru - don`t you think that biasing with "half the supply voltage" applies to an amplifier with unity DC gain only? Here, we have a DC gain of (1+3/1.2)

The DC gain is unity.

[Audioguru]

Of course there must be a coupling capacitor feeding the 1k input resistor of the opamp so that the opamp has a DC gain of only 1. It also might need an output coupling capacitor.

[Andy Watson]

Of course there must be a coupling capacitor feeding the 1k input resistor of the opamp so that the opamp has a DC gain of only 1.

It already has    C1 and C2. Remove these from the circuit (as would be the case for DC) and it becomes obvious that the op-amp has unity gain, DC feedback - via two paths.

It also might need an output coupling capacitor.

Yep!

[Zero999]

Audioguru - don`t you think that biasing with "half the supply voltage" applies to an amplifier with unity DC gain only? Here, we have a DC gain of (1+3/1.2)

The DC gain is unity.

Of course it is. The DC voltage at the inverting and non-inverting inputs closely follow the output.