0
\$\begingroup\$

I've built the circuit shown in the picture on the breadboard. The simulation works as expected but when I build it i don't get any oscillations. I've measured all the DC voltages and they are as expected. The measure hfe of the transistor seems to 216 while I assumed 100 in my calculations.

I've done a continuity check using my multi meter and all seems to be in order. Do you guys have any idea why this might be happening?

BJT is biased at 5mA and Collector voltage at 6V.

Edit1: Added a 10uF capacitor. Also added a 0.1uF emitter bypass capacitor to increase gain. Still no go.

enter image description here

enter image description here

enter image description here

\$\endgroup\$
  • \$\begingroup\$ I'm assuming that you're using an oscilloscope to watch the oscillations (or lack thereof)? If you are, can you get a screen shot of that? \$\endgroup\$ – KingDuken Mar 13 at 2:56
  • \$\begingroup\$ @KingDuken Just did that. Thanks! \$\endgroup\$ – Pratik Kunkolienkar Mar 13 at 3:04
  • \$\begingroup\$ Did you compute Q? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 13 at 5:32
  • \$\begingroup\$ Using a trim potmeter to adjust the bias might help. If the transconductance of the transistor is too low, the circuit will not oscillate. Also, the emitter degeneration may be too high. Bypass the emitter resistor. \$\endgroup\$ – Bart Mar 13 at 7:22
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 If i'm doing this right, my inductor resistance was measured to be 0.2 Ohms, so \$0.2*\sqrt{\frac{820}{7.5}} = 2.0913\$ \$\endgroup\$ – Pratik Kunkolienkar Mar 13 at 13:32
0
\$\begingroup\$

Where is your VDD Cbypass cap?

You need a 0.1uf from the top of R1 to ground.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ if you mean the 47u, its the blue thing above the transistor \$\endgroup\$ – Pratik Kunkolienkar Mar 13 at 4:12
  • \$\begingroup\$ @PratikKunkolienkar, no that is the coupling capacitor. Analogs' answer is valid. Omitting the power supply decoupling capacitor can complete void the functioning of a circuit like this. \$\endgroup\$ – Bart Mar 13 at 7:20
  • \$\begingroup\$ good answer . . . . \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 15 at 16:15
0
\$\begingroup\$

The problems that I see with this circuit:

  • No power supply decoupling.
  • The gain at the collector is 12 at most (1200/100), which may be too low. Loading at the collector by the feedback pi-filter, (and the oscilloscope!) will further reduce the gain.
  • No adjustable bias voltage. The emitter bias current controls the gm (transconductance) of the transistor. This value is important for the correct functioning of the circuit.

I suggest replacing the 1.2k resistor by a choke which resonates at the intended frequency of the oscillator, maybe bypassing the emitter resistor to increase the gain, and making the base voltage adjustable with a potmeter.

I recently built a 16MHz Colpitts VCO using this approach, albeit in common base configuration. Getting it to oscillate took a minute of adjusting.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ Added a 10uF capacitor this morning. Also added a 0.1uF emitter bypass capacitor to increase gain. Still no go. How do i spec the choke? Isn't a choke just an inductor? how do i make sure it resonate at the right frequency will that be its self resonating frequency? \$\endgroup\$ – Pratik Kunkolienkar Mar 13 at 12:39
  • \$\begingroup\$ It is just a coil that has parallel resonance at the working frequency so the collector will see it as a very high impedance. electronics-tutorials.ws/oscillator/colpitts.html \$\endgroup\$ – Bart Mar 13 at 12:45
  • \$\begingroup\$ Just realised that i might need a lower ESR bypass capacitor so it works correctly at higher frequency. I was using an electrolytic \$\endgroup\$ – Pratik Kunkolienkar Mar 13 at 13:34
0
\$\begingroup\$

In 1975 when I graduated working for Bristol Aerospace, a gold medalist EE Bill Whitehead was my mentor and he showed me these RLC Impedance graphs. Since we used a slide rule before the LED calculator came out in the early 70's I understood how to do log math on a slide rule.

Since I assume you have some Impedance math skills, you ought to be able to do the same.

This is a test. Does this filter attenuate? What is the loop gain?

enter image description here

enter image description here

Final question: What is the phase shift at resonance and is it correct?

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ Hint: -90 before and +90 after \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 15 at 6:14
  • \$\begingroup\$ Well it will attenuate with a 180° (should be 90°?) phase shift won't it? Isn't that the point? and then the transistor amplifies it to satisfy the Barkhausen criterion. right? \$\endgroup\$ – Pratik Kunkolienkar Mar 16 at 0:18
  • \$\begingroup\$ keep in mind jumpers are about 0.5~1nH/mm and where is Vcc cap? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 16 at 1:19
  • \$\begingroup\$ I've added a 0.1uF after someone on here reminded me about it. Someone also said that i didn't have enough gain. So I've ordered some 33uF inductors and 15pF caps. \$\endgroup\$ – Pratik Kunkolienkar Mar 16 at 2:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.