2
\$\begingroup\$

I'm trying to build the LC oscillator show on the bottom of this page for some school project and I'm having a bit of problems translating the theoretical circuit into a real-world one.

Here's the schematic:
enter image description here

I think I get how the circuit works on paper. We have a LC resonant circuit which will produce a certain output frequency. Because the energy in the circuit gets transferred out due to differences between theoretical components and real world components, the transistor is there to keep adding the energy into the circuit. The transistor is controlled by the coupled coil L2 in which we get current induced by the coil L which keeps switching the transistor

For the real world circuit I can use more or less whatever components I want, so my main limitation is that I only have a 1.2 mH 1:1 transformer available.

In some simulations I used that transformer and a \$ 470 \mu F\$ capacitor (I just happen to have one in my drawer) and the expected frequency should be around 212 Hz.

For some reason, the simulations don't seem to be working as expected and I can't figure out why.

For example here's a link to falstad circuit simulator and it's showing bad connections. I also only get the bottom half of the sinusoid.

In Multisim, I'm getting flat line on the oscilloscope with this circuit:

circuit in multisim

I don't know how to pick a right transistor for this circuit so I just took 2N2714 randomly.

Here's my question: How do I figure out why this doesn't simulate well and how do I determine what I need to look for when picking a transistor for the circuit?

UPDATE Here's a new link for falstad simulator and it's an improvement. I'm still not getting expected oscillations from it though.

\$\endgroup\$
2
\$\begingroup\$

Transistor is zero biased for DC.

Just out of head but try.

  • Lift ground end of T1 primary off ground then

  • Connect 1k to ground from end of winding.

  • From same point connect 4k7 to V+.

If no action or not ideal adjust say 4k7 up and down and observe.

BUT

  • As above plus.

  • Disconnect emitter from ground and connect 100 ohms to 1k from emiter to ground.

  • And connect a capacitor from emitter to ground.


The transformer at 1:1 is feeding back FAR more than is needed.

Here's another variant:

  • Disconnect BASE end or primary from base.

    • Connect a 22k and 1k in series to ground from primary end.
      (Primary end - 22l - 1k - ground

    • Connect a 0.1 uF from 22k/1k tap to base.

    • Connect a say 100k from base to V+_

    • May need another resistor base to ground such that vbase is about 0.7V nominal DC.

    • Adjust 22k in 22k/1k divider to change feedback magnitude.

This is not an ideal way to vary feedback level and I have not tried to calculate impedances of inductors at frequency etc - very much 'out of head' values BUT I'd expect it to "sort of work".

If you get amplitudes which are either too small and decay or which grow, try connecting an NTC thermistor or lightbulb in series with the cap to the base. You'll need to play with levels and values. There are better ways of doing this but the Red Queen is chasing me so ... .

Report back ...

\$\endgroup\$
  • \$\begingroup\$ When I lift the T1 of the ground, I get what looks like stable oscillations in the simulator. If I use 4k7 connected to V+, I get huge positive feedback and the oscillation amplitude quickly goes sky high. If I add resistor or resistor+capacitor to emitter, I get what looks like stable oscillations even without lifting T1 off the ground. Even in that case, if I use the 4k7, amplitude keeps increasing very quickly. \$\endgroup\$ – AndrejaKo Dec 5 '11 at 13:29
2
\$\begingroup\$

The falstad simulator takes some getting used to.

You can't just join a wire in to the middle of another wire/component. Wires must meet at their ends.

So, to make a "tee" junction you need three "ends" - be that ends of wires, or ends of component connections - and they all join at the same spot.

\$\endgroup\$
  • \$\begingroup\$ I got a new falstad link, but it still appears that I'm not getting the expected oscillations. Any ideas what could be wrong? \$\endgroup\$ – AndrejaKo Dec 5 '11 at 12:32
1
\$\begingroup\$

You seem to understand the basics, sortof. Yes, the output signal is fed back into a amplifier with a gain greater than 1. In this case the amplfier also inverts (gain is actually below -1), so you invert the feedback signal to compensate. However, some of your comments don't make sense and show confusion on your part:

Because the energy in the circuit gets transferred out due to differences between theoretical components and real world components

No, it doesn't oscillate just because the parts aren't close to theoretical. This oscillates quite deliberately, and would work fine with a ideal transformer and capacitor.

the transistor is there to keep adding the energy into the circuit

No. The transistor provides gain. Any additional energy the output might have that is not coming from the input comes from the power supply. Transistors are never sources of power.

Now to why it doesn't work. I see two reasonably possible causes:

  1. You have the transformer flipped. Your schematic is wrong in that it doesn't show a dot on one side of each transformer winding so you can see polarity. These can be left out when polarity doesn't matter, like when a power transformer is feeding a full wave rectifier. In your circuit, the polarity matter so it's wrong not to show it. Flip the wires of one of the windings and see if that helps.

  2. The circuit is wired correctly but hasn't been kicked to start it. Note that the transformer winding connected to the base is shorting the base to ground in the DC case. That keeps the transistor off, which never drives the other winding, which never feeds anything into the transistor to turn it on. Another way of saying this is that all off is a stable state. To get around this, briefly touch a wire between the collector and ground. That will force a pulse thru the transformer which should start things going. That is, of course, assuming the transformer polarity is correct (see point #1 above).

\$\endgroup\$
  • \$\begingroup\$ For the first italics, I meant that in real-world, with just an LC circuit without the transistor, we'd get damped oscillations which would stop after some time, so the transistor is needed to allow the power supply to bring in more power. For the rest, the 2. helped a lot, so thanks a lot. \$\endgroup\$ – AndrejaKo Dec 5 '11 at 13:12
1
\$\begingroup\$

There are two problems with your Falstad simulation:

  1. You don't have any way of kick-starting the simulation. In reality, there will usually be enough ambient noise to start a good oscillator. In simulators, you have to provide that "kick" to get it going. For this, I wired a push-switch from the top leg of the transformer to V+

  2. You need to limit the current to the transistor. A lot of these models are simple and don't include ohmic resistances in the models; and will happily conduct kiloamps for a device usually in a TO92 package. If you don't introduce some base resistance, (or some way to limit the circuit), your current can actually increase unbounded. See this simulation for an example of this unbounded behavior.

    In reality, this current would be limited by many things, such as:

    • Non-zero impedance of the DC power supply
    • Non-zero series resistance of the transformer/inductor
    • Beta saturation of the transistor at high current
    • Explosion of transistor at high current

See this simulation for a working oscillator example. All I did was place a 100 ohm resistor in series with the base. The problem with "ideal" circuits is that often times the model is too simple or omits critical details as "non-ideal".

\$\endgroup\$

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.