Capacitor n oscillator

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zachtheterrible

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I’ve got some questions. Here they go:

1. I know that the leads of a capacitor are supposed to be as short as possible, but do they make a really huge difference? How much?
2. I’m not sure how an oscillation circuit works. If you could explain this one that I have here, I would be very greatful.
3. Using just a capacitor and an inductor, what is about the highest frequency that is possible?
 

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zachtheterrible said:
I’ve got some questions. Here they go:

1. I know that the leads of a capacitor are supposed to be as short as possible, but do they make a really huge difference? How much?
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At low frequencies they make no difference, but they have more and more effect as the frequency gets higher - which is why VHF construction is fairly critical, and above that things get rapidly worse!.

2. I’m not sure how an oscillation circuit works. If you could explain this one that I have here, I would be very greatful.
Click to expand...

All an oscillator requires is positive feedback, in this case it's given by the small capacitor from collector to emitter. The tuned circuit merely makes sure it oscillates at the frequency you wish.

This particular circuit works (as an oscillator) in common base, the base is grounded by a decoupling capacitor, the output is from the collector, and the input is to the emitter.

3. Using just a capacitor and an inductor, what is about the highest frequency that is possible?
Click to expand...

It depends completely on construction techniques, but once you get to UHF frequencies inductors become simple straight pieces of wire, once you hit microwave frequencies you're into plumbing rather than wiring, but it's still a resonant circuit using capacitance and inductance.
 
Using just a capacitor and inductor, I agree. If you actually want to make an oscillator, you have to add gain. If you use a bipolar transistor, the maximum frequency possible becomes dependent on transistor parameters, primarily Ft (cutoff frequency) and junction capacitances.
 
Thanx for the answers, but I'm not sure I totally get the oscillator yet . When you say that the base is grounded by a decoupling capacitor, the base is connected to positive, so how does that work? Also, Could you go into some more detail about positive feedback? I sort of know about the 180 degree phase shift, but don't have a complete understanding. How is the input the emitter? I thought that the base is the only thing that can control the transistor. Lastly, when you say that when getting into microwave frequencies, you get into plumbing, what does that mean?? Thanx for all the help !
 

C1 grounds the base, even though it connects to the positive rail, the positive rail and ground are the same as far as AC is concerned.

Transistors can be used in three basic modes:

1) Common emitter, input to base, output from collector.

2) Common collector, input to base, output from emitter.

3) Common base, input to emitter, output from collector.

Common base is often used for high frequency RF circuits, as it has various advantages at high frequencies.

What I mean by 'plumbing' is the tuned circuits involved, they become pipes and tubes at microwave frequencies - you can't even use coaxial cable any more, the signals are fed down 'wave guides' - which are basically pipes!.
 
zachtheterrible said:
Thanx for the answers, but I'm not sure I totally get the oscillator yet . When you say that the base is grounded by a decoupling capacitor, the base is connected to positive, so how does that work?
Click to expand...
You need to understand the difference between DC ground and AC ground. Ideally, your battery (power supply) has zero impedance at all frequencies, i.e., a change in current will not cause a change in voltage. If this is true, then +9V in your circuit becomes AC ground. Of course, it will never be absolutely zero impedance, but we try to get as close as possible by adding capacitors from the supply voltage(s) to GND. BTW, do you have a small cap (around 0.01uF - 0.1uF) from +9v to GND, with the leads as short as possible? You should.
There is no phase shift from collector to emitter in the common base oscillator. What is needed in an oscillator is 0 (or 360) degrees phase shift around the loop, with some sort of gain. In the common base oscillator, the gain is voltage gain. The impedance loking into the emitter is low, the impedance at the collector is high, and the current gain from emitter to collector is nearly equal to one. This yields voltage gain, and hence oscillation. In common emitter oscillators, the transistor inverts (180 degrees), so an additional 180 degrees of phase shift is required in the feedback network in order to get a total of 360 degrees. Voltage and current gain are provided by the transistor.
Lastly, when you say that when getting into microwave frequencies, you get into plumbing, what does that mean?? Thanx for all the help !
Click to expand...
Microwaves are generally routed locally by waveguides ("plumbing") instead of wires. Do a Google search for "waveguides" or "waveguide plumbing" (without the quotation marks).

Edit: Looks like Nigel and I were working on answering your questions at the same time. Two heads are better than one!
 
Hmmm, I think I'm startin' to understand this. If you could explain impedance a little more indepth, that would be great! Also, I'm not sure how the common base amplifier works. How can a signal @ the emitter control the circuit? I've been trying to put a .01 uf capacitor across the battery but haven't gotten 'round 2 it. thx
 
When you get around to it, it might make your circuit work if it doesn't now. Don't literally connect it across the battery. Connect it across the points on the circuit board where the battery connects. That may be what you meant anyway. The reason for this is that the wires to your battery have inductance, and inductance has impedance that rises linearly with frequency. Remember we want the impedance from +V to GND to be as low as possible, especially at the oscillation frequency.
The common base amplifier is difficult to understand at first, and it is equally difficult to explain, at least for me. Do a Google search for "common-base" (without the quotation marks).
A simplified explanation is that a small emitter-base voltage change causes a relatively large change in emitter current. Most of this change (99% typically) will show up at the collector. The impedance at the collector is the parallel combination of the transistor's collector impedance (usually 10's to 100's of kohms) in parallel with the load impedance. If this parallel combination is higher than the source impedance driving the emitter, voltage gain results, because both impedances see (nearly) the same current change.
There - are you sufficiently confused? I know that the concepts of emitter impedance and collector impedance are not easy to understand.
 
I guess that in time I'll understand this. Anyways, I've got some other questions. what is the forumla to deterime the inductance of an inductor, and also the formula to determine the frequency of a resonant circuit? Or give me a website or something. Thanx
 

The formula for resonance is:

F = 1/(2*PI*Square root(L*C)).

Where F is in Hertz, L in Henries, and C in Farads.

Determining the value of an inductor is a lot more complicated, the only really accurate way is to measure it.
 
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