# Help understanding circuits

Discussion in 'Electronic Basics' started by AJ, Mar 6, 2008.

1. ### AJGuest

Hi,

I was wondering if someone might be able to answer some questions on the
following video transmitter circuits...
http://www.ajpotts.fsnet.co.uk/Circuits.html

1. In the top circuit, what is the idea of the diode on the video input and
why isn't it on the bottom circuit?

2. I have never really understood how this configuration of oscillator
works, mainly the purpose of the 10pF cap from the collector to emitter. I
assume it is a variation of the colpitts and by applying a voltage on the
base of the transistor varies the CE capacitance which in turn varies the
frequency for modulation but im not sure? What I am mainly confused about
is how this capacitor turns the transistor on and off and how its value was
derived. I must have looked at 100 circuits and descriptions but nothing
explains it in a way I understand, I feel like a bit of a fool.

3. What are the capacitors on the emitters of the transistors on the bottom
circuit for? I am guessing they are to bypass the emitter resistors to
provide higher gain at the oscillator frequency but if that's the case then
why is there one on the frequency doubler circuit made up from Q2? Also,
why does the oscillator on the bottom circuit have the bypass cap while the
top one doesn't?

4. How does this frequency doubler work? Hints on what to search form in
google would be a great help as I have been unable to find much.

Any help would be greatly appreciated.

Best regards

AJ

2. ### Michael BlackGuest

If you mean by the "top circuit", the one at
http://www.ajpotts.fsnet.co.uk/videotx.gif
then that 1N4148 after the 4.7uF capacitor is a clamping diode. Look it
up in a dictionary. The video signal is AC coupled in order to take out
any DC voltage, but the transmitter actually needs that video signal
to be not a negative and positive going signal that you get after the
coupling capacitor, but a voltage that varies from about ground potential
to a more positive level. The diode "clamps" the voltage at just below
ground level (because the diode has a voltage drop, the signal can go
somewhat below zero volts), so the video signal fluctuates from there to a
positive voltage.

One decided they'd use the diode, the other decided not to.

An oscillator by definition is an amplifier with feedback, you need to
take the output signal from somewhere and feed it back to the input of
the amplifier. In both circuits, the base of the oscillator transistor
is at ground for radio frequencies, the 470pF capacitors on the top one
the 1nF capacitor on the bottom one, which means there can't be feedback
to the base. So instead, the output from the collector is fed back to
the emitter, the transistor acting as a common base transistor. If you
don't have that feedback capacitor there, the circuit may not oscillate.
I say "may not" because if you are high enough up in frequency and other
factors are right, there is enough inherent capacitance there to do
the feedback without an explicit capacitor.

The value of the capacitor is enough so that there is enough feedback
to get the amplifier oscillating and keep oscillating. The lower the
frequency, the larger that capacitor will ahve to be, though some of
the rest of the circuit comes into play in determing the exact
value (though "exact" is relative, since it will oscillatoe with
the capacitor varying over a reasonable range.)

Take note that neither of these transmitters are FM. They are both
amplitude modulated, the modulating video signal going to the emitter to
control the voltage there which varies the amplitude of the signal. (In
the top one, the video signal is applied to the emitter of the actual
oscillator, so there is likely to be incidental frequency modulation, but
that is just because it's a really simple circuit.

Your question about FM may be because a common "FM wireless mic" circuit
is to use such an oscillator, and then feed the audio into the base of
the transistor, which does cause the frequency to vary. But that works
because the audio signal is far below the frequency of the actual
oscillator, so the bypass capacitor that puts the base at ground won't
be large enough to "short" the audio signal to ground. But, a video signal
is larger, and by the time you get a low enough bypass capacitor there, if
you were frequency modulating the oscillator, you might find it's too low
and will affect operation of the oscillator.
The top one is just a modulated oscillator, nothing else. If you bypass
the emitter of that one, it will limit the frequency response, and like
I said, a video signal requires a wider frequency response and any bypass
capacitor there would start affecting the input video frequency.

Plus, if you bypass that emitter in the oscillator, you will kill the
oscillation since there will be no place for the feedback signal from
the collector to go but ground.

I see, there is a small value capacitor on the bottom one. That's just
a way of adjusting the feedback, the small value doesn't act as a "short"
at that frequency, but it will reduce the signal at that point, which is
often what you want (since too much feedback causes problems). Think of
those two capacitors, the one from the collector to the emitter and the
one from the emitter to ground, as a voltage divider, similar to two
resistors acting as a voltage divider.

I said above that in some circuits at some frequencies you don't need
the capacitor from collector to emitter, there's enough inherent capacitance
do do the job. Likwise, there is inherent capacitance from the emitter
to ground anyway, so you can see such oscillators without a capacitor
from emitter to ground.

Any amplifier needs to be biased at DC, but what's good for that may not
be good for the AC response of the stage. You could just short the
emitter to ground, which would give the most AC gain, but might not
be the best way to bias the circuit. Put a resistor in the emitter,
it in effect puts the emitter at "ground" for AC signals, allowing full
AC gain despite the resistor.
It in effect distorts the input signal to generate harmonics (ie multiples
of the input frequency) and then has a frequency selective element to
pick off the desired frequency. So let's say the oscilator puts out
a sinewave. There will be relatively little harmonic content (how little
dependong on how pure the sinewave is), but drive a stage hard enough (ie
overload it) and it will generate harmonics of that input frequency. Then
you put a tuned circuit in the collector, to pick off the desired harmonic,
ie multiple of the input frequency.

So in that circuit, the second stage has the output tuned to twice
the input, generating the desired signal in the UHF tv band.

Michael