More than one legitimate ham brick manufacturer produced a CB amp under a
different "corporate" name. I assume they used similar engineering
"expertise" with the bias circuits.
BTW, a 1N5408 on top of an output transistor makes a fair temperature
regulator. Not perfect but its cheap and simple as a retrofit of an existing
brick.
Carl
KM1H
----- Original Message -----
From: "Peter Chadwick" <g3rzp@g3rzp.wanadoo.co.uk>
To: <amps@contesting.com>
Sent: Wednesday, October 10, 2007 12:55 PM
Subject: Re: [Amps] CB Amps or full spectrum???
>> If you look at the tank circuit of these amps, you see a pi network that
>> is usually no different than any other amp, except the coil is small
>> turns wise because it is for 11 meters (and close to 10 meters as well).
>> So what kind of "filtering" are you talking about?
> If the pi network is not different than, say, a 30L-1 or an SB 200 or any
> other amp with a pi network, why would harmonics be any more of a problem
> than with other amps?<
> Firstly, if you do the sums for a Class AB tube amplifier with the usual Q
> values, you'll see that you don't meet either the FCC requirements for
> harmonics (46dB) nor the Radio Regulations requirements, which are 4 dB
> tighter at 50dB. Which is why the manufacturers these days use a Pi-L
> network. Not that anyone worries too much about existing equipment, which
> from memory, is, at least as far as the Radio Regs are concerned,
> 'grandfathered' until 2012 anyway. The solid state CB amplifiers, besides
> being pretty awful on IMD, usually have minimal harmonic filtering. In
> fact, you can, in a solid state push pull amp, have reasonable IMD, but a
> 3rd harmonic only 13 dB down. So this is why you need decent low pass
> filters. Now some transistors, presented with the input impedance of a
> filter at the 3rd harmonic, happily oscillate. The Swan 100MX had a
> tendency to do this on 15metres with some antenna loads.
> Biasing the 12 volt transistors is fun anyway. From a DC viewpoint, you'd
> like a constant current source of bias that reduced as the transistors got
> warm and the beta went up. But a constant current source is ideally an
> infinite impedance, and as soon as you apply RF, the rectification then
> alters the bias point. So in practice, you have to end up with a constant
> voltage, very low impedance bias source, which has to fall in voltage as
> the die temperature goes up at approximately 2mV/degree C plus a bit more
> to allow for the increase in beta as temperature goes up. Add to this the
> fact that the RF saturation voltage of the 12 volt transistor isn't that
> negligible, so the available voltage swing is not quite so big as you
> would hope - quite likely down to about 20 volts peak to peak at the
> best - and you can start seeing why the CB amps are so dreadful. At 100
> watts, with 20v p-p, which is about 6 volts rms, the load impedance is
> down to around 0.36 ohms, so losses start to bec
> ome a major problem. So this why people go to 24 or 48 volts and even
> better, to FETs, which tend to be a bit more linear anyway. You can use a
> constant current source of bias with pure Class A, but the bias must not
> change when the RF drive is applied - this is done in integrated circuits
> with driver stages on board for linear cellphone applications, but not
> above about +8 or +9dBm output power.
> To avoid the instability problems, you need to look at the S parameters at
> the harmonic frequencies and do either a Linville or a Stern stability
> criteria calculation when loaded with the impedance that the filters
> present at those frequencies, as well as an all phases VSWR of whatever
> your worst case VSWR is at the operating frequency. At least, this is what
> proper designers do, not the CB amp makers, most of whom, I suspect, have
> never even heard of Linville or Stern or stability criteria! The exercise
> also tells you how much margin you have on the stability. There are
> stability criteria equations for tubes, too.
> 73
> Peter G3RZP
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