>
>At 09:33 AM 2000-02-14 -0800, Steve Thompson wrote:
>>In my experience, the formula holds at 0.5-2dB gain compression, but only
if
>>the amplifier if truely linear up to when it starts to compress. If the
>>formula is mathematically derived, it has to assume linearity up to V/2,
>>then look at compression at full output. It can't differentiaite between
>>hard and soft compression either (although the effect in practice wasn't
>>that great in my tests).
>
>It is not commonly realized that "soft" compression is due to harmonic
>energy. Power meters cannot differentiate between the fundamental and
>harmonic power so the harmonic energy contributes to the total power
>measured. In addition, grounded-grid amplifiers feed through driver energy
>which is also measured.
>
>The only way to measure fundamental power without harmonic contribution is
>with a tuned receiver or spectrum analyzer.
>
>>Many power mosfets have gain which drops
>>continuously with power (Po/Pin is banana shaped)
>
>If this is being observed with an untuned power meter, it is likely because
>MOSFETs and FETs readily generate copius amounts of odd-order harmonic
>energy, most notably 3rd and 5th harmonics. Helge Granberg wrote about this
>characteristic extensively in the earlier Motorola RF Power Transistor data
>book application notes.
>
>>.. latest LDMOS has an 'S' shaped Po/Pi characteristic at most bias
levels.
>
>Likely the same problem: untuned power meter and harmonics being measured.
>
>It's also not commonly realized that spectrum analyzer measurements are
>notoriously inaccurate at amplitude measurement; even the latest Hewlett
>Packard instruments with digital readout of the cursor's location are
>specified to no better than about +/- 1 dB accuracy. This inaccuracy can be
>reduced considerably by comparing the power measured by the analyzer to a
>known, clean signal source level established by a power meter.
>
>>Most bipolar amps are
>>dreadful because the particular transistor isn't suited to linear
operation
>>and/or the biassing arrangements are rubbish.
>
>Bipolar RF power transistors are built with and without emitter resistance
>which introduces negative feedback. The emitter resistance helps raise the
>real part of the base resistance for easier matching. Motorola called these
>devices "Controlled Q". A very-low-Q, low-pass network is usually used to
>connect the base junction to the base lead of the package and raises the
>base impedance further (in V/UHF transistors, this network is often the
>reason why the high frequency response drops drastically beyond the
>intended operational frequency range).
>
>>Tubes aren't exempt - poor bias and/or anode voltage regulation, or wrong
>>loading can mess things up a treat.
>
>Tetrodes driven into drawing grid current will cause the grid bias to shift
>further negative if the bias supply does not have sufficiently-low source
>impedance, both positive AND negative. Take the common 4CX250B, for
>example. The typical grid bias regulator is a zener with a shunt 10K
>potentiometer, the bias taken from the pot wiper. These tubes typically
>require a grid bias of between -50 and -80 volts, and the zener voltage
>most-commonly used is 100 or 105 volts. Thus, there is a differential
>voltage of 20 to 50 volts between the control grid and the zener diode
>itself. A 10K ohm potentiometer used to adjust the bias voltage results in
>a resistance of between 2000 and 5000 ohms in series with the grid to the
>zener-regulated source. If the grid current is just 1 milliampere, Ohm's
>Law shows that the voltage developed between the grid and zener will be
>between 2 and 5 volts, a barely-acceptable variation. If the grid current
>is 5 milliamperes, the voltage difference between the grid and zener will
>rise to between 10 and 50 volts, clearly a very undesirable state of
>affairs. A shunt regulator is necessary to prevent the bias from being
>more-negative.
>
Preventing grid-bias from becoming more negative increases grid current.
Grid current means distortion. Why not adjust the grid bias potential
so that there is zero grid current with full drive? ZSAC is set by adj.
screen potential.
cheers, Steve
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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