Grounded Grid Amplifiers
Also see: Tuned input systems
I’ve constructed RF power amplifiers since I started in amateur radio nearly
50 years ago, my first transmitters and receivers were all homebrew. My first
actual designs were in a technical college electronics lab in the late 60’s,
where as a lab project associated with engineering courses I designed a grid
driven neutralized 7094
PA stage. The 7094 was quite a large tube for me back then, and
it was interesting learning how to use information in data sheets to calculate
and estimate component values.
Over the years, especially with the recent decline in vacuum tube quality,
I’ve come to firmly believe the only place to connect the grid in a grounded
grid amplifier is directly to ground!
The grid not only shields the input from direct RF feedback from the anode,
it is also a good shield to prevent or minimize the anode voltage that might
appear on the cathode during tube arcs. Floating the grid above ground is bad
for RF, and bad for arc protection.
The only thing preventing full anode voltage from
appearing on the cathode of the 3-500Z is the ground on the control grid.
The control grid ground is also the single most critical connection for
stability. The grid connection to ground should always be as wide and short as
possible, and use as many pins as possible.
One indication of a good design and/or knowledgeable designer is how well the
grid is grounded.
Grounded Grid Amplifiers
An amplifier with the input applied between the cathode and grid and the
output between the anode and grid is called a grounded-grid amplifier. This is
true even when the amplifier does not have a directly grounded grid.
The grid RF reference point, which is the chassis ground, is the common
reference for both input and output power.
Grounded Grid Amplifier Power and Efficiency measurements
The input and output circuit of a grounded grid amplifier are
connected in series through the tube. Plate current is common through both
cathode and anode, and only dc plate voltage is not.
Back when we measured power as plate input power and not RF
output power, the FCC even had a rule similar to this. The FCC wanted driver
plate input power to be included as a full part of power amplifier plate input
power. Thus kilowatt grounded-grid amplifiers, like the Heathkit SB220, when
driven by 100-watt exciters could only run 900 watts input if the operator
wished to comply with FCC rules.
Probably based more on FCC conservatism than actual operation,
a few widely accepted handbooks and authorities claim driver power adds to
output power via feedthrough and is not accounted for in the metering system.
Thinking unclearly, these books propose full driver power be deducted from output power
when calculating efficiency.
In actual operation, all extra current
contributed by the exciter is fully accounted for in the plate current metering.
The only thing not accounted for is a portion of the average cathode-to-grid
voltage, which directly adds to the anode-cathode voltage during negative
cathode swings. During positive cathode voltage excursions the gird is more
negative compared to the cathode, so the tube cuts off. Since the tube is just
“coasting”, the positive cathode swing does not detract from effective operating
anode voltage. This asymmetrical tube conduction causes the RF voltage between
cathode and grid to contribute to amplifier output by adding effective
anode-to-cathode voltage without the additional voltage showing on meters.
If we look at this circuit we can see what that happens,
because the anode and cathode are indeed in series! As in all series circuits,
current is the same at all points in a mesh or loop. We only have to insert an
anode current meter in the anode or in the negative rail of the HV supply to
measure the full effect of drive power on plate input power. The metering
shortfall is confined to measuring effective anode-to-cathode voltage, since the
meter connects from HV to ground (to the grid, not to the cathode). The meter
also cannot read the time-varying cathode voltage accurately because it is the
wrong type of meter (it can’t read average voltage) and it is connected across
the wrong two points.
HV is read from grid to anode, while the signal source is in series with the
HV supply, adding grid-cathode RF voltage to effective high voltage on negative
swings of the cathode. Since the tube conducts heavily during negative cathode
swings, that is also when the extra voltage provides the largest contribution to
output power.
Gain of a Grounded Grid Amplifier
With the output and input in series, a grounded grid amplifier
has large amounts of negative feedback. This negative feedback reduces
distortion and stage gain.
The
dc plate current of a grounded grid stage is found by Ip =
(µ+1)Es / Rp + (µ+1) R1 + Zload
The input impedance of the stage, neglecting
capacitances, is Zin = Eg / Ip = Rp + Zload / µ + 1
Thus the input impedance is the total plate
circuit impedance (rp + Zload) divided by µ+1.
Power gain is given by Eout^2/Rload
/ Eout^2 (µ+1) / (Rp + Rload)
The larger the ratio of Zin to Zout and the higher the mu, the greater the
stage gain. This means two things influence gain of a grounded grid stage:
The ratio of R2 (output load impedance at the
anode plus input resistance) to input resistance (driving impedance) of the
tube. The lower cathode driving impedance is, for a given anode operating
impedance, the higher the stage gain. µ primary is important because it
determines driving impedance of the grounded grid stage.
Thus we understand why high µ tubes, especially
when the tube also operates with high anode voltages (to create a high anode
impedance), have low input
impedances and high gain. The 3CX800A7 would fit this category and is a fairly
high gain tube with relatively modest anode voltages. The 3CX1200 series
and electrically similar 3-1000Z are lower gain tubes, even when operating at more than twice
the anode voltage of a 3CX800. The 4-1000A is an especially low gain grounded
grid tube for a given anode voltage, because it has low µ and that causes a high
cathode drive impedance.
The YC-156 3CPX5000A7 has a particularly low
cathode drive impedance, and thus has very high gain in grounded grid at high
anode voltages.
Changing µ (mu) has the following
effects:
| Reducing µ generally | Increasing µ |
| increases the effct of tank tuning and load impedance changes on input impedance |
decreases variation in input impedance with load and tank system variations |
| increases negative feedback, which reduces distortion | decreases negative feedback, which increases distortion |
| increases idle or quiescent current for a given bias and anode voltage |
decreases idle or quiescent current for a given bias and anode voltage |
| increases grid current needed for a given power, which increases distortion |
decreases grid current needed for a given power, which decreases distortion |
| decreases gain | increases gain because higher mu decreases driving impedance |
Cathode voltage swing depends on the grid to cathode voltage required to move
the tube through the conduction desired or required.
Grounded Grid Tetrode General Example
Let’s assume we have a tube operating in AB1 with a grid-to-cathode bias
voltage of -50 volts. As long as the grid voltage is not very much positive with
respect to the cathode, the grid-cathode path will not show grid current. Sine
we know the negative grid to cathode voltage is 50 volts, we know the negative
cathode swing cannot exceed around 50 volts on peaks. Otherwise the control grid
will start looking positive with respect to the cathode.
This means the cathode could not have much more than 50 volts negative on
peaks. With a sine wave, this is 100 volts peak-to-peak voltage at the cathode.
This would take the grid right up to the point of zero current, where any
additional voltage would cause grid current (because the grid would go
positive).
So the cathode voltage would be about 35 volts RMS, or 50 volts peak, or 100
volts peak to peak.
In this schematic, the AC source couples through C1 and superimposes AC
voltage across L1. This makes the end-to-end voltage of L1 swing between -50 and
+50 volts.
When A is positive with respect to B, the grid is more negative with respect
to the cathode. This is because Eb adds in series aiding with the voltage from A
to B. It is just like two batteries in series aiding at that instant of time.
This cuts the tube plate current off, because bias in now -100 volts.
When the drive (source) voltage swings negative, A is negative with respect
to B. This is now like two batteries in series, the voltage across L1 and the
voltage across Eb, but opposing each other. At the driver positive sine wave
crest, these voltages subtract to zero volts. This is not enough to cause grid
current (grid does not go positive with respect to cathode) but it does cause
very high plate current.
Note that the more positive direction cathode voltage swing reduces effective
screen to cathode and anode to cathode voltages, while a negative swing (the
same direction that tends to decrease negative control grid bias) increases
effective screen and anode voltages to cathode! This works in the same direction
as control grid to cathode voltage, also tending to increase or decrease anode
current in step with the effects on the control grid. This could be considered
positive feedback, although the amount is slight for higher anode and screen
voltage tubes (higher compared to grid bias) .
Distortion and Gain Reducing Negative Feedback
All plate (and screen) current flows through L1 in the cathode system. If we
have an effective RMS time-varying signal
anode current of 1 ampere, the generator (source) would have to force 1
amp of RMS current to counter act that current. This is the current varying
throughout the RF cycle, not the steady or average indicated dc plate current.
If you pay attention to polarities, you will probably be able to see how this
works. The anode path RF current is out-of-phase with what grid excitation
requires to create that RF anode current. Since the control grid draws no
current, we are left only with the out-of-phase anode current and a negligible
amount of screen current. This system requires around 35 volts RMS into the
cathode to swing the grid-cathode voltage to the edge of grid current (AB2).
Thus we would have about 35 volts at 1 amp RMS, or 35 ohms driving impedance.
Driving impedance in this case would be 35 ohms, which is 1 amp RMS, 35 volts
RMS, and of course 35 watts average power.
From this you can see how the grid bias point necessary for proper bias ties
into gain and drive power. If the tube required 100 volts or so bias in AB1, and
had the same anode effective RMS current, drive power would double. We would
need 70.7 volts RMS drive voltage, instead of 35 volts, at the same 1 ampere
RMS.
The RMS voltage into the tank would be the effective one ampere RMS signal
current times anode impedance presented to the anode by the tank system and
load. Let’s assume that impedance presented to the anode is 3000 ohms. From
this, we can see load power is 3000 watts. It also follows gain is 3dB less if
anode current is the same and we double bias, because we have twice the required
grid voltage (cathode voltage swing) for the same anode current when the tube
requires double bias voltage. I’ve neglected minor things like screen current
and amplification factor to make this simple.
From this we can seen why gain of any
cathode driven amplifier varies greatly with the ratio of anode load impedance
and cathode driving impedance.
As an interesting side note, this is why the gain of most cathode-driven tetrode
amplifiers does not change greatly with or without use screen and grid bias
voltages. I learned this lesson when I had a pair of 4-1000A tubes cathode
driven. They were difficult to drive to full output because cathode voltage
swing required to move the tubes through the normal operating load line was so
great. In grounded grid without screen voltage, the driving impedance per tube
was just over 100 ohms.
In an attempt to improve linearity and reduce drive power requirements, I
built a screen and bias supply. These tubes required about 130 volts of control
grid bias when screen voltage was applied. After designing and constructing a
very good bias and screen supply, vacuum tube regulated with 6146 regulator
tubes, drive requirements barely changed. Of course IMD products were reduced
greatly because the tubes had much better ratios of screen to control grid
current (control grid current went to zero mA).
The only way to increase gain was to move the high voltage up over 6000 volts,
which of course would increase anode impedance for the same RF power level.
Grounded grid tetrodes are much cleaner than grid driven tetrodes. This he
negative feedback in the anode current flowing through the input system. This is
also why we want a tuned input right at the cathode, not three feet or 15 feet
away like many assume is OK. A tuned input a long distance away can look good
for SWR and drive power, but it can increase IMD and can put a big bite on
efficiency. People who never measure anything but SWR will pipe up and say “I
use an external tuner and it works fine” or they don’t use a tuned input at all,
because they only measure SWR.
When we don’t measure or attempt to observe problems, we assume there are no
problems.
Metering of a Grounded Grid Amplifier
See this link: