by Richard Lloyd Measures, AG6K.

as of 7/11/98


The following manuscript was originally written under contract for the Amplifier and Projects Chapter (13) of the 1995 ARRL Handbook. About a month before the due date on the contract, I received a phone call from the ARRL Handbook Editor. He told me to stop writing.


Amplification can be defined as: The process of increasing the magnitude of a variable quantity--especially the magnitude of a voltage and/or current, without substantially altering any other quality.

The (pure) sine wave is the only periodic waveform that contains no harmonic energy. All of the energy in a sine wave is contained in the fundamental frequency. In other words, a pure sine wave is coherent. Maintaining the quality of sine waves during amplification is a major concern in the design and operation of amplifiers.

Even though the following discussion pertains to the operation of gridded electron tubes, some of the concepts apply to power FETs because both are voltage-driven amplifying devices.

In the discourse that follows: if a voltage is said to be negative-going, that does not necessarily mean that the voltage is negative. It could be a positive voltage that is moving in the negative direction. If a voltage is positive-going, or moving in the positive direction, it could be a negative voltage or a positive voltage that is moving in the positive direction.

In an electron tube, the cathode emits a cloud of electrons. Since electrons carry a negative charge, and unlike charges attract, electrons are strongly attracted by the positive voltage that is applied to the anode. Unless something is placed in the way, much current flows between the cathode and the anode.

The term "grid" describes appearance--not function. A grid is made from a number of closely spaced wires or bars--like a bird cage. The grid is placed close to the cathode. As a result, the grid has more influence over the cathode's electrons than does the more distant anode. Thus, a small change in grid-voltage produces a large change in the flow of electrons. Because the voltage applied to the grid controls the flow of electrons from the cathode to the anode, in function, the grid acts like a valve or gate. The grid requires virtually zero current to perform its job. As a result, the power gain of a gridded electron tube is theoretically high.

Since like charges repel, a sufficiently negative grid can stop electrons from traveling to the anode. As the grid is made less-negative, the flow of electrons to the anode steadily increases. In other words, a positive-going, albeit negative polarity, grid-voltage causes an increasing flow of electrons between the cathode and the anode. The reverse is also true: a negative-going grid-voltage causes a decreasing flow of electrons from the cathode to the anode. As long as the grid remains negative with respect to the cathode, the relationship between grid-to-cathode voltage and anode-current is fairly linear.

When an appropriate load resistor is connected between the anode and its positive voltage source, the changes in anode-current produced by changes in grid-voltage create a proportional, typically much larger, voltage change across the resistor. The ratio of changes in voltage across the load resistor to changes in grid-voltage is the voltage amplification factor. It is designated by the Greek letter Mu. Since Mu is higher at high anode-voltages than it is at low anode-voltages, average Mu is a more meaningful number than maximum Mu. Average Mu ratings vary from about 2 to 240. Mu is partly determined by the spacing between the grid wires and the distance between the grid and the cathode..

Classes of Operation

The duration of anode-current conduction per cycle determines the class of amplifier operation. A conduction angle of 360 degrees means that the anode is conducting current during 100% of the input sine wave cycle. A conduction angle of 90 degrees means that the anode is conducting current during 25% of the input sine wave cycle. Long conduction angles produce a more linear representation of the input sine wave. Short conduction angles produce more efficiency--and less linearity.

Class A is defined as a conduction angle of 360 degrees. Class B is defined as a conduction angle of 180 degrees. Class C is defined as a conduction angle of less than 180 degrees. The subscript 1 indicates that no grid-current flows. The subscript 2 indicates that grid-current flows--the result of driving the grid into the positive voltage region.

When the conduction angle is less than 360 degrees, the missing part of the sine wave must somehow be filled in. One way of filling in the missing part of the sine wave is by utilizing the flywheel effect of an output tank circuit. Another way to produce a smooth sine wave is to use a push-pull configuration. If each device in a push-pull circuit conducts for at least 180 degrees, a smooth sine wave can be produced.

Class A Operation

Class A is the most linear class of amplifier operation. Class A amplifiers produce only about 1/100,000 part, or minus 50dB, distortion. The theoretical efficiency of a Class A amplifier is 50%. The practical efficiency is slightly lower. Class A is used mainly in low level amplifiers--where efficiency is not much of an issue. Since Class A operates with continuous (360 degrees) conduction, no tank circuit is needed to complete the sine wave. Class A is ideal for wide band amplification.

The zero-signal anode-current [ZSAC] in Class A is set to roughly half of the electron tube's maximum anode-current rating. Although the meter-indicated anode-current remains constant from zero signal to maximum signal, the instantaneous anode-current typically varies from just above zero to c. 6-times the meter-indicated anode-current.

The maximum available power in Class A is roughly equal to the anode-dissipation rating of the electron tube.

The Class A amplifier can be compared with a gas turbine engine. Both have a smooth, continuous power stroke--and neither one is very efficient.

Class AB1 Operation

Class AB1 amplifiers are roughly 60% efficient. The trade-off for increased efficiency is slightly more distortion--roughly 1/10,000 part, or minus 40db. Since most transceivers produce more than minus 36db of IMD, such an amplifier would not add significant distortion.

The anode-current in Class AB1 varies in proportion to the grid-voltage for about 60% of the input sine wave cycle. Thus, the anode-current is off for about 40% of each input sine wave cycle. The missing 40% in the output sine wave is filled in by the flywheel effect of the output tank circuit.

In Class A or Class AB1 a somewhat unusual relationship exists between the work being performed by the tube and the grid voltage. The grid is operated in the zero to negative voltage region. Maximum instantaneous anode-current, maximum anode voltage swing and maximum peak power output coincide with an instantaneous grid-voltage of zero--i.e., maximum stoke equals zero grid volts.

The grid must not be allowed to become positive. If the grid became positive, electrons from the cathode would begin flowing into the grid. Whenever grid-current flows, the linear relationship between grid-voltage and anode-current deteriorates.

Since there is zero grid-current in Class A or Class AB1 operation--and any voltage multiplied by zero amperes is zero watts--the driving power is usually stated as zero on the tube manufacturer's technical information sheet. However, because charging and discharging a capacitor requires current flow, in the real world of conductor RF resistance, charging/discharging the grid capacitance at an RF rate consumes some power. In a typical HF amplifier, the drive power required for Class AB1 MF/HF operation is roughly 1% to 2% of the output power. Thus, the typical power gain is roughly 50 to 100. As frequency increases, conductor resistance increases due to skin-effect. More R causes more I^2 R loss. As frequency increases, the amount of current needed to charge and discharge the grid capacitance also increases--causing even more I^2 R loss. These losses can only be compensated for by adding more drive power.

Drivers (usually a transceiver) require a resistive load--which the capacitive grid does not provide--so a suitable resistance must be connected from the grid to RF-ground..

The zero-signal anode-current [ZSAC] in a Class AB1 amplifier is normally set to about 20% of the maximum-signal single-tone anode-current.

Tubes that are designed for Class A and Class AB1 service produce high peak anode-current when the instantaneous grid-voltage is zero. Typically, the peak anode-current is about three times the maximum rated (average) anode-current. Most of the tubes that are used in Class A and Class AB1 RF amplifier service are tetrodes and pentodes--devices that have the advantage of grid-to-screen amplification. Triodes are seldom used because the only grid designs that can produce high anode-current with zero grid volts are those that have a Mu of 2 to 5. Low Mu triodes require much more driving voltage than a comparable tube with a screen requires. Since imposing a high RF voltage across the capacitive grid is difficult, low Mu triode Class A and Class AB1 power amplifiers are only practical up to a few hundred kHz.

Class AB1 Cathode-Driven Operation

The most common configuration for Class AB1 operation is grid-driven. Since grid amplification as well as grid-to-screen amplification takes place, the resulting power gain is high. Class AB1 amplifiers can also be cathode-driven if the tube is a tetrode or pentode. The grid is tied to the cathode. Thus, the grid-voltage is always 0V--so no grid-current flows. The screen is grounded. The input signal is applied to the cathode/grid. Because input signal voltage is applied between the grid and the screen, grid-to-screen amplification takes place. However, since the grid is tied to the cathode, no grid amplification takes place. Although the power gain is relatively low, linearity is excellent. The Collins 30S-1 is an example of a cathode-driven Class AB1 amplifier.

The Class AB1 amplifier is like a 2-cycle, single-cylinder engine. The power stroke is roughly half of each crankshaft revolution--it has a flywheel (the tank circuit) that supplies power between power strokes--and it is more efficient than a gas turbine (Class A).

Class AB2 Operation

Class AB2 is similar to Class AB1 except that the grid is driven into the positive voltage region during a part of the anode conduction period. A Class AB2 amplifier can be grid-driven or cathode-driven.

Grid-Driven Class AB2 and Negative-feedback

When the grid is driven positive, it attracts and accelerates the cathode's electrons. Some of the electrons stick to the grid, resulting in grid-current. Electrons that miss the grid travel to the anode. The accelerated head start causes a sharp increase in instantaneous anode-current--and a sharp decrease in linearity. The distortion products from a single-ended, Class AB2 grid-driven amplifier are roughly 1/100 part [minus 20db]. In SSB service, this level of distortion is virtually certain to cause interference to other stations using adjacent frequencies. However, by limiting grid-current and by adding an unbypassed, low-L cathode feedback resistor (to develop an out-of-phase {negative} feedback voltage) it is possible to achieve acceptable linearity in grid-driven Class AB2 operation--but only if the grid current produced is small.

An unbypassed cathode resistor is also useful for improving linearity in Class AB1. For example, the 4CX250B has a somewhat objectionable distortion level in Class AB1, SSB service. Adding a 25 Ohm resistor between the unbypassed cathode and chassis ground improves linearity. The trade-off is that slightly more grid drive voltage is needed to achieve the same output level. Cathode negative-feedback is also useful with TV sweep tubes--devices that were originally designed for switching--the opposite of linear amplification. When an appropriate cathode resistor is used with a sweep tube, reasonable linearity can be achieved.

Class AB2 Grounded-Grid Operation

Even though Class AB2 cathode-driven/grounded-grid operation produces grid-current, it is never the less fairly linear due to the laundering effect of negative-feedback. This is the result of the input and output signals being in series with each other and out of phase. Due to the negative-feedback, the distortion level in Class AB2 grounded-grid service is low--typically about 40db below PEP.

High-Mu triodes work well in Class AB2 grounded-grid operation. Medium-Mu triodes can be used, but they have less power gain. Tetrodes and pentodes usually work well in grounded-grid operation. Since tetrodes and pentodes typically have a grid-to-screen amplification factor of about 5, its easy to assume that they offer an advantage over triodes in Class AB2 grounded-grid operation. However, RF-grounding the grid and the screen stops grid-to-screen amplification. Applying DC screen-voltage does NOT increase gain because grid-to-screen amplification can not take place unless input signal voltage is applied between the grid and the screen.

The maximum available power in Class AB2 is roughly double the anode-dissipation rating.

Class B Operation

Class B is defined as a conduction angle of 180 degrees. Class B RF amplifiers produce unacceptable distortion in SSB operation.

Class C Operation

Class C is defined as an anode conduction angle of less than 180 degrees. In Class C, the amplifying device is deliberately not operated linearly. Instead, it is operated as a switch in order to reduce resistance loss. The anode conduction angle in Class C operation is usually made as short as is possible. In effect, the tank circuit makes the RF output sine wave--like a bell that is struck at a constant rate by a hammer. This is similar to the principal behind the spark transmitter.

The efficiency of a typical Class C amplifier is high. When compared to a Class AB1 or Class AB2 amplifier operating at the same power input, a Class C amplifier will deliver a received signal increase of about 1db--in other words, 1/6 of 1 S-unit. However, significant trade-offs are required to achieve that 1/6 of 1 S-unit. As is the case with Class B operation, the distortion from Class C operation is so high that SSB operation is precluded. Only CW, FM or FSK operation is practical. The harmonic output level from a Class C amplifier is substantial. Extra filtering is usually needed to control harmonic radiation.

The maximum available power in Class C operation is roughly three to four times the anode-dissipation rating of the electron tube.

Class D Operation

Class D is used up to about 1.6MHz--mostly in AM broadcast service. In Class D operation, the amplifying device rapidly switches on and off at a fixed rate--like a switching power supply--except that the output voltage varies at an RF rate. The amplitude of the RF is controlled by varying the on period of the switch. Smoothing is accomplished by a complicated filter that converts some of the odd-harmonic energy from the rectangular waves back to fundamental frequency energy. Class D is highly efficient--but it is limited in frequency capability and frequency agility.

Amplifier Design Considerations

Arriving at an amplifier design that will give years of surprise-free service involves many considerations. Merely copying circuits from published amplifier designs or commercial amplifiers is not necessarily the best approach. Doing so may result in copying someone's mistakes. The best approach is to learn what you can about each section of an amplifier, discuss it with others--then reach your own conclusions.

Basic prerequisites for getting a handle on what's going on inside an amplifier are an understanding of Ohm's Law, inductive and capacitive reactance, impedance, resonance, how gridded electron tubes function, and some knowledge of L and pi networks.

A useful book on amplifier design is Eimac®'s Care and Feeding of Power Grid Tubes. I will minimize the discussion of topics that are covered adequately in this book.

Tubes vs. FETs

When the first RF power FETs were introduced, it was commonly thought that FETs would eventually replace bipolar transistors and gridded electron tubes in HF power amplifiers. Since RF power FETs work better at 50V than they do at 12V, FETs have not replaced bipolar transistors in 12V mobile applications. Another difficulty with FETs is cost. A pair of FETs that can produce 1200W PEP at 29MHz cost about six times more than an electron tube, or tubes, that can do the same job. The FETs' input power requirements are 50V at 50A, i.e., 2500W--so there's considerable heat to dispose of. Meeting the cooling requirement is not nearly as easy as it is with tubes because tubes operate quite happily at surface temperatures that destroy silicon devices.

In low power applications at room-temperature, solid-state devices can last 100 years. However, at the junction temperatures encountered in high power applications, the P and N doped layers slowly diffuse into each other--thereby steadily eroding the device's amplifying ability. A relatively-large rigorous cooling system is needed to achieve a reasonable operating life from high power solid-state RF amplifying devices.

Another difficulty with solid-state high power RF amplifiers is their power supply requirements. Tubes are quite tolerant of moderate variations in their anode supply voltage. Transistors, however, are fatally sensitive to over-voltage. It is much easier to build a 3000V, 0.8A unregulated supply for a tube than to build a regulated 50V, 50A power supply with over-voltage, over-current and over-temperature protection circuitry for high-power solid-state devices.

The bottom-line is that 1500W, HF, gridded electron tube amplifiers are more efficient, more forgiving, easier to cool, more compact, weigh less, are more tolerant of high SWR and are less costly than 1500W HF semiconductor amplifiers. For instance, a pair of legal-limit FETs cost about $800 from Motorola®. The efficiency is about 10% less that what one can achieve with gridded electron tubes.

Grounded-Grid Versus Grid-Driven

For at least the last three decades, the vast majority of amateur radio amplifier designs have been Class AB2 cathode-driven--a.k.a. 'grounded-grid'. One reason for this is simplicity--or at least the appearance of simplicity. Ground the grid(s), drive the cathode. Only three supplies are needed--the T-R switching supply, the filament supply and the anode supply. Neutralization is theoretically not needed because the grounded grid(s) shield the output element, the anode, from the input element, the cathode. This theory works almost perfectly. Grounded-grid amplifiers are virtually always stable at the operating frequency because the reactance of the feedback C is too high at HF to allow regeneration. This is fortunate because there is no way of neutralizing a single ended grounded-grid amplifier. Another advantage is flexibility. Almost any tetrode, pentode, or high-Mu triode from the junk box will work. Linearity is usually good and the typical power gain--10db to 14db--is acceptable. So far, so good. Now for the trade-offs.

What goes on inside a grounded-grid amplifier is not as simple as it looks. The AC component of the anode-current and the grid-current, i.e., the RF cathode current, passes entirely through the cathode coupling capacitor and the tuned-input circuit--so the input circuit is in series with (and out of phase with) the output circuit. The components in the tuned circuit must be able to handle a substantial amount of RF current. Manufacturers of tubes that are designed for grounded-grid operation typically recommend using a tuned input pi-network with a Q of 2 to 5. To maintain an acceptable SWR and Q when the operating frequency changes appreciably, all three reactances in the tuned input must change proportionally. However, if Q is allowed to change, L can be left as is providing that C1 and C2 are retuned. [For more information on this problem, see the section titled "Tuned Input Circuits."]

Even though HF grounded-grid amplifiers are stable at their operating frequency, at VHF the grid looses its ability to shield the input from the output. HF grounded-grid amplifiers have a less-than-pristine reputation for VHF stability.

For wide frequency coverage, the Class AB1 grid-driven amplifier requires a much simpler tuned input than a grounded-grid amplifier requires. Typically, grid-driven amplifiers have more power gain than grounded-grid amplifiers. One Class AB1 grid-driven amplifier has about as much gain as two Class AB2 grounded-grid amplifiers in series. The trade-off is two additional DC supplies--a grid bias supply and a screen supply. Both of these supplies need to be adjustable. HV power FETs make this task easy.


There are two types of cathodes--directly-heated and indirectly-heated. In a directly-heated cathode, a ditungsten carbide layer on the hot (c.1800 degrees K) tungsten, alloyed with about 1.5% thorium--a.k.a. 'thoriated-tungsten', filament wire emits electrons. In an indirectly-heated cathode, the filament (a.k.a. heater) heats a metal cylinder that is coated with strontium oxide and barium oxide. This coating is relatively frangible--but highly emissive.

Ditungsten carbide is commonly formed by heating tungsten in an atmosphere of acetylene (C2H2) gas. Carbon atoms in the gas break their electron bonds with hydrogen atoms and bond with tungsten atoms to form ditungsten carbide on the surface of the filament wire. Since it is atomically linked to the underlying tungsten, the ditungsten carbide layer is very durable. During use, the process reverses. Ditungsten carbide gradually looses carbon and changes back to tungsten. Extra heat exponentially accelerates this process. A cathode is worn out when the carbon is mostly used up.

After their cathodes grow tired of emitting electrons, large external-anode amplifier tubes are commonly "recarburized" with acetylene, vacuum-pumped and resealed. This restores full emission. Although it is possible to recarburize a 3-500Z, doing so is not economically feasible. The smallest tube that is currently being recarburized is the 3CX1000A7.

Each type of cathode has advantages and disadvantages. Indirectly-heated cylinder [8877] and planar [3CX100A5] cathodes have much less inductance than a directly-heated cathode made from wires. Thus, indirectly-heated cathodes are more frequency-capable. Some indirectly-heated cathode tubes can perform satisfactorily at 2500MHz. The 3CX100A5 is an example.

Directly-heated/thoriated-tungsten cathodes are more resistant to damage from electrons that bounce off the anode. It's possible to use up to 22kV with the larger thoriated-tungsten cathode tubes. Electrons that have been accelerated by such voltages move at very high velocities. When they strike the anode, they produce X-rays.

A thoriated-tungsten cathode typically warms up in one second, while few indirectly-heated cathodes can warm up safely in one minute--and three to five minutes is not uncommon. For HF operation, indirectly-heated cathode tubes have a much higher cost to watt ratio than thoriated-tungsten cathode tubes. For VHF and especially for UHF operation, indirectly-heated cathode tubes are often the only choice. For super-power HF operation, thoriated-tungsten cathode tubes are the only choice.

Cathodes deserve respect. Filament-voltage and filament inrush current are the prime areas for concern.

Filament / Heater Considerations

For optimum life from a thoriated-tungsten cathode, the filament-voltage should be just above the voltage where PEP output begins to decrease. As a thoriated-tungsten cathode ages, filament-voltage needs to be increased incrementally to restore full PEP. By using this technique, commercial broadcasters typically achieve an operating life of more than 20,000 hours from thoriated-tungsten cathode tubes.

According to Eimac®'s Care and Feeding of Power Grid Tubes, "every 3% rise in thoriated-tungsten cathode filament-voltage results in a 50% decrease in life due to carbon loss." Each additional 3% rise in filament-voltage decreases the life by half. Thus, cathode life is proportional to [E1/E2]^23.4 where E1 is the lowest filament-voltage at which normal PEP output is realized--and E2 is the increased filament-voltage. However, for heater-type oxide cathodes, if the heater potential is allowed to fall below the specified level, the emissive material may flake off of the cathode, and cause a cathode-grid short. On the other hand, excessive heater potential causes barium migration to the grid - which results in primary grid emission.

Controlling Filament-Voltage

It's simple to make the filament voltage adjustable when the filament is powered by its own transformer. All that's needed is a small rheostat in series with the primary. For dual voltage, dual primary transformers, a dual ganged rheostat is required. However, when the filament is powered by a winding on the HV transformer, making the filament-voltage adjustable is more difficult since a dual ganged, very low resistance, high current rheostat must be connected to the low-voltage high-current secondary winding. The typical value needed for a pair of 3-500Zs would be (2) adjustable 0.01 Ohm @ 30A--most definitely not a common rheostat. A reasonable substitute can be made from a double pole, c.10 position, 30A rotary switch and short lengths of resistance wire or ribbon bridging the fixed contacts.

An indirectly-heated cathode can be ruined by operating it below the rated minimum filament-voltage. When operated above its maximum filament-voltage rating, an indirectly-heated cathode boils off emissive material (principally barium) onto the grid and other parts. This results in decreased cathode life and undesirable grid-emission when the grid warms up during transmit. This condition is indicated when the output power steadily drops off in AØ (max. signal, key-down, a.k.a. NØN) operation. The decrease in power normally begins within two seconds.

For maximum cathode life in HF communications service, an indirectly-heated cathode should be operated at the rated minimum filament-voltage. This can be accomplished best with a regulated DC supply.

Controlling Filament-Voltage During Receive

In a typical amateur radio amplifier, the filament-voltage rises about 5% during receive due to decreased load on the electric-mains. Not only is a 5% increase in filament-voltage during receive useless, it uses up thoriated-tungsten cathode emission life about three times as fast as during transmit. However, if an appropriate resistor is placed in series with the filament transformer primary, and shorted out by a relay during transmit, the filament-voltage will not change appreciably between transmit and receive. An appropriate type of relay for shorting the resistor on transmit is a 'power' reed-relay.

Filament Inrush Current

Thoriated-tungsten filaments commonly consist of two vertical intermeshing helices (coils) of tungsten wire that are suspended by their ends. (see Sept. 1990 QST, p.15) The conductance of tungsten at room temperature is about 8.33 times the conductance at the normal operating temperature. Thus, the start-up current for a 15-ampere filament can exceed 100 amperes. Needless to say, 100 amperes makes for a dandy electromagnet.

In a high amplification triode such as the 3-500Z, the filament helices clear the grid cage by a matter of thousandths of an inch. If the position of the filament changes, a grid-to-filament short may result. Therefore, it is prudent to limit filament inrush current in order to minimize thermal and magnetic stress.

Since the grid-to-cathode clearance in an indirectly-heated cathode is not affected by movement of the heater inside the rigid cathode cylinder, indirectly-heated cathodes are not affected by inrush current.

For many of its smaller thoriated-tungsten cathode amplifier tubes--such as the 3-400Z and 3-500Z--Eimac® recommends that filament inrush current be limited to no more than double the normal current. This rating is easily exceeded unless a special current-limiting filament transformer or a step-start circuit is used.