On July 1, 2020, this post will be obsolete. See the corresponding post from my 2020 version of No Nonsense Extra Class License Study Guide.
There are three fewer questions in this section. The question on klystrons, for example, was removed. Some of the other questions were changed…Dan
E7B – Amplifiers class of operation; vacuum tube and solid-state circuits; distortion and intermodulation; spurious and parasitic suppression; microwave amplifiers; switching-type amplifiers
There are several classifications of amplifiers, based on their mode of operation. In a class A amplifier, the transistor is always conducting current. That means that the bias of a Class A common emitter amplifier would normally be set approximately half-way between saturation and cutoff on the load line. (E7B04)
In a class B amplifier, there are normally two transistors operating in a “push-pull” configuration. One transistor turns on during the positive half of a cycle, while the other turns on during the negative half. Push-pull amplifiers reduce or eliminate even-order harmonics. (E7B06)
A Class AB amplifier operates over more than 180 degrees but less than 360 degrees of a signal cycle. (E7B01) Class B and Class AB amplifiers are more efficient than Class A amplifiers.
Class C amplifiers conduct over less than 180 degrees of the input signal. This type of operation distorts the output signal, but it is very efficient. Up to 90% efficiency is possible.
A Class D amplifier is a type of amplifier that uses switching technology to achieve high efficiency. (E7B02) The output of a class D amplifier circuit includes a low-pass filter to remove switching signal components. (E7B03)
Amplifiers are used in many different applications, but one application that is especially important, at least as far as signal quality goes, is RF power amplification. RF power amplifiers may emit harmonics or spurious signals, that may cause harmful interference.
One thing that can be done to prevent unwanted oscillations in an RF power amplifier is to install parasitic suppressors and/or neutralize the stage. (E7B05) An RF power amplifier can be neutralized by feeding a 180-degree out-of-phase portion of the output back to the input. (E7B08) Another thing one can do to reduce unwanted emissions is to use a push-pull amplifier.
In order to preserve signal integrity, amplifiers used as the final amplifier in an amateur radio transceiver, or as an external amplifer, are Class A or Class AB linear amplifiers. The use of non-linear Class C amplifiers is not a good choice. The reason for this is that signal distortion and excessive bandwidth is a likely result when a Class C amplifier is used to amplify a single-sideband phone signal. (E7B07)
Although transistorized linear amplifiers are becoming more common, many high-power amplifiers still use vacuum tubes. These amplifiers require that the operator tune the output circuit. The tuning capacitor is adjusted for minimum plate current, while the loading capacitor is adjusted for maximum permissible plate current is how the loading and tuning capacitors are to be adjusted when tuning a vacuum tube RF power amplifier that employs a pi-network output circuit. (E7B09)
The type of circuit shown in Figure E7-1 is a common emitter amplifier. (E7B12) In Figure E7-1, the purpose of R1 and R2 is to provide fixed bias. (E7B10) In Figure E7-1, the purpose of R3 is to provide self bias. (E7B11)
In Figure E7-2, the purpose of R is to provide emitter load. (E7B13) In Figure E7-2, the purpose of C2 is to provide output coupling. (E7B14)
Thermal runaway is one problem that can occur if a transistor amplifier is not designed correctly. What happens is that when the ambient temperature increases, the leakage current of the transistor increases, causing an increase in the collector-to-emitter current. This increases the power dissipation, further increasing the junction temperature, which increases yet again the leakage current. One way to prevent thermal runaway in a bipolar transistor amplifier is to use a resistor in series with the emitter. (E7B15)
RF power amplifers often generate unwanted signals via a process called intermodulation. Strong signals external to the transmitter combine with the signal being generated, causing sometimes unexpected and unwanted emissions. The effect of intermodulation products in a linear power amplifier is the transmission of spurious signals. (E7B16) Odd-order, rather than even-order, intermodulation distortion products are of concern in linear power amplifiers because they are relatively close in frequency to the desired signal. (E7B17)
One type of amplifer that is often used as a power amplifier is the grounded-grid amplifier. Grounded-grid amplifiers are relatively easy to build, and they are very stable in operation. One characteristic of a grounded-grid amplifier is low input impedance. (E7B18)
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