Amplifiers
Most of the modulator and oscillator circuits you’ll use in amateur radio put out a fairly small amount of wattage. It’s the job of the RF amplifier circuits to increase the signal strength of your output. In some cases, you’ll also find pre-amplifiers for increasing gain in your received signal. There are also voltage amplifiers for increasing overall power in your circuits.
There are 6 different classes of power amplifiers, but we will only focus on four for the exam. Those are linear amplifiers in class A, class AB and class C; along with a switching class D amp.
A class A amplifier uses 100% of the input signal and conducts, or amplifies it, over the entire cycle. This may sound good, but it is actually very inefficient. What’s the operating point of a Class A common emitter amplifier? It’s approximately halfway between saturation and cutoff.
Schematic of class A amplifier
The class AB amplifier is a hybrid of class A and class B amps. While class A amps are active for 360-degrees of the cycle, a class B amp is only active for 180-degrees. What’s unique in an ideal AB amplifier? It works on a signal wave in parts. So, the portion of the signal cycle that each active element in a push-pull Class AB amplifier conducts is more than 180 degrees but less than 360 degrees.
Schematic of class AB amplifier
A Class C amp uses less than 50% of the signal and requires a tuned circuit. Class C is not usually used to amplify a single-sideband phone signal. This is because signal distortion and excessive bandwidth is likely to result.
Schematic of class C amplifier
A Class D amplifier is our switching amp. Class D is an amplifier that uses switching technology to achieve high efficiency. Switching amplifiers are more efficient than linear amplifiers because of their power usage. That means the switching device is at saturation or cutoff most of the time.
A switching amplifier has some technical considerations. It should have a filter to remove harmonic content at its output.
Troubleshooting time for amps. How do you prevent unwanted oscillations in an RF power amplifier? You install parasitic suppressors and/or neutralize the stage.
Before we move on from amps, let’s cover three other styles.
- A grounded-grid amplifier is based on a tube design. One characteristic it has is low input impedance.
- An emitter follower, or common collector, amplifier has the characteristic of having input and output signals in phase.
- An operational amplifier is also called an op-amp. An op-amp is a high-gain, direct-coupled differential amplifier with very high input impedance and very low output impedance.
Exploring more on op-amps now. They are DC based and used mostly in the power management side of radios and other equipment.
Schematic of operational amplifier
Terms to know related to op-amps:
- The typical input impedance of an op-amp is very high.
- The typical output impedance of an op-amp is very low.
- “Op-amp input offset voltage” is the differential input voltage needed to bring the open loop output voltage to zero.
- The gain-bandwidth of an operational amplifier is the frequency at which the open-loop gain of the amplifier equals one.
- The gain of an ideal operational amplifier does not vary with frequency.
Matching Network
So, your amplified signal is ready for output to your antenna. Now, a matching network is used to ensure the signal is at an expected impedance, commonly 50-ohms. Remember impedance is resistance in an AC circuit, like your antenna. The basic circuit is the L-matching circuit. L for inductance matching. There are many other versions as well.
Schematic of a L-matching circuit
Sometimes impedance matching is more complicated. Complex impedance is a specific term. It’s when an impedance has a phase shift between the voltage across it and the current through it. How does an impedance-matching circuit transform a complex impedance to a resistive impedance? It does it by canceling the reactive part of the impedance and changing the resistive part to a desired value.
Remember, the “reactive” part of the impedance comes from capacitors and inductors. Impedance matching will allow us to transform the signal to purely resistive. Use inductors or capacitors to cancel out the reactive part of the impedance.
What if you are working with a final amplifier and a 50-ohm unbalanced network in vacuum tube radio? You should use a Pi-network with an additional output series inductor. This is a type of Pi-L network. A tip to remember is that L is the circuit symbol for an inductor. The reason you add an inductor in this case is to get greater harmonic suppression.
