Amplifiers come in all shapes and sizes. The key characteristic of virtually every amplifier is its gain and fidelity of the output relative to the input. Gain, denoted by β (Greek letter beta), is the ratio of output voltage, current or power to input. An amplifier by definition has a power gain greater than one. As opposed to signal gain, power gain can be elaborated as operating power gain, transducer power gain or available power gain, in all cases average rather than instantaneous.
A two-port operating power gain is:
Gp = Pload/Pinput
where Gp is the operating power gain, Pload is the maximum power delivered to the load, averaged over time, and
Pinput is the time-averaged power at the network input. The transducer power gain, GT, is:
GT = Pmaximum load/Pmaximum source
where Pmaximum load is the maximum available power at the load, Pmaximum source is the maximum power available from the source.
Gain and fidelity stand in an inverse relationship. In other words, higher fidelity comes at the expense of lower gain and vice versa. Fidelity, however, is a subjective measure of the agreement of input and output in regard to several properties not including gain. These properties are, in part:
Bandwidth, the span of useful frequency range
Frequency response, related to bandwidth but measured on a different scale
Linearity, a comparison of the proportion between input and output that is the same for high amplitude and low amplitude input
Noise, a measure of undesired signal(s), internal or external, present in the output
Slew rate, the maximum rate of change in the output
Rise time, settling time, ringing and overshoot
Stability, freedom from oscillation
Gain is normally specified for the linear portion of the amplifier operating range where the change in output power is linear with respect to a corresponding change in input power. Increasing the power level of an input signal forces the amplifier to start transitioning into a nonlinear mode and generate spurious frequency components. These undesired components, which include harmonics and intermodulation products, represent intermodulation distortion (IMD) at the amplifier output. The ability of an amplifier to handle different input power levels without introducing significant distortion is its linearity. Linearity can be expressed in terms of parameters that include:
The output 1 dB compression point (OP1dB), which defines the output power level at which the gain of a system decreases by 1 dB.
The saturated output power (PSAT), the power level when a rise in input power does not change the output power level.
The second-order intercept point (IP2) and third-order intercept point (IP3), which are hypothetical points for the input (IIP2, IIP3) and output (OIP2, OIP3) signal power levels at which the power of the corresponding spurious components would reach the same level of fundamental components.
The process of selecting an amplifier typically involves trading off different design parameters. The trade-offs have given rise to some specific categories of amplifiers. Among the most common types:
Low-noise amplifiers (LNAs) are optimized to amplify weak signals. A typical application for an LNA is with low-level transducer signals or in the first stage of an RF receiver front-end where it boosts the signal power coming from the antenna. Typical LNAs incorporate an impedance-matching block for the input/output section and an amplification block. The general idea is to minimize the noise of the amplifier for a given signal source impedance to approach the transistor minimum noise figure. Because silicon FETs have a low inherent noise, they are often used in LNAs.
Low-phase-noise amplifiers are often found in RF signal chains requiring high signal integrity. Phase noise is close-in noise to the carrier which appears as jitter characterized by small fluctuations in the phase of a signal in the time domain.
Basically every component in the system can degrade the phase noise. There are certain tricks of the trade employed to minimize the contribution of phase noise sources. For example, the power transformer’s magnetic field can cause problems, so designers often use a toroid design, perhaps with an internal electrostatic shield. Even system power supply noise can modulate devices enough to hamper performance. Some devices have a reputation for generating noise on grounds, so designers will route the current paths of noisy devices to keep them away from sensitive components.
Wenzel Associates gives a quick way to estimate phase noise when a predominant carrier is present: The phase noise far Sθ from this carrier may be estimated by adding the amplifier noise figure to -174 dBm and subtracting the signal level (all in dB). For example, a +3 dB noise figure amplifier with a -20 dBm input level would exhibit an Sθ of -174 + 3 – (-20) = -151 dBc. But for complex signals or frequency offsets near a carrier the MMIC will exhibit 1/f or “flicker” phase modulation.
One particular type of amplifier crops up in RF superheterodyne circuits, the high linearity amplifier. The high linearity refers to amplifier behavior specifically within the superheterodyne circuit and requires a bit of explanation.
All superheterodyne receivers use an intermediate frequency, IF, between the input of the antenna and the amplification stages. When a single tone is received it is processed so the signal can be more easily demodulated. What’s called the theoretical third-order intercept is determined when two signals go into the input signal port of the receiver. These signals then pass through a mixer. The mixer produces tones that consist of the differences or sums of those signals and which are referred to as the IM (intermodulation) products. Some of these tones are within the IF frequency band. Consequently, they will pass through the IF amplifier (IFA) which follows this mixer.
One particular IM product plays a pivotal role in the signal processing: The second harmonic, which is related to the third-order tones, which then leads to the third-order intercept, IP3. As signal strength is increased by 1 dB, the IM products also rise but by 3 dB. The third-order intercept point is the output signal level (through extrapolation) at which the third-order tones would have the same amplitude level as the desired input frequencies. If the RF system reaches this theoretical point, the mixer output signal becomes distorted. Then stronger input signals won’t generate bigger output signals.
This is a problem because the frequency near the signal to be detected has distorted the information on the carrier signal. The IM products cause this distortion and the noise that follows, not the level of the
signals. The third-order intercept point is a calculation of the level of the signal vs. that of the unwanted IM products.
Enter the high-linearity amplifier. Higher linearity offers less distortion. The better the IP3, the better the receiver’s ability to separate and discern several tones that are processed simultaneously inside the passband.
The Third Order Intercept Point is determined by plotting input power vs. the output power. The two curves are then drawn, one for a nonlinear product, the other for the linear amplified signal. Alternatively, the IP3 is a figure-of-merit that characterizes receiver tolerance to several signals that are present simultaneously inside the desired passband. The IP3 is a power level, typically given in dBm, and is closely related to the 1 dB compression point.
There are a few other widely used amplifier types that should be mentioned. Variable gain (sometimes called voltage controlled) amplifiers, or VGAs, find use where there’s a need to accommodate wide signal level variations, i.e. a high dynamic range. Examples include ultrasound, speech analysis, radar, wireless communications, and instrumentation. Gain that can be changed either digitally in steps or continuously by using analog control. This type of amplifier often finds use in automatic gain control (AGC) and gain-drift compensation caused by variations in temperature or the qualities of other components.
Wideband amplifiers are typically RF amps. As the name implies, they work over a range of frequencies that can span hundreds of megahertz or more. The main difference between an ordinary RF amp and a wideband version is typically in the input and output matching networks. These must be designed to present the necessary complex conjugate impedance over a broad range of frequencies to keep the amplifier stable. The result is generally a large gain bandwidth product that usually comes at the cost of mediocre efficiency and noise.
Finally, we should mention gain blocks. Gain blocks basically amplify an input signal by a fixed gain that is flat over a specified frequency range. Gain blocks typically operate in the RF range, but there are different types. Traditional RF gain blocks usually are designed for 50/70-Ω terminations. There are also IF gain devices usually designed with 200-600-Ω terminations for use with saw, crystal, ceramic and mechanical filters. Gain blocks often include matching and biasing circuits to minimize the number of external components needed to integrate them into a signal chain.