Positive Gain Slope Amplifiers Compensate for Gain Roll-Off in Wideband Systems
Meeting gain roll-off and gain flatness requirements over frequency is a common problem in many modern-day discrete RF transceivers. Ideally, the gain in the signal path of an RF transceiver should be flat over frequency in the band of interest. However, each component in the RF line-up has a finite bandwidth, which can cause the overall system gain response to roll-off over frequency. This is seen as negative slope in a graph of gain versus frequency. This behavior makes meeting gain flatness specifications for these transceivers very challenging to achieve, particularly over wide bandwidths.
Consider the simplified receiver chain shown in Figure 1. The RF line up consists of a low noise amplifier (LNA) and two gain blocks (RF Amp #1 and RF Amp #2). The finite bandwidth and negative gain slope of the three amplifiers will affect overall system gain and bandwidth of the cascaded line-up. The figure assumes all three amplifiers have the same gain and bandwidth for simplicity. At each stage, the blue curve denotes the gain response of the amplifier on its own. The red curve shows the cumulative response of the LNA and the first gain block, while the turquoise curve shows the cumulative response of all three amplifiers. Each of the RF blocks contribute to gain errors over frequency due to the composite gain roll-off of the RF path line-up.
In practice, designers have at least two techniques to compensate for gain roll-off. One approach is to use fixed equalizers in the signal chain to flatten the gain response by adding attenuation with a frequency response slope roughly inverse to the gain slope. This approach is discussed in detail in Flattening Gain Slope with MMIC Fixed Equalizers. The other approach is to use an amplifier with positive gain slope over the desired bandwidth.
This article will focus on the benefits of positive gain slope amplifiers for managing gain variation over frequency. Pros and cons relative to the equalizer approach will be discussed, and examples from Mini-Circuits’ catalog will be presented. Lastly, applications of positive gain slope amplifiers will be explored.
Equalizers vs. Amplifiers
Fixed equalizers can be a very useful building block for managing negative gain slope in the signal chain. Because they come in a wide variety of precise attenuation slope values, they offer designers flexibility to match the right attenuation slope to their system gain slope to produce a desired combined response. Mini-Circuits MMIC equalizers are available with frequency ranges from DC to 6, 20 and 45 GHz in 2x2mm QFN or bare die format, so they require relatively little board space.
But equalizers come with several trade-offs that must be considered. One tradeoff is that overall RF signal chain gain will be sacrificed in exchange for gain flatness over a wider usable bandwidth. Adding the equalizer in the RF signal chain will sacrifice noise figure performance for an RF receiver, and when used near the PA, will sacrifice transmitter output power. If these limitations are critical to a given system’s performance, it is preferable to use a gain flattening technique that provides gain rather than attenuation.
A positive gain slope amplifier allows the RF receiver to maintain the RF link budget performance for gain, noise figure and dynamic range, and at the same time meet gain flatness specifications over frequency. On the transmitter side, this kind of amplifier avoids reducing the PA power output, again while levelling the system frequency response. Figure 2 shows a generic illustration of the gain response of a wideband receiver, a positive gain slope amplifier and the combined response of the two. Note the flattening effect and increase in overall gain in the composite response. Mini-Circuits has addressed the need for this capability with a unique set of wideband MMIC amplifiers with positive gain slope.
Mini-Circuits’ Positive Gain Slope Amplifiers
One example of a positive gain slope amplifier from Mini-Circuits’ catalog is the PMA-183PLN+. This amplifier covers the 6 to 18 GHz frequency range with roughly +0.21 dB/GHz slope from 6 to 15 GHz and +0.55 dB/GHz slope from 15 to 18 GHz. The gain amplifier gain response is shown in Figure 3. This model also provides excellent noise figure of 1.2 dB and 33 dB directivity. It comes in a 3.5 x 2.5mm QFN package or in bare die format.
A second amplifier in this product category is the AVA-183P+, which covers an even wider bandwidth from 0.5 to 18 GHz. This model exhibits a gain slope of roughly +0.13 dB/GHz from 0.5 to 10 GHz and +0.15 dB/GHz from 10 to 18 GHz. The amplifier gain response is shown in Figure 4.
Both of these products can be used in the RF signal path line-up to compensate for receivers’ and transmitters’ gain roll-off over frequency and widen the overall bandwidth of the signal chain.
Common Applications of Amplifiers with Positive Gain Slope
An example of an RF transceiver chain is shown in Figure 5. The finite bandwidth of the LNA, mixer, IF amplifier, and the PA all contribute to gain errors over frequency due to the composite gain roll-off of the RF and IF signal path line-up over frequency. This especially becomes a problem due to the bandwidth limitations of the available commercial-off-the-shelf (COTS) components on the market to construct these transceivers.
Using an amplifier in the RF line-up with a positive slope can help to widen and flatten the gain response of the RF signal chain, similarly to the effect shown in Figure 2. Using an amplifier in the RF signal chain line-up with a positive slope (blue line) will combine with the receiver response (red line) and produce the composite response (green line) with an extended bandwidth and flat gain response to higher frequencies than the receiver on its own.
Other applications beyond transceiver architectures can also benefit from Mini-Circuits’ positive gain slope amplifier products. One such application is an amplifier driving a long coaxial cable run or Category 3 or Category 5 twisted pair, into a 50W termination load. The bandwidth of long coaxial cables is limited, and the frequency response of the cable rolls off toward the upper regions of the operating bandwidth. Figure 6 plots the attenuation of several different types of coaxial cables over frequency. The Y-axis plots the attenuation, in dB, per 100 feet of several different coaxial cable types versus the frequency on the X-axis. The attenuation slope for the different coaxial cable types in Figure 6 is increasing for increasing frequencies.
A positive gain slope amplifier can be used to compensate for the bandwidth limitations of driving the long coaxial cable. The combination of the positive slope amplifier and long coaxial cable can produce a wideband, flat gain response. Figure 7 shows a schematic illustration of this application.
Negative gain slope is a common and well-known characteristic of wideband transceivers, cable runs and other applications. While designers have multiple techniques to compensate for gain roll-off, amplifiers with positive gain slope have some distinct advantages. Mini-Circuits has developed a unique product family of positive gain slope amplifiers to support these applications. If you have questions about the amplifiers presented in this article or specific requirements for your application, please contact our applications team.