Discrete General Purpose Amplification Overview

Discrete General Purpose Amplification Overview
Discrete General Purpose Amplification Overview:
Design Considerations, the Traditional Darlington
Solution and Better Alternatives
September 12th, 2013
Written By Alan Ake, Skyworks Solutions, Inc.
Even with today’s increasing levels of RF integration, there remain countless applications
for discrete amplifier design including intermediate frequency (IF) amplifier chains at a
few hundred MHz, high performance receive and transmit RF chains well into the GHz
range, amplifier chains for cellular repeaters requiring 80 dB of cascaded gain and LO
buffer applications requiring high isolation and low noise figure (NF) to name a few.
The term “general purpose amplifier” could be defined as a device in the amplifier chain
mainly providing sufficient gain, acceptable cost and with NF and linearity levels that are
adequate from a cascaded system perspective. Still, every attempt should be made to use
the highest performance solution available given the cost constraints of the application.
The classic solution for general purpose amplification is the broadband Darlington
architecture amplifier, and many designers favor it because of the following
1) Ease of Use: RF matching and biasing of these devices is usually trivial, requiring
only DC blocking caps on the input and output along with a bias inductor large enough to
act as a broadband choke. Internally matched to 50 ohms, these devices typically display
return losses >10 dB over a wide range of frequencies. In the end, the minimal up front
effort required to use these devices may be their main advantage
2) Flat Gain: Through resistive feedback and careful transistor sizing, Darlington
amplifiers can display flat gain and good return losses simultaneously over bandwidths of
a few GHz.
3) Low Cost: At the lower end of the performance scale, Silicon or Silicon Germanium
Darlington devices can be very low cost. Higher performance InGaP HBT Darlington
devices with integrated bias are significantly more expensive but still much less than 1
That said, there are several key disadvantages that radio designers should bear in mind
when considering the use of a Darlington amplifier:
1) Inefficient: Given comparable bias voltages and currents, common emitter (CE) based
InGaP HBT linear amplifiers can offer much higher third order intercept points and 1 dB
gain compression points.
2) High out of band gain: The vast majority of applications only require percent
bandwidths of < 20% thus the broadband gain characteristics can be detrimental to
system stability especially at high frequency. Percent bandwidth is simply the useable
bandwidth divided by the center frequency. For a 140 MHz IF amplifier application, of
what benefit is the high gain of the Darlington at 5 GHz?
3) Poor Directivity: Defined as |S(2,1)+S(1,2)| in dB, this parameter gets to one of the
key deficiencies of the Darlington amplifier. A typical Darlington gain block may have
an S(2,1) of 20 dB and an S(1,2) of -24 dB for a directivity of only 4 dB. This low
directivity makes Darlington amplifiers poor choice for buffer applications. Common
emitter (CE) based amplifiers typically have directivities of 7 dB or more and cascode
structure amplifiers in either HBT or more commonly pHEMT typically display
directivity values of 15 dB or more. As directivity increases, the amplifier becomes more
and more unilateral and thus the input and output matching can be considered separately.
This high isolation from input to output implied by high directivity also implies greater
stability for the device.
Considering the above advantages and disadvantages of using a Darlington device in a
design, are there better alternatives to use for most of these applications? The answer is
yes and here are two of the best solutions for general purpose amplifier applications:
1) CE based InGaP HBT linear drivers: Although sometimes offered in the three lead
SOT-89 package, look for devices in small DFN and QFN packages that offer such
features as bias current control that is independent from the supply voltage as well as a
low current power down feature. With high efficiency and superior linearity, these
solutions typically have lower NF and higher directivity than Darlington devices.
Although they do require some simple reactive matching, these devices can typically
achieve high performance tunes of greater than 10 percent bandwidth which is sufficient
for most applications. One item regarding device selection: make sure to consider the
input RF power handling capability of any HBT device you choose to avoid potential
ruggedness issues.
2) Cascode pHEMT Amplifiers: Typically marketed for their low noise characteristics,
devices of this type are, for many applications, the ideal solution for high performance,
general purpose amplification. Requiring simple external matching, these devices
typically offer bandwidths of 30% or more with excellent linearity and return loss
performance. Noted previously, these cascade amplifiers typically offer high directivity
of 15 to more than 20 dB in some cases yielding an extremely stable and essentially
unilateral design. Again, look for devices offered in QFN or DFN packages with the
ability to set the device current independent from the supply voltage. This allows Idd to
be adjusted to suit the linearity requirements of a design resulting in optimal flexibility
and efficiency. Furthermore, these cascade pHEMT devices display excellent thermal
performance and ruggedness allowing operation in temperature environments as high as
105 degrees C and with RF inputs greater than +20 dBm. The sub 1 dB NF is an added
Skyworks offers a large portfolio of excellent InGaP HBT and GaAs pHEMT amplifiers
that are ideal choices for high performance general purpose applications. These devices
are offered as families of related parts all using the same package and layout to cover
applications from a few hundred MHz up to 4 GHz. These solutions offer linearity, NF,
efficiency and directivity that are far superior to Darlington amplifiers at comparable
price points.
Skyworks’ applications engineering regularly provides custom solutions which are
tailored to specific application requirements and we will be happy to assist with device
selection and to optimize any of our devices to your specification.
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