The Evolution of the Push-Pull RFPA
Steve C. Cripps
Cardiff University, Cardiff, UK
Abstract — The evolution of the push-pull power amplifier is
summarized. The technique would appear to have originated
about 100 years ago, and quickly became the default circuit
configuration for early vacuum tube audio amplifiers. Its use has
also been widely adopted for implementing power amplifiers at
radio frequencies (“RF”), but the key requirement for near-ideal
magnetically coupled transformers has halted the progress of the
push-pull amplifier into the GHz bands. When transformers are
replaced by balun structures, based on the properties of
transmission lines, the assumed low frequency advantages of
push-pull operation can quickly be lost. In particular, little
attention has been paid to the response of balun stuctures at
harmonic frequencies.
Index Terms — power amplifiers. Baluns, transformers.
frequencies as to which “benefits” a push-pull PA can still
offer.
II. THE CLASSICAL PUSH-PULL PA
The basic classical push-pull amplifier is shown
schematically in Fig. 1. A pair of identical devices is
connected between two transformers which have ideal
coupling properties.
I. INTRODUCTION
The push-pull RFPA is an iconic electronic configuration,
whose origins are difficult to pin down with any certainty.
Priority is usually assigned to Colpitts, from a 1915
U.S,Patent [1]. As such we can celebrate the centenary of the
push-pull amplifier this week. The patent, optimistically
entitled “System for the Transmission of Intelligence”, does
not however appear to recognise the specific benefits of pushpull operation, despite showing clear use of the configuration
in the various schematics. An earlier patent from 1895 [2]
predates commercially available vacuum tubes but appears to
show a deeper appreciation of the principles and benefits of
push-pull operation. The push-pull configuration was clearly
already in wide use when early amplifiers and radio sets using
vacuum tubes became consumer items in the 1920’s; the RCA
“Radiola” being a well cited example, of which wellpreserved specimens in good condition are still offered for
sale on antique radio websites.
The evolution of push-pull operation into higher radio
frequency applications has not been universal. Indeed today it
is only quite recent research that has reported successful
implementations at GHz frequencies, whereas in the HF/VHF
frequency range its use appears to be, essentially, mandatory.
The divergence of default design approaches (“single-ended”
vs. push-pull) is largely due to the properties of the passive
structures used to combine the output of the two differentially
excited devices. At higher frequencies it becomes difficult to
implement all of the functions of a near-ideal transformer that
is readily available at lower frequencies. As such, higher
frequency implementations tend not to utilize all of the
classical benefits of the push-pull configuration; indeed there
remains some confusion amongst RFPA designers at GHz
Fig. 1. Basic push-pull circuit configuration.
In the first instance, this is appears to be a simple
differential configuration; the input transformer produces a
differential input so that the two devices operate in antiphase.
The output transformer presents a resistive load to each
device, and the magnetic fields generated by the two devices
are additive, thus inducing a combined current to flow in the
output secondary transformer winding, as shown in Fig.2.
Output
Fundamental
Fig. 2. Combining action of output transformer
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At first sight, therefore, the push-pull PA appears to be little
more than a differential power combiner, and its advantages
limited to providing a means of doubling the power from a
single device. This is indeed the case when the devices are
being run in a Class A mode, and the device output waveforms
are essentially sinusoidal. At higher frequencies however there
is an additional benefit in that the detrimental effects of
common lead parasitic reactance are theoretically cancelled.
This is recognized in Fig.1, where in the ideal limiting case
the two source connections do not have to be grounded
directly due to the cancellation of the opposing currents. In the
present historical vein, it is worth recalling that the
cancellation of common-lead parasitics became something of
a “cause celebre” in the early era of GaAs MMIC technology
development, whereby it was widely claimed that push-pull
operation could eliminate the need to fabricate through-vias.
In hindsight, things did not turn out to be quite so
straightforward and thru-via technology has become a
mandatory feature in GHz power processes.
It is when the two devices are run in reduced conduction
angle, or Class AB mode, that the core benefit of push-pull
operation comes into play. The device current waveforms are
now truncated sinewaves, and have a substantial second
harmonic component. Although this waveshaping is highly
beneficial for efficiency, any second harmonic appearing on
the corresponding device output voltage is likely to be
detrimental, and classical Class AB theory calls for a short
circuit at the device output in order to force a sinusoidal
voltage. Fig. 3 illustrates how the ideal action of the output
combining transformer can achieve this. The second harmonic
current components are in phase and the magnetic fields
produced by the two devices cancel, thus realising an
“implied” short circuit termination at even harmonics. Thus,
over the frequency range that the transformer functions in a
near-ideal manner, a specific harmonic shorting network is not
required, which in turn leads to broadband high efficiency
operation.
Fig. 3.
Output combining action at even harmonic frequencies.
At audio frequencies, transformers are easy to construct, and
the “zero-bias Class B push-pull amplifier” became a mainstay
of the audio amplifier industry; a position it still largely
retains, in part through the replacement of unipolar active
devices with complementary semiconductor pairs. It should
also be noted that the requirements for the input transformer
are less critical, and as such is not always present in push-pull
amplifiers, being replaced by some kind of active phase shifter
to create the differential input signal.
III. THE PUSH-PULL PA AT HF AND VHF
Up to about 1GHz, the push-pull “culture” is almost
endemic, but the utilized benefits are not quite so clear. In
particular, the need to implement matching often requires the
provision of impedance transformation networks that restrict
bandwidth and complicate harmonic frequency effects. At
audio frequencies the availability of near-ideal transformers in
effect reduces the matching problem to one of settling on a
specific turns ratio. But the realisation of transformers having
such ideal properties becomes increasingly more difficult as
the HF and VHF frequency range is traversed. Mainly through
the more recent development of ferrite materials that have
suitably low loss characteristics, the differential circuit
environment allows the use of impedance matching structures
such as the Ruthroff and Guanella transformers [3,4]. In
designing such structures the focus shifts to realising the
required impedance transformation (up to 100:1 ratio in some
cases), and although the structures are still essentially
“balanced”, the key property of even harmonic cancellation
becomes less obvious and in many cases one has to speculate
may not be often realised. Although commercial amplifiers
have been available in this frequency sector for many years,
offering octave through to decade bandwidths, the efficiencies
over a substantial portion of the acclaimed bandwidth are
often substantially lower than the specified capability of the
individual devices.
IV. THE PUSH-PULL PA AT GHZ FREQUENCIES
Most RFPAs at GHz frequencies are single-ended designs.
Microwave PA designers are often taken to task, usually by
engineers more familiar with lower frequency techniques, as
to why they appear to shun the advantages of push-pull
operation. In fact, the microwave designer could well respond
by asking the VHF designer for “proof” that their designs
actually leverage the full benefits of a classical push-pull
amplifier to any significant extent. In the rather limited
literature that is available for VHF PAs, little attention seems
to be paid to the properties of the matching structures at
harmonic frequencies. Indeed, the benefits of push-pull
operation appear to be largely “assumed”; an act of faith based
on the use of a differential configuration.
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But to a microwave PA designer push-pull appears to offer
some attractions. Matching is usually major issue, with the
compulsory 50 Ohm output termination and the rather limited
matching range offered by practical distributed passive
networks. A center-tapped transformer presents a terminating
impedance of 25 Ohms to the device and its matching
network, a valuable factor of 2 from a single-ended approach
using the same device, and a factor of 4 compared to a powercomparable parallel pair of similar devices. But the problem
now lies almost entirely in the realisation of the transformers.
At GHz frequencies the use of magnetically coupled windings
finally becomes impractical, as are the losses in the ubiquitous
magnetic materials used at lower frequencies. Transformers
transmogrify into “Baluns”, structures that seek to replicate
the action of a transformer but using coupled transmission
lines.
At higher GHz frequencies the 3-D structure needs to be
transformed into a planar format, and historically multi-layer
structures have not been an allowable option. The basic balun
in Fig. 4 can be made into a single layer structure using edgecoupled microstrips to form the main balun line, as shown in
Fig.5. The main problem with this planar implementation is
that there is now a second “parasitic” transmission line which
is formed between the inner strip and the ground plane. This is
obviously absent in the 3-D structure that uses a coaxial line,
and as such is not considered in Marchand’s original analysis .
This extra transmission line appears in parallel with one of the
balanced outputs, and can be shown to be very detrimental to
the balun performance, causing imbalanced “trace separation”
over the entire operating bandwidth [6].
A. Microwave Balun Evolution
The evolution of the microwave balun is a story in itself, as
witnessed by over 500 US patents, not to mention over 2000
hits on ieeexplore. The main step, and a very critical one, is to
use the properties of transmission lines rather than coupled
inductors. The starting point for this process is the basic balun
structure shown in Fig.4. A 50 Ohm transmission line is
“floated” above a ground plane, thus forming two distinct
transmission lines; the original line itself (which at lower
frequencies can be a physical co-axial cable), and a shortcircuited stub formed between the outer of the main line and
the surrounding ground plane. This structure will deliver a
matched output to a balanced 50 Ohm load placed at the
“floated” end of the line. This structure has a bandwidth which
is mainly dependent on the characteristic impedance of the
SCSS, and as such many variants have been proposed which
seek to increase this impedance to a high value. It has been
Fig. 4 Basic transmission line balun .
used extensively at UHF lower GHz frequencies essentially in
this basic form, with a thin semi-rigid cable as the main line
and some form of void in the groundplane underneath. As
such the basic structure can be used quite successfully as a
differential combiner in a “quasi” push-pull amplifier, and
bandwidths in excess of an octave can be obtained. Marchand
[5] proposed a widely-cited refinement which can improve the
bandwidth of the basic structure, through the use of an extra
open-circuited stub connected in series with one of the output
balanced ports.
B. Planar Evolution
Fig. 5. Planar version of 3-D balun shown in Fig. 4.
Some recent work [6] has shown that the Marchand
configuration, transformed into planar form, can actually
cancel this detrimental effect, provided certain key
relationships are maintained between the impedances of the
characteristic impedances of the main balun line and the
Marchand stub. This has resulted in a structure which has
demonstrated double octave performance using a single
dielectric layer. Variations on the planar Marchand balun
having comparable performance have also been reported using
mutli-layer planar structures [7].
C. Harmonic Behaviour of Microwave Baluns
It seems to be a widespread assumption that once a suitable
balun structure has been proven, a push-pull amplifier can be
easily formed around it, and all of the benefits of push-pull
operation will be effortlessly exhibited. But despite a wealth
of literature on this subject, it is quite rare that the
performance of this structure at harmonic frequencies is
considered; this being the very core benefit of push-pull
operation using reduced conduction angle modes. In fact,
rather than presenting a desirable even harmonic short circuit
at the balanced outputs, the basic structure in Fig. 4 will
present an open circuit. Furthermore, at higher GHz
frequencies there will be some extensive matching circuitry
between the balanced balun port and the device itself, so that
over a wide bandwidth the second harmonic impedance will
trace a sweeping path around the edge of the Smith Chart.
Although there is some utility for the PA designer in having a
reactive, rather than a resistive termination port at the critical
second harmonic frequency, it is certainly still a challenging
circuit design task to control the range of the second harmonic
reactance such that power and efficiency performance can be
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maintained, and/or even to show worthwhile improvement
over comparable single ended designs.
D. Recent Results
Interest in realising push-pull PA designs has shown a
significant increase in the last few years. Several results have
been reported for PAs covering in excess of octave
bandwidths with efficiencies in excess of 50% [8,9]. It is
however interesting to note that these results coincide with the
availability of commercial GaN transistors, whose higher
voltage operation and low parasitic capacitance per watt of
power makes them much more “friendly” in terms of
impedance matching for optimum power and efficiency. Such
a result is shown in Fig.6; although the power and efficiency
show useful performance over greater than an octave of
bandwidth, it is clear that optimum efficiency is only being
achieved over a considerably smaller bandwidth.
ended design approach when optimum performance is
required for narrower band applications. It is a matter of
speculation as to whether the same bilateral approach could be
profitably adopted at lower frequencies, where cited efficiency
performance from push-pull designs often appears to be
substantially less than optimum values.
REFERENCES
[1] U.S.Patent 1,137,384, 1915 “System for the Transmission of
Intelligence,”, (Colpitts,Westinghouse Corp.)
[2] U.S. Patent 549,477, 1895, “Local Transmitter Circuit for
Telephones”, (Dean, Bell Telephone Labs.)
[3] C.L. Ruthroff,, “Some Broadband Transformers,” Proc. I.R.E.,
47, no.8., pp 1337-1342, 1959.
[4] G. Guanella, “Novel Matching System for High Frequencies”,
Brown-Boveri Review, Vol. 31, pp327-329, Sept.1944.
[5] N. Marchand, “Transmission Line Conversion Transformer”,
Electronics, 47, pp142-146, Dedc. 1944.
[6] T.Canning, J.R.Powell, S.C.Cripps, “Optimal Design of Single
Broadband Marchand Baluns Using Single Layer Planar
Technology”, IEEE Trans. Microwave Theory & Tech., vol. 62,
no.6., pp1183-1191, June 2014.
[7] A.V.Pham, M.J.Chen, K.Aihara, “LCP for Microwave Packages
and Modules”, Cambridge University Press, 2012.
[8] R.M.Smith, J.Lees, P.J.Tasker, S.C.Cripps, “A 40W Push-Pull
PA for High Efficiency Decade Bandwidth Applications at
Microwave Frequencies, 2012 MTT-S, Int’l Microwave
Symposium Digest.
[9] J.J. Yan, H. Young-Pyo Hong, S.Shinjo, K.Makai, P.M.Asbeck,
“Broadband High PAE GaN Push Pull Power Amplifier for
500MHz-2.6 GHz Operation, 2013 MTT-S, Int’l Microwave
Symposium Digest.
(a)
Fig.6. (a) 3:1 band push-pull PA design, (b) measured
performance.
V. CONCLUSIONS
The push-pull PA configuration has been a mainstay, and a
default approach, at audio and low MHz frequencies, and
justifiably so. At higher frequencies, the critical design issues
change considerably, and the benefits of push-pull become
increasingly inscrutable. Rather than overlooking the potential
benefits of push-pull operation, GHz designers use single-
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