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Complementary Push-Pull
Amplifiers for Active Antennas:
A Critical Review
by
Chris Trask / N7ZWY
Sonoran Radio Research
P.O. Box 25240
Tempe, AZ 85285-5240
Senior Member IEEE
Email: christrask@earthlink.net
20 February 2008
Revised 10 June 2008
Revised 2 December 2008
Revised 4 July 2013
Revised 15 September 2013
Trask, “Push-Pull Amplifiers Rev E”
1
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Introduction
Active antennas generally require ampli-
fiers of exceptional intermodulation distortion
(IMD) performance, as well as good noise fig-
ure (NF) performance coupled with sufficient
gain to at least overcome transmission line
losses to the receiver. IMD performance be-
comes increasingly important as one ventures
downward into the HF and then the MF and LF
broadcast band spectrum, whereas there is
less emphasis in NF performance as terres-
trial and galactic background noise dominates
the noise environment and renders good NF
performance in receiver front ends as being a
secondary design goal.
Many designs exist for active antenna
amplifiers, and the majority of them suffer from
poor IMD performance but are still useful for
general purposes. More demanding users
properly see good IMD and NF performance
as being essential characteristics in an an-
tenna/receiver system, and spare little expense
in the pursuit of good equipment.
There is a great deal of interest in active
antennas that make use of small antenna ele-
ments, such as short verticals and dipoles as
well as ferrite-cored magnetic field loop anten-
nas and electric field loops. The amplifiers as-
sociated with these antennas must not only have
exceptional IMD performance and good NF
performance, but should also be affordable and
make use of components that are readily avail-
able worldwide.
One approach to the design of such am-
plifiers makes use of a MOSFET or JFET de-
vice operating as a source follower as the first
stage to provide a high impedance for the elec-
trically small antenna element. Such a stage is
then followed by a second stage that couples
the signal to the low 50- or 75-ohm load im-
pedance of coaxial cable, preferrably with
some gain but most certainly without signal level
loss. A suitable choice for the second stage is
Trask, “Push-Pull Amplifiers Rev E”
2
an emitter follower, which will easily
accomodate the low cable load impedance
while providing a fairly high load impedance for
the source follower first stage. Although such
designs do not offer any signal gain, they are
capable of very high IMD performance, which
can be an acceptable trade-off.
The KAA 1000 Amplifier
The origins of this series of active antenna
amplifiers goes back at least to a Warsaw Pact
active monopole antenna known as the KAA
1000 (1). Shown in functional form in the sche-
matic of Fig. 1, this amplifier uses a single-gate
MOSFET as the input device, biased at a fairly
high current of 48mA. The potentiometer R2 is
adjusted as part of a test procedure described
in the manual. The inductor L1 provides a high
signal impedance to the MOSFET source.
The complementary output transistors are
operated in class AB with a collector current of
5mA, which is adjusted by varying potenti-
ometer R6, again as part of the test procedure.
Diodes D1 and D2 provide bias stabilization
over the specified temperature range of -25ºC
to +80ºC.
Resistors R8 and R9 provide additional
bias stabilization, and resistor R10 aids in pro-
viding a source impedance to the 75-ohm ca-
ble. Not shown in Fig. 1 is a large inductor used
to pass the supply power from the cable to the
Vcc line of the amplifier. Current drain for the
KAA 1000 is specified as being less than
100mA.
The design of the KAA 1000 is, of course,
somewhat dated, however is is very informa-
tive in terms of concept and execution. Single-
gate MOSFETs for small-signal applications
pretty much faded away after the RCA 40673
went out of production due to RCA selling off
all of their semiconductor fabrication facilities
almost three decades ago.
4 July 2013
It is difficult to comprehend why anyone
would resort to using class AB in a small signal
application, especially in an active antenna
amplifier where exceptional linearity and NF are
highly desired. Such a topology is usually rel-
egated to power amplifiers where power effi-
ciency and linearity are simultaneous design
goals. The designer of this unit had gone to
considerable trouble to provide for good per-
formance by heavily biasing the MOSFET and
stabilizing the biasing over temperature, and
then spoiled it by not using class A in the output
stage.
As it stands, the KAA 1000 has a third-
order output intermodulation point (OIP3) of
under +30dBm (into 75 ohms), and the three
resistors R8, R9, and R10 actually degrade the
gain of the unit.
The Lankford Complementary
Push-Pull Amplifier
A recent adaptation of the KAA 1000 with
improved IMD performance was devised by
Dallas Lankford (2), the functional schematic
of which is shown in Fig. 2. Here, the MOSFET
has been replaced by a more contemporary
JFET, the biasing of which is adjusted by
potentiometer R2. The two output transistors
are biased as class A, and this combination
provides an excellent degree of linearity, the
OIP3 being in the vicinity of +50dBm.
The overall design does have one seri-
ous shortcoming, which is the low load imped-
ance seen by the JFET due to the lack of a suit-
able inductor in series with the 180-ohm resis-
tor R4. This results in a gain loss of approxi-
mately 3.5dB, which detracts from the poten-
tial NF of the circuit and the overall signal-to-
noise (SNR) performance of the receiver sys-
tem.
Also impairing the circuit is the lack of
temperature compensation diodes in the bias
string for the output transistors (R5, R6, and R7).
Just as the diodes in the KAA 1000 are essen-
tial for maintaining the class AB bias point over
temperature, they are equally important in main-
Fig. 1 - KA 1000 Complementary Push-Pull Active Whip Antenna (from Reference 1)
Trask, “Push-Pull Amplifiers Rev E”
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taining the bias point of class A amplifiers that
operate at appreciable collector currents.
A serious inconvenience exists with the
PNP output transistor, which is a 2N5160.
Despite it’s good linearity performance (see
Fig. 3), the device has been rendered obso-
lete as a consequence of the availability of bet-
ter performing and less expensive devices, as
well as the fact that very few designers consider
PNP devices in RF design due to the overall
lack of suitable devices plus the overriding
prejudice towards designs that incorporate only
NPN devices.
As it is, the 2N5160 is currently only avail-
able from Microsemi, as part of it’s ever-grow-
ing line of replacement semiconductors. That
product line grew substatially a few decades
ago when Motorola suddenly decided that it
was no longer going to be a participant in the
discrete semiconductor market. The 2N5160
now costs around $US10 each in small quanti-
ties, and will likely increase as the demand for
replacement devices such as this naturally de-
Fig. 3 - 2N5160 Characteristic Curves
(horizontal 1V/div, vertical 2mA/div,
20µA/step)
creases with time.
A Pair of Updated Complementary
Push-Pull Amplifier Designs
Both of the circuits discussed thus far
Fig. 2 - Complementary Push-Pull Active Whip Antenna as Designed
by Dallas Lankford (from Reference 2)
Trask, “Push-Pull Amplifiers Rev E”
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have both positive and negative points. One
positive aspect that they have in common is the
use of a MOSFET or JFET source follower in
the first stage so as to provide a high input im-
pedance. Another is the use of a complemen-
tary pair push-pull output stage.
The KAA 1000 uses a high value inductor
in the source load to avoid signal level loss
whereas the Lankford design omits this com-
ponent and subsequently has a moderate loss
in signal gain, impacting both NF and SNR,
which are important considerations in the de-
sign of receiver systems.
The Lankford design uses a class A bias
level in the output stage, while the KAA 1000
uses class AB, which results in significantly
lower IMD performance. However, the Lankford
design omits the thermal compensation diodes
of the KAA 1000, even though both designs
require them, each for their own reasons.
Lastly, there is the nagging inconvenience
of the cost and availability of the 2N5160 tran-
sistor.
Parts List
C1, C2, C3, C4, C5 - 0.1uF
D1, D2 - 1N914 or 1N4148
Q1 - J309, J310, or U310
Q2 - J309
Q3 - 2N2222, 2N4401, MPS6521, or BFQ19
(see text)
R1, R2 - 1.0M
R3 - 120 ohms
R4, R6 - 10K
R5 - 3.3K (estimated)
R7 - 22 ohms
Q4 - 2N2907, 2N4403, MPS6523, or
BFQ149 (see text)
Fig. 4 - Complementary Push-Pull Amplifier with
Single-ended Input Stage
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