Push-pull transformer coupled RF amplifier with response to DC

A transformer-based coupling network permits a push-pull amplifier to be used with single ended signals, while providing constant gain and, if desired, bidirectional impedance matching, from RF down to DC.

FIELD OF THE INVENTION 
The present invention relates to amplifier circuitry, and more particularly 
relates to a transformer coupled push-pull amplifier that provides flat 
response down to DC. 
BACKGROUND AND SUMMARY OF THE INVENTION 
A common requirement of amplifiers is high efficiency, low distortion 
amplification of essentially sine wave signals. In communication and 
instrumentation applications, the source and load are usually single 
ended. However, the low distortion requirement is usually best satisfied 
by a balanced, push-pull circuit configuration. In such circuit 
configurations, the even harmonic distortion components cancel, leaving 
the third harmonic component as the dominant distortion source. For 
amplifiers employing bipolar transistors, this third harmonic component is 
relatively small. 
The usual way to employ push-pull amplifiers with single ended signals is 
to use a pair of push-pull emitter coupled or grounded emitter transistors 
for the amplifier, with the input power coupled to the bases through a 
single ended-to-balanced transformer, and the output power coupled from 
collectors to load through a balanced to single ended transformer. 
Examples of this prior art methodology are found in Application Notes 593, 
779, 1028 and 1024, Motorola RF Device Data Manual, Volume 2, April, 1988. 
This arrangement provides the requisite power efficiency and low 
distortion for frequencies above the low frequency cutoff of the input and 
output transformers. However, in cases where it is necessary to maintain 
both constant gain and matched terminations down to DC, conventional 
transformer coupled circuits cannot be used. 
In accordance with the present invention, a push-pull amplifier circuit is 
provided with a new coupling circuit that connects the push-pull outputs 
to a single ended load. This circuit delivers the output power of both 
amplifying devices to the load at high frequencies, provides constant gain 
and output power down to DC, and, if desired, provides constant 
bidirectional output impedance matching from RF to DC. 
The foregoing and additional features and advantages of the present 
invention will be more readily apparent from the following detailed 
description, which proceeds with reference to the accompanying drawings.

DETAILED DESCRIPTION 
Referring first to FIG. 1, an amplifier 10 according to one embodiment of 
the present invention comprises a pair of differentially coupled bipolar 
transistors Q.sub.N, Q.sub.P having first and second input terminals 12, 
14, and first and second balanced output terminals 16, 18. A first 
resistor R.sub.IN couples the first balanced output terminal 16 to a first 
power source V.sub.TN. Second and third series connected resistors 
R.sub.TF, R.sub.LF couple the second balanced output terminal 18 to a 
second power source V.sub.TP. A node 20 between the second and third 
resistors is shunted to ground through a capacitor C.sub.LF. 
The amplifier 10 further includes a coaxial transmission line balun 
transformer T.sub.BU having first and second input terminals 22, 24, and 
first and second output terminals 26, 28. The first and second input 
terminals 22, 24 are coupled to the first and second balanced output 
terminals of transistors Q.sub.P and Q.sub.N, respectively. The first and 
second transformer output terminals 26, 28 define a single ended amplifier 
output port 30. 
Resistors R.sub.TN, R.sub.TP, R.sub.LF, capacitor C.sub.LF, and balun 
transformer T.sub.BU define a coupling network 32. Resistors R.sub.TN and 
R.sub.TP serve as bias and reverse termination resistors. Resistor 
R.sub.LF and capacitor C.sub.LF form a crossover network that maintains 
constant amplifier output to DC. The characteristic impedance of T.sub.BU 
equals R.sub.load . The values of R.sub.TN, R.sub.TP and R.sub.LF, are 
half that of R.sub.load to provide an output reverse termination equal to 
R.sub.load. When such a reverse termination is not needed, these resistor 
values may be increased to reduce loss of output power therein. 
The illustrated amplifier 10 further includes two transmission lines 
T.sub.P, T.sub.N. These lines are not essential, but may be used to 
transmit the output power physically away from the amplifier transistors 
Q.sub.N, Q.sub.P and can serve as a convenient means to mount the 
amplifier output devices. If used, their impedances should be half of 
R.sub.load. 
The outer conductor of the transmission line balun T.sub.BU is grounded at 
its output end 28 and is surrounded along its length by magnetic cores 34 
(illustrated in cross section). These cores cause the outer conductor to 
present an inductance L.sub.BU to ground to signals applied at input end 
24. At high frequencies, the reactance of L.sub.BU is high, so the balun 
presents a floating impedance of R.sub.load across nodes 22, 24. The 
amplifier output power is delivered via T.sub.BU to R.sub.load. 
The amplifier 10 is illustrated with circuit values and bias 
voltages/currents selected to yield an output of 5 volts peak to peak into 
a 50 ohm load, and to provide a fully matched back termination. 
Idealized Operation 
The ideal linearized small signal behavior of the amplifier 10 is 
considered at high frequency, DC, and in the crossover frequency range 
using the equivalent circuits of FIGS. 2-4. It is assumed for simplicity 
that the amplifier's output can be modelled by a floating current source 
I.sub.rf 36, which is a first-order approximation to the collectors of a 
push-pull emitter-coupled amplifier. 
The high frequency equivalent circuit is shown in FIG. 2. Here capacitor 
C.sub.LF, effectively grounds the second end 20 of R.sub.TP, and the 
impedance of L.sub.BU effectively disconnects the input end of the balun's 
outer conductor 24 from ground. Some of each end of I.sub.rf flows into 
the 25 ohms presented by T.sub.P and T.sub.N, and the rest into resistors 
R.sub.TN and R.sub.TP. To maintain AC balance, i.e. equal and opposite 
signal voltages at nodes 18 and 16, R.sub.TN and R.sub.TP must be equal. 
The 50 ohm impedance presented across the inputs 22, 24 by the balun 
transformer T.sub.BU fully matches the two 25 ohm in series output 
impedances of T.sub.P, T.sub.N. Similarly, R.sub.load fully terminates 
T.sub.BU. Thus, all of the power entering T.sub.N, T.sub.P is delivered to 
R.sub.load. The signal voltage across R.sub.load is the same as across 
nodes 18, 16, and is therefore twice the voltage from node 18 or 16 to 
ground. 
Any energy reflected by R.sub.load enters T.sub.BU at output terminals 26, 
28 and propagates through a fully matched path comprising T.sub.BU, 
T.sub.P and T.sub.N (if present), and is partly absorbed and partly 
reflected by resistors R.sub.TN and R.sub.TP. 
If the sum of R.sub.TP and R.sub.TN equals 50 ohms (i.e. 25 ohms each for a 
balanced system) then T.sub.BU (and T.sub.P and T.sub.N if present) are 
terminated, i.e. the amplifier is fully back terminated. For this case, 
the total load impedance across I.sub.rf is 25 ohms, giving a total 
voltage across it, and across R.sub.load, of .+-.2.5 volts for the 
illustrated .+-.100 mA signal current. 
The DC equivalent circuit of amplifier 10 is shown in FIG. 3. At DC, the 
transmission line segments are short circuits. R.sub.TN and the left end 
of I.sub.rf are grounded by L.sub.BU. The right end of I.sub.rf drives a 
load consisting of R.sub.load in parallel with the series combination of 
R.sub.TP and R.sub.LF. The resulting voltage across R.sub.load is the same 
as in the high frequency analysis, because the load across the output 
current I.sub.rf is the same as at HF for these component values. For DC 
gain equal to the HF gain, R.sub.LF must be equal to R.sub.TP. Capacitor 
C.sub.LF is effectively an open circuit, so the back-termination is the 
series combination of R.sub.TP and R.sub.LF. If R.sub.TP and R.sub.LF both 
equal 25 ohms, then the back termination provides a match at DC. 
The equivalent circuit in the crossover frequency range is in FIG. 4. Here 
the impedances of capacitor C.sub.LF and inductor L.sub.BU are neither 
very low nor very high as compared with the resistors. However, the 
frequencies are low enough that the transmission line T.sub.P, T.sub.N 
delays and capacitances are negligible and are omitted. Circuit analysis 
and simulation show that the power delivered to the load R.sub.load in the 
crossover range of frequencies is constant provided that: 
EQU L.sub.BU /R.sub.TN =C.sub.LF *R.sub.LF (1) 
This condition also maintains a constant resistance looking back into the 
amplifier from R.sub.load. If all resistors are half of R.sub.source (i.e. 
25 ohms), then full back-termination is again maintained. 
Single Ended Input Embodiment 
A similar coupling network can also be used to connect the input of a 
differential amplifier to a single ended source, as shown in FIG. 5. The 
single ended signal source is represented by V.sub.source and its 
impedance by R.sub.source. 
In the illustrated single ended-input amplifier 38, R.sub.source is assumed 
to be 50 ohms, and V.sub.source is .+-.2 volts. 
As will be recognized from the preceding discussion, the impedance looking 
into balun T.sub.BU at node 40 for the fully terminated case is a constant 
50 ohms from DC on up. Therefore, the voltage at node 40 is a constant 
.+-.1 volt. The voltage across the amplifier's inputs 42, 44 is also a 
constant .+-.1 volt. If R.sub.TP, R.sub.TN, and R.sub.LF are not 25 ohms, 
then a constant signal amplitude is still applied across the amplifier's 
input, from DC to high frequency, but some of the signal is reflected back 
to the source, i.e. the input is not fully forward terminated. 
From the foregoing, it will be recognized that an amplifier according to 
the present invention permits use of push-pull amplifier topologies with 
single ended signals while providing constant gain and, if desired, 
constant bidirectional impedance matching from RF to DC. 
Having described and illustrated the principles of my invention with 
reference to two preferred embodiments, it will be apparent that the 
invention can be modified in arrangement and detail without departing from 
such principles. For example, while the invention has been illustrated 
with reference to single ended-to-balanced, and balanced-to-single ended 
forms, it will be recognized that single ended-to-single ended, and 
balanced-to-balanced forms can be constructed by combining the teachings 
of the FIGS. 1 and 5 embodiments. Similarly, while the invention has been 
described with reference to a push-pull amplifier employing bipolar 
transistors, it will be recognized that the principles thereof are equally 
applicable to a number of other wide band circuits. 
In view of the many possible embodiments to which the principles of my 
invention may be put, it should be recognized that the detailed 
embodiments are illustrative only and should not be taken as limiting the 
scope of my invention. Rather, I claim as my invention all such 
embodiments as may come within the scope and spirit of the following 
claims and equivalents thereto.