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Патент USA US3029404

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April 10, 1962
Filed May 25, 1960
+6 #5
' ' CAP/164010056,
Q n‘
COLL E6701‘?
44,0;- 7"
I nvenlar
A ttorr'ze y
.r' H
Patented Apr. 10, 1962
connected to collector and base respectively of the ground
Alan John Finnamore, Cogenhoe, England, assignor to
National Research Development Corporation, London,
England, a British corporation
Filed May 23, 1969, Ser. No. 30,833
12 Claims. (Cl. 330-3)
This invention is a transistor ampli?er intended prim
arily for the ampli?cation of direct current, although 10
it is also capable of amplifying alternating currents from
ed emitter transistor J and to current sources for operating
it, the coil polarities being such that the arrangement
forms a free-running blocking oscillator.
The use of a
transistor in this way can produce relatively large pulses
of current from low voltage supply sources, since the
transistor can be allowed to “bottom” in operation, and
the cores are thereby driven into effective saturation if
the number of turns and currents in them are designed
The control windings n1 are connected in series op
very low frequencies up to a frequency determined by
position, as are also the output or pick-off windings mi.
the characteristics of the transistor and associated circuitry.
The junction of the two windings n4 is ea-rthed and each
The main problem in the design of direct current amp-li
pick-off winding feeds a separate recti?er, MR1 and MR2, '
?ers is to minimise the drift (random changes in para 15 and a separate reservoir condenser, the two recti?ed out
met rs) introduced by the ampli?er, since this sets a limit
puts being opposed so that their difference is the output
to the useful sensitivity of the ampli?er. This is because
the output resulting from a signal comparable in magnitude
from the ampli?er. By rectifying the outputs from the
pick~off coils in this way before opposing them to obtain
with drift can not be distinguished with certainty from
their difference, the recti?ers MR1 and MR2, which may
that resulting from drift.
20 be silicon junction diodes or other metal recti?ers, are
' Drift in transistor ampli?ers may be caused by varia
always driven well into the conducting region independ
tions with temperature of the interelectrode potentials and
ently of the presence or absence of a signal and hence
operate at high e?iciency, which is not always attained with
a single recti?er fed with the differential output only.'
currents in the transistors, and to prevent these effects
being cumulative over a number of stages in cascade it
is necessary to convert the direct current input signal into 25 With the independent recti?ers shown, the output/input
alternating current by modulation.
characteristic is quite linear over a useful range of polaris
ing currents.
' The modulation commonly involves the generation of a
pulse waveform which is applied to some non-linear
In the absence of any polarising flux from an input
device, such as an electromechanical relay or non-linear
signal the voltages induced in the two opposed pick-off
resistance in series or parallel with the signal to be ampli 30 windings n4 are equal and no output is obtained, but an
?ed. An ampli?er is then required for the alternating
input signal applied to the windings n1 produces a control
current ?ux which aids the transistor collector current
current output from the modulator, and ?nally the alter
nating current output is demodulated to obtain an ampli
flux in one core and opposes it in the other. One core
?ed copy of the input.
therefore saturates before the other, and as a result the
voltages induced in the two pick-off windings n4 differ.
The transistor ampli?er described below utilises modu
lation of the input signal, but avoids the need for separate
FIG. 2 illustrates the difference between the shape of
switching waveform generator, modulator and ampli?er,
the voltage waveforms at the transistor collector ‘and at
in such a way that drift can not occur, except as a second
the pick-off windings n4.
order e?‘ect, due to the random variations in transistor
It is important that the impedence connected across the
control windings n1 should be high at the relevant frequen
cies, but for high sensitivity the DC. resistance R in
series with these windings should be low. These con?ict
parameters (or battery supply) with temperature.
The main object of the invention, therefore, is to provide
a transistor ampli?er capable of stable operation down to
zero frequency (direct current) in which a single transistor
ing requirements are best met by connecting a low resist- _
ance choke L in series with the control windings. Opti
of ‘an input signal.
mum sensitivity is achieved when the input resistance is
equal to the resistance of the source.
The invention comprises an ampli?er for stable opera
Although the above description mentions separate cores
tion down to zero frequency and incorporating a transistor
T1 and T2 each carrying its own set of windings, it is
having its operating currents fed to it through tightly
obviously possible to use common windings over the two ,
coupled feedback windings to constitute a free-running
cores, provided the correct winding sense is preserved,
blocking oscillator, said windings being on a magnetic
acts at the same time as both oscillator and modulator
transductor core or cores magnetised towards saturation
and the two cores shown can then be regarded as limbs
by the blocking oscillator current pulses, input signal
of a common core and may be so in fact.
windings on said transductor and arranged so that an input
FIG. 3 shows the effect of adding a capacitance C1
from collector to earth. This is a useful means of keep
signal produces control current ?ux which aids the mag
netic ?ux of the oscillator current in one part of the
transductor core but apposes it in ‘another part of the
core, and a pick-off system responsive to the flux changes
ing the reverse voltage swing at the collector within the
limits speci?ed for the particular transistor, without wast
ing power as occurs if a diode limiter is used.
Maximum sensitivity is obtained when the On/Oif ra
so produced. Preferably there is a pick-off coil on said
tio is approximately unity and this condition can be pro
one part of the core and a recti?er fed from said pick-off
coil, another pick-off coil on said other part of the core 60 cured by adjustment of VB and the transistor base resistor
and feeding a separate recti?er and the two recti?ed cur-'
The sensitivity can generally be increased by “tuning”
rents are then opposed to give their difference as the
the control and/ or output windings, but care is required
output from the ampl?er.
to avoid undesired resonances, e.g. with leakage induct
The accompanying diagrams illustrate the invention in
ances, which are necessarily high. Tuning may also de
detail. FIG. 1 is a circuit of the ampli?er; FIGS. 2 and 3
grade the linearity and tends to worsen the drift due to
are waveforms illustrating the operation of the circuit,
variations in any parameters which also effect the op
and FIG. 4 illustrates a possible core structure and winding
Referring to FIG. 1, T1 and T2 represent identical
transductor magnetic cores each wound with four windings
n1, n2, .143 and n4. Windings m2 and 113 are connected in
series aiding fashion and are the coupled feedback windings
erating frequency.
Linearity may be improved by‘ adding a further con
trol winding and feeding back part of the recti?ed out
put to this winding. This may also be used to increase
the input impedance, so that variations of input resist
and cause said core means to be magnetized towards sat
ance, e.g. in the leads to a thermocouple, may be re
uration by oscillator signals, input signal windings on said
core means arranged so an input signal to said input
Satisfactory performance has been obtained from fer
rite pot cores and ferrite rings, but highest sensitivity
windings produces control flux which aids the magnetic
?ux produced by the oscillator signal ?ux in one part
seems to be given with mumetal or permalloy. The lat
ter is very sensitive to ambient magnetic ?elds however,
and unless a magnetic screen is used signi?cant de?ec
of the core means but opposes it in another part of the
core means, and output windings disposed on said core
means to produce differing output signals in response to
the control ?ux aiding and opposing said oscillator signal
tions are produced by changing the orientation of the
ampli?er in the earth’s magnetic ?eld. This property
enables the device to be used as a magnetic ?eld detector 10 flux.
2. An ampli?er as in claim 1 and further including re
0r meter.
The space occupied may be made very small by using
spective rectifying means for said output windings for
modulator being eliminated.
(c) The input impedance may be readily adjusted to
potential point for limiting the reverse voltage swings
rectifying said output signals, and means for opposing
small ferrite rings in an arrangement such as that shown
the recti?ed output signals against each other to obtain
in FIG. 4.
Pot cores are preferable if minimum sensitivity to am 15 their difference.
3. An ampli?er as in claim 1 wherein said oscillator
bient magnetic ?elds is desired, since these are largely
is a blocking oscillator.
4. An ampli?er as in claim 3 wherein the blocking
In the ampli?er above described the processes of am
element includes an active clement, an input terminal and
pli?cation, waveform generation and modulation are
output terminal of which are respectively coupled to said
combined in a manner which has a unique combination
feedback windings.
of advantages in that:
5. An ampli?er as in claim 4 wherein said active ele
(a) The drift of transistor parameters with tempera
ment is a transistor and said input and output terminals
ture, and small variations of supply voltage, produce only
thereof are respectively base and collector electrodes.
a second order effect on the output signal.
6. An ampli?er as in claim 5 including a condenser
(b) Since only one transistor is involved, the reliability
connected between said collector electrode and a common
is improved, drift and unreliability due to a separate
of said collector electrode.
7. An ampli?er as in claim 1 wherein said feedback
suit the source impedance.
windings comprise at least two oppositely wound wind
(d) A number of input signals, possibly at different
ings for operating in said oscillator and on said core
means as aforesaid, said input windings being in series
impedance levels, can be easily added.
(e) Overall efficiency can be very high, due to the
well known suitability of transistors for switching func
tions (i.e. current low when voltage high, voltage low
when current high). Average power consumption can
therefore be of the order of a few milliwatts, permitting
operation from small batteries suitable for portable test
opposition and said output windings being in series oppo
8. An ampli?er as in claim 1 and further including
means for increasing both the input signal impedance and
ampli?er sensitivity.
9. An ampli?er as in claim 1 wherein said input wind
ings are in series opposition and in series with a choke.
(1‘) The circuit lends itself readily to cascading of am
10. An ampli?er as in claim 1 wherein said core means
pli?er stages, so that any prescribed power gain can be 40
includes two saturable cores each carrying four windings
achieved. Matching between stages is easy because of
the ?rst respective two of which are in series opposition
the property (0) above.
and form said input windings, the second respective two
(g) By increasing the gain and using negative feed
of which are in series aiding relationship with the third
back, linearity can be improved to any extent necessary
to make linearity commensurate with zero drift.
(11) Because of the simplicity of the circuit the num
ber of possible sources of drift is small and in any par
ticular application it should be an easy matter to de
termine which are important. This is a valuable feature
in cases where the ultimate possible zero stability is re
quired. The zero stability referred to the input achieved
so far without special selection or matching of transistors
or other components is of the order 10-11 watts over the
temperature range 20-55 ° C. This is believed to be bet
ter than has been achieved in any other type of transistor CH O!
D.C. ampli?er so far reported.
(j) Although the device was primarily designed for
ampli?cation of low power direct current signals, such
as those from thermocouples, strain gauges etc., or for
respective two being in series aiding but in opposition
respectively to said second two windings and tightly cou
pled thereto to form said feedback windings, the fourth
respective two of said windings being in series opposi
tion and forming said output windings, and a grounded
emitter transistor having collector and base electrodes
connected in series respectively to said second two and
third two windings to form therewith a blocking oscil
lator as said free-running oscillator.
ll. An ampli?er as in claim 10 wherein said cores are
respectively ferrite rings.
1.2. An ampli?er as in claim 10 and further including
respective rectifying means for said output windings for
rectifying said output signals, and means for opposing the
recti?ed output signals against each other to obtain their
stabilising the zero of transistor operational ampli?ers, 60 difference.
its bandwidth is limited only by the operating frequency.
References Cited in the ?le of this patent
What I claim is:
l. A stable-operating push-pull type DC. or A.C.
self~contained magnetic ampli?er comprising magnetic
transductor core means, a free-running oscillator having
on said core means tightly coupled feedback windings
for receiving supply signals to operate said oscillator
Alexanderson _________ __ July 9, 1957
Paynter _____________ __ Mar. 11, 1958
Pittman et al. ________ __ Sept. 20, 1960
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