The structural association with elements coupled with amplifiers can be classified either in H03F or in the subclass covering the element (or in both the subclasses), depending on which aspect is more relevant.

In particular H03F units represented as "black boxes" in a specific application (e.g. a servo loop control circuit for a motor) are in general not classified in H03F, unless a specific technical effect which is relevant for H03F is achieved (e.g. linearity of amplification).

Components covered by specific subclasses (e.g. resistors, inductors, transmission lines, etc.):

As general remark, it must be noted that multiple classification symbols may be given. The philosophy is to classify documents in several sub-groups as the case may be, i.e. the classifier should not stop the classification task once that the first suitable symbol is found, but he should continue to assign symbols until all the aspects have been properly classified.

The subclass has a very large number of related technical areas, e.g.:

Indexing Code orthogonal classification

In addition to one or more classification symbols relating to the invention information, where appropriate, one or more Indexing Code symbols relating to orthogonal classification (H03F 2200/03 - H03F 2200/555), i.e. covering aspects which are spanning over one or more groups, should be allocated for the following ranges:

H03F 1/00, H03F 1/565

H03F 3/181, H03F 3/213

H03F 3/24, H03F 3/72

Vacuum tube amplifiers classification

The following IPC groups, which are mostly related to discharge tube amplifiers or general type amplifiers, are used to classify circuit topologies based also on vacuum tubes, valve amplifiers, due to historical reasons (see vacuum tube amplifier in the "Glossary of terms" section below):

H03F 1/04, H03F 1/07

H03F 1/13

H03F 1/16

H03F 1/20

H03F 1/24

H03F 1/28

H03F 1/33

H03F 1/36

H03F 1/40

H03F 1/50

H03F 1/54

H03F 3/02

H03F 3/181

H03F 3/189

H03F 3/22

H03F 3/28

H03F 3/36

H03F 3/40

H03F 3/44

H03F 3/48

H03F 3/52

H03F 3/64

Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements, wherein said details are addressing a specific technical effect for the amplification:

Raise the efficiency

Reduce detrimental influences of internal impedances of the amplifying elements

Reduce influence of noise generated by amplifying elements

Reduce influence of variations of temperature or supply voltage or other physical parameters

Reduce non-linear distortion (except when using negative feedback)

Negative feedback with or without positive feedback

Positive feedback without negative feedback

Bandwidth extension

Protection

Modification of input or output impedances

The technical effect is specified according to the IPC definitions for the individual subgroups of H03F 1/00 which follow hereinafter.

A document may be temporarily allocated in the main group H03F 1/00 only for EPO internal circulation, e.g. to receive it from neighbouring field classifiers via the classification code.

Modifications of amplifiers to raise the efficiency, wherein said modification comprises the following techniques:

dynamic bias control based on the input/output signal

supply control based on the input/output signal

selection of one or more amplifiers from a plurality of amplifier

reduction of the number of DC current paths

use of particular circuit topologies, e.g. Doherty amplifier, LINC Amplifier

use of discharge tube amplifiers acting also as modulators.

Modification of amplifiers to achieve frequency stabilisation or signal isolation among amplifying stages, wherein said modification comprises the following techniques:

use of particular circuit topologies, e.g. transimpendance amplifier, cascode amplifier

use of amplifying elements with multiple electrode connections

use of attenuating means

use of neutralising means, e.g. Miller effect compensation circuitry, pole/zero cancellations in the transfer function

use of distributed coupling

Modifications of amplifiers to reduce influence of variations of temperature or supply voltage, or other physical parameters, wherein said modifications comprise:

bias stabilisation circuits

drifting protection circuits

using a switching device and eventually digital means

case of switching on or off a power supply

case of push-pull configuration

Modifications of amplifiers to reduce non-linear distortion, wherein said modifications comprise:

using feedforward circuits

using predistortion circuits, which are implemented with feedback, or via multiple paths, or in audio amplifiers, or with nonlinear elements, or acting on cartesian and polar parameters

case of differential amplifiers

case of single ended push pull amplifiers

Indexing Code orthogonal classification

In addition to one or more classification symbols relating to the invention information, where appropriate, one or more Indexing Code symbols relating to orthogonal classification (H03F 2201/3203 - H03F 2201/3236), i.e. covering aspects which are spanning over one or more EC subgroups, should be allocated for the following sub-groups range: H03F 1/32 - H03F 1/3241.

Negative-feedback-circuit arrangements with or without positive feedback, wherein said arrangements comprises:

adaptations for reducing the non-linear distortion

hybrid or directional couplers

transformers.

Modifications of amplifiers to extend the bandwidth, wherein the modifications comprise:

periodic amplifiers

aperiodic amplifiers, i.e. wherein aperiodic means that there is no resonant circuit present.

Circuit arrangements for protecting such amplifiers, wherein the arrangements comprises:

using redundant amplifying elements

using protective devices, e.g. diodes.

Modifications of input or output impedances, not otherwise provided for, wherein said modifications may comprise:

matching and tuning circuits which are specifically designed for RF amplifiers

circuit adaptations in class E,F,E/F amplifiers.

Electronic amplifiers based on tubes or semiconductors.

Amplifiers are classified according to topology (e.g., push-pull, emitter follower, differential) and/or according to use (e.g., LF, HF) and/or according to the amplifying element (e.g., tubes, tunnel diodes) and/or according to special functionality (e.g., multi-channel amplifiers, gated amplifiers).

Amplifiers with both tubes and semiconductors are classified in H03F 5/00.

Amplifiers based on other amplification principles such as magnetic, dielectric, mechanical/acoustic, superconductivity, etc., are classified in subgroups H03F 7/00-H03F 99/00.

A document may be temporarily allocated in the main group H03F 3/00 only for EPO internal circulation, e.g. to receive it from neighbouring field classifiers via the classification code.

Amplifiers with switched capacitors used as synthesized resistors.

Amplifiers controlled by light, that is, for optically generated or transferred signals, e.g., transimpedance amplifiers for fiber optic links, optical isolation amplifiers.

Amplifiers controlled by light, wherein the amplifier comprises FETs, e.g., JFET or CMOS transimpedance amplifiers for fiber optic links.

Amplifiers controlled by light, wherein the amplifier comprises optically coupled stages, e.g., optical isolating amplifiers.

Amplifiers controlled by light, wherein the amplifier has functional units such as op-amps, i.e., not showing transistor level circuit details.

Amplifiers using diodes as an amplifying element, e.g., negative resistance diodes such as Gunn, IMPATT, Esaki (tunnel) diodes.

Amplifiers using Esaki (tunnel) diodes as an amplifying element.

Amplifiers with semiconductor amplifying devices having more than three electrodes, e.g., dual-gate MOSFETs, back-gate controlled MOSFETs, multi-emitter/collector BJTs. Note: by semiconductor amplifying devices is understood transistors; opamps and the like are not included, even if they have more than three terminals. Cascode amplifiers using dual-gate MOSFETs are classified in H03F 1/223.

Amplifiers with semiconductor field-effect amplifying devices, e.g., MOSFETs, JFETs, MESFETs.

Amplifiers with semiconductor junction field-effect amplifying devices, e.g., JFETs, MESFETs (but not MOSFETs).

Low frequency amplifiers, in most cases which are suitable for amplification of audio signals.

High frequency amplifiers, e.g. radio frequency amplifiers, as detailed circuits or integrated circuit blocks.

Power amplifiers in general. e.g. class B amplifiers, Class C amplifiers, Class D amplifiers, Class E amplifiers, Class F amplifiers, Class G amplifiers, Class H amplifiers, Class S amplifiers, or combination of power amplifiers, or integrated circuits wherein the circuit topology is based on lumped elements.

Documents which are addressing a power amplifier in general are usually not classified in this subgroup, but are allocated to the corresponding subgroups:

H03F 3/21

H03F 3/211

H03F 3/213

H03F 3/217

H03F 3/22

H03F 3/24

Indexing Code deep-indexing classification

In addition to one or more classification symbols relating to the invention information, where appropriate, one or more Indexing Code symbols relating to deep-indexing classification (H03F 2203/21103 - H03F 2203/21196), i.e. covering embodiment aspects, should be allocated for the following sub-group: H03F 3/211

Amplifiers using switches as amplifying elements, e.g., class D audio amplifiers, class E, class F, class S RF amplifiers, as well as control circuitry such as PCM-PWM conversion etc.

Multiple classification symbols may be given, e.g., a stereo full-bridge class D audio power amplifier with protection means and anti pop-noise means would be classified in H03F 3/68, H03F 3/2173, H03F 1/52, H03F 1/305, and possibly also somewhere in H04S. A class D MRI gradient amplifier would be classified in both G01R 33/3852 and H03F 3/217. An RF amplifier with a class D amplifier used as a tracking power supply (aka class S) would be classified in the relevant H03F 1/02 sub-group, as well as in H03F 3/217 (provided that the class D amplifier is shown in some detail), and possibly also in H03F 3/189, for example.

Switching amplifiers using FET switches as amplifying elements.

Switching amplifiers with half- or full-bridges.

Switching amplifiers for digital input signals, or having explicit ADCs or DACs.

Class E RF amplifiers.

Switching amplifiers with one or more switches (e.g., segmented transistors in RF amplifiers) or output stages (such as in staggered phase class D amplifiers) in parallel or in series, cooperating to produce a common output signal.

Power amplifiers with tubes (thermionic valves only).

Push-pull amplifiers (see the push-pull configuration in the "Glossary of terms" section above) and phase-splitters therefor, i.e. the phase splitter circuitry which is suitable for driving them.

Single-ended push-pull amplifiers (see the SEPP configuration in the "Glossary of terms" section above) and phase-splitters therefor, i.e. the phase splitter circuitry which is suitable for driving them.

It must be noted that the expression "common drain output", which is present in the definition of subgroup H03F 3/301, relates to a common drain configuration for the output transistors (see the "Glossary of terms" section above), therefore it relates to the source terminals of the output transistors being tied together to form the output terminal.

Similar consideration applies for the expression "common source output", which is present in the definition of subgroup H03F 3/3022, and which relates to a common source configuration of the output transistors (see the "Glossary of terms" section above), i.e. the drain terminals of the output transistors being tied together to form the output terminal.

Furthermore, in the following subgroups the expression "symmetrical driving of the end stage" is present:

H03F 3/3016

H03F 3/3028

H03F 3/305

H03F 3/3059

H03F 3/3064

H03F 3/3076

From a circuital topology point of view, it relates to the presence of symmetry in the driving stage circuitry of the SEPP, i.e. if the driving signal path for one of the output transistor has the same/corresponding/complementary elements of the driving signal path of the other output transistor.

As clarifying example the following circuit topology is shown.

Example of SEPP with symmetrical driving of the end stage (drawing extracted from US2002109548).

Mostly of the times the presence of symmetry can be easily detected on the circuit topology by considering a virtual axis (see the dotted line "Symmetry axis" in the drawing above) passing through the output terminal to determine two sub-circuits which are located above and below said axis, respectively, and by examining the similarities/corresponding elements among the two sub-circuits.

If no symmetry is detected then the SEPP circuit is of the type having "asymmetrical driving of the end stage" and it has to be classified in one of the following subgroups:

H03F 3/3011

H03F 3/3023

H03F 3/3037

H03F 3/3042

H03F 3/3045

H03F 3/3057

H03F 3/3062

H03F 3/3067

H03F 3/3071

H03F 3/3088

Furthermore, it must be noted that, when two SEPPs are configured as class B or AB bridges, they are classified in H03F 3/3081 and when they are implemented with FETs they are classified in H03F 3/3061. Switching amplifiers in bridge configuration are classified in H03F 3/2173.

Indexing Code orthogonal classification

In addition to one or more EC symbols relating to the invention information, where appropriate, one or more Indexing Code symbols relating to orthogonal classification (H03F 2203/30003 - H03F 2203/30156), i.e. covering aspects which are spanning over one or more EC subgroups, should be allocated for this subgroup.

DC amplifiers which are formed by IC blocks, semiconductors or tubes, in which all the amplifying stages are DC coupled, e.g. without using capacitive or switching elements, which are not specific for RF amplifiers or audio amplifiers and which have circuit topologies that are not related to other H03F classification symbols.

DC amplifiers wherein only the AC signal is passed through the amplifying units, the DC signal is decoupled via switching elements, e.g. see the Chopper amplifier configuration in the "Glossary of terms" section above.

In most cases totem-pole amplifiers, see the "Glossary of terms" section above, which are implemented via semiconductor elements or tubes.

Differential amplifiers in general and characterised by the technology used, the circuit topologies, and the way that the common mode signals are rejected.

Pseudo differential amplifiers (see the "Glossary of terms" section above) are usually classified in H03F 3/4508, H03F 3/45179, H03F 3/45278, H03F 3/45376.

Furthermore, it is common practice at EPO, when classifying semiconductor differential amplifiers, to consider first the technology of the amplifying transistors and the circuit topology information, in order to classify under one of the subgroups belonging to H03F 3/45076.

About circuit topology, the expression "Complementary long tailed pairs having parallel inputs and being supplied in parallel" is present in the following subgroups:

H03F 3/45112

H03F 3/4521

H03F 3/45309

H03F 3/45408

As clarifying example the following circuit topology is shown:

Example of differential amplifier with complementary long tailed pairs having parallel inputs and being supplied in parallel: the first transistor pair (711,712) and the second transistor pair (721,722) constitute two complementary transistor pairs, which, together with the corresponding current sources (710,720) constitute long tail pairs (see the "Glossary of terms" section above), and wherein said pairs have parallel inputs (see the common input terminals (VinN, VinP) and are supplied in parallel (see the common supplies VDD, VSS), (drawing extracted from US2005040889).

Similar consideration applies to the expression "Complementary PI types having parallel inputs and being supplied in parallel", wherein the complementary transistor pairs are forming differential amplifiers of the PI type, (see the "Glossary of terms" section above), and which are classified in the following subgroups:

H03F 3/45125

H03F 3/45224

H03F 3/45322

H03F 3/45421

About the expression "Complementary long tailed pairs having parallel inputs and being supplied in series", which is present in the following subgroups:

H03F 3/45139

H03F 3/45237

H03F 3/45336

H03F 3/45434

As clarifying example the following circuit topology is shown:

Example of differential amplifier with complementary long tailed pairs having parallel inputs and being supplied in series: the first transistor pair (Q221,Q222) and the second transistor pair (Q223, Q224) constitute two complementary transistor pairs, which, together with the corresponding current sources (234,236) constitute long tail pairs (see the "Glossary of terms" section above), and wherein said pairs have parallel inputs (see the common input terminals (VIN-, VIN+) and are supplied in series (see the series connecting path from VD to GND, which is formed by element 234, the first transistor pair (Q221,Q222), the second transistor pair (Q223, Q224), element 236), (drawing extracted from FR2814554).

Similar consideration applies to the expression "Complementary PI types having parallel inputs and being supplied in series", wherein the complementary transistor pairs are forming differential amplifiers of the PI type, (see the "Glossary of terms" section above), and which are classified in the following subgroups:

H03F 3/45152

H03F 3/45251

H03F 3/45349

H03F 3/45448

Once that the technology and the circuit topology related to the differential amplifier are classified, a possible classification under one of the subgroups belonging to H03F 3/45479 has to be considered. It must be noted that the expression "common mode signal rejection", which is present in the definition of subgroup H03F 3/45479, has to be interpreted with a broader meaning than the one used in common practice (see the "Glossary of terms" section above). In practice common mode rejection has to be meant as operating point/DC level control/offset reduction, and said control can be implemented at sub-circuit level.

Hence all the subgroups of H03F 3/45479 are addressing all the possible implementations of said common mode rejection within the sub-circuits that are forming the differential amplifier architecture (for a detailed definition of said sub-circuits see the following section).

Indexing Code orthogonal classification

In addition to one or more EC symbols relating to the invention information, where appropriate, one or more Indexing Code symbols relating to orthogonal classification (H03F 2203/45002 - H03F 2203/45732), i.e. covering aspects which are spanning over one or more EC subgroups, should be allocated for this subgroup.

Said aspects are mainly based on the following sub-circuits, which can be determined in a long tailed pair, PI type, and, although with limitations, in special types of differential amplifiers, e.g. pseudo-differential amplifier:

AAC = Active Amplifying Circuit, i.e. the elements comprising the amplifying transistors.

CSC = Common Source Circuit, i.e. the elements forming the long tailed circuit or the PI circuit.

IC = Input Circuit, i.e. the elements coupled to the input terminals of the amplifying transistors.

LC = Loading Circuit, i.e. the elements coupled to the output terminals of the amplifying transistors.

CMCL = Common Mode Controlling Loop, i.e. the elements which are controlling the common mode(s) of the differential amplifier.

FBC = Feed-Back Circuit, i.e. the elements which are introducing feedback signal(s) for the differential amplifier.

Here is an example of some of said sub-circuits above for a long tailed differential pair.

drawing derived from US2006055463

Reflex amplifiers, see the "Glossary of terms" section above, which are implemented via semiconductor elements or tubes.

Amplifiers in cathode follower, source follower, emitter follower configuration (see the "Glossary of terms" section above).

Indexing Code deep-indexing classification

In addition to one or more classification symbols relating to the invention information, where appropriate, one or more Indexing Code symbols relating to deep-indexing classification (H03F 2203/5003 - H03F 2203/5045), i.e. covering embodiment aspects, should be allocated for this subgroup.

Amplifiers using transit-time effects, mostly Klystrons or Travelling Wave Tubes (TWT) see the "Glossary of terms" section above.

Amplifiers having distributed coupling networks, e.g. transmission lines, the so called microwave amplifiers.

Two-way amplifiers, which are implemented via semiconductor elements or tubes and which are typically employed to amplify the signal levels of forward and reverse signals.

Amplifying-mixing devices for achieving a special technical effect, e.g. interference reduction.

Stereo amplifiers or multi-channel amplifiers in the low frequency range.

Multi-band amplifiers in the high frequency range, wherein multiple outputs are present.

Charge amplifiers, see the "Glossary of terms" section above.

Amplifiers which are enabled or disabled by means of a control signal, e.g. via a controlled switch.

Indexing Code deep-indexing classification

In addition to one or more classification symbols relating to the invention information, where appropriate, one or more Indexing Code symbols relating to deep-indexing classification (H03F 2203/7203 - H03F 2203/7239), i.e. covering embodiment aspects, should be allocated for this subgroup.

Parametric amplifiers, i.e. wherein a component parameter such as capacitance or inductance is varied to achieve amplification.

Parametric amplifiers where inductance/permeability is varied to achieve amplification.

Parametric amplifiers where capacitance/permittivity is varied to achieve amplification.

Amplifiers using saturable reactors as amplifying elements (aka mag-amps).

Magnetic amplifiers where an AC load current flows in a main coil, controlled by the current in a control coil.

Magnetic amplifiers where a load current flows in one direction in a main coil, and typically in the other direction in another main coil, controlled by the current in a control coil, e.g., a control coil in a center leg of a transformer controlling two main coils in two outer legs.

Magnetic amplifiers where the load current is auto-commutated by the main coil, the core being periodically reset by a control coil.

Amplifiers using non-linear (hysteretic) dielectric elements to achieve amplification.

Amplifiers with intermediate mechanical or acoustic energy transfer .

Amplifiers relying on galvano-magnetic effects, i.e., which arise when a conductor or semiconductor placed in a magnetic field carries current, such as the Hall-effect.

Amplifiers relying on intermediate transformation into light to achieve amplification.

Amplifiers not covered elsewhere, e.g. relying on radioactivity or using esoteric semiconductor structures.