GB1590647A - Transformer connected amplifying stages - Google Patents

Description

(54) TRANSFORMER CONNECTED AMPLIFYING STAGES (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company, of 190 Strand, London W.C.2, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to transformer connected amplifying stages.

A junction transistor is a typical threeterminal amplifying device having an input terminal, output terminal and common terminal. Other devices such as thermionic valves and field effect transistors (FET) also have three terminals with corresponding functions. Combinations of such devices may also be constructed to form threeterminal amplifying circuits, such as the well-known Darlington pair, the complementary pair or the cascaded pair. Such circuit combinations may be used to substitute for the single basic three-terminal device so that some aspect of performance may be improved.

Considering the basic transistor amplifying stage, three modes of operation are known. These are respectively termed "common emitter", "common collector" and "common base".

The common emitter connection provides both current and voltage amplification. The common collector connection, which is also referred to as operating in a voltage follower mode, has 100% negative feedback voltage giving less than unity voltage amplification but does provide a high current gain. The common base connection on the other hand gives less than unity current gain but provides substantial voltage gain.

According to the invention there is provided a three terminal current (voltage) amplifying stage with negative feedback having approximately unity voltage (current) gain including a transformer having one winding connected in the load circuit of the stage and a second winding connected in the input circuit of the stage, the respective polarities of and ratio of turns between the windings being such that the negative feedback voltage (or current) is reduced to provide greater voltage (current) gain.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 illustrates a basic three terminal amplifying device in a common emitter mode, Fig. 2 illustrates a basic three terminal amplifying device in a common collector mode, Fig. 3 illustrates a modification of the circuit Fig. 2 with a transformer coupling between input and output.

Fig. 4 illustrates a modification of the circuit of Fig. 3, and Fig. 5 illustrates a modification to a basic common base configuration with a transformer coupling between input and output.

Fig. 1 shows a commonly used transistor output stage of linear amplifiers with a common emitter configuration. It has an input terminal 1, an output terminal 2 and a common terminal 3. There is generally a resistor in series with terminal 3 to render the stage more linear. This arrangement wastes part of the output power in this series resistor. The arrangement of Fig. 2 is more linear than that of Fig. 1 because the load itself serves as the series resistor.

This also eliminates any waste of output power in the resistor. It is also desirable that the substrate material of the output transistor should be attached to an effective heat conductor to remove excess heat. In bipolar transistors the substrate is generally electrically connected to terminal 2 and since metals are the most effective heat conductors as well as the most effective electrical conductors it is very desirable to arrange for this terminal to be electrically grounded. Fig. 2 allows this to be done. However, the arrangement of Fig. 2 provides no voltage gain and creates such difficulties in the design of the amplifier circuitry to drive it, that it has been little used in high frequency amplifiers.

Fig. 3 shows the addition of a transformer T1 in a special connection to Fig. 2. The winding across the load senses the volt age developed across the load, and by suitable choice of turns ratio subtracts part of it from the voltage fed back to the input.

This effectively provides the stage with voltage gain, which greatly eases the design of the high frequency amplifier required to drive it. If the ratio of turns is n:l for the load and input windings respectively then the input winding substracts 1/n of the output voltage from the feedback voltage requirement at the input of the stage. A device that gives a high voltage gain in a common emitter mode can now provide a voltage gain approaching 11(1 - 1/n) in the common collector mode. In many cases a transformer or inductor will already exist at the output stage which can be modified to provide the action of T1.

A typical application is depicted in Fig. 4.

In this case improved performance is obtained by the use of two transistors. These may be formed on the same semiconductor substrate so that terminal 2 is shared. Transformers T1 and T2 form a broadband directional coupler derived from a design by C. G. Sontheimer et al (U.S. Patent 3,426,298) to split the output power into a transmission line at output 1 and a feedback network at output 2. Impedances Z1 and Z2 are frequency dependent and may be used to disconnect the effect of T1 from the input circuit at frequencies the amplifier is not required to carry.

Where a common case connection is required to have a current gain of greater than unity a similar transformer coupled arrangement between the input and the output can be used, as shown in Fig. 5. Here the load winding of the transformer is in series with the load and the other winding shunts the input. A part of the input current requirement is supplied by current induced in the input circuit by the transformer when the load draws current through the other winding.

WHAT WE CLAIM IS:- 1. A three terminal current (voltage) amplifying stage with a negative feedback having approximately unity voltage (current) gain including a transformer having one winding connected in the load circuit of the stage and a second winding connected in the input circuit of the stage, the respective polarities of and the ratio of turns between the windings being such that the negative feedback voltage (or current) is reduced to provide greater voltage (current) gain.

2. An amplifying stage according to claim 1 having a transistor as an amplifying device, the transistor being connected for operation in a common collector mode, the one winding of the transformer being connected in parallel with the load and the second winding being connected in series with the input.

3. An amplifying stage according to claim 2 in which a second transistor is connected with the first transistor to form a Darling pair, the second winding and the load being connected to the emitter of the second transistor the base of which is connected to the emitter of the first transistor, the collectors of the two transistors being conneced together.

4. An amplifying stage according to claim 3 in which the load circuit includes a first winding of a second transformer in series with the load, the second winding of the second transformer being connected at one end thereof to the common collector connection of the two transistors to provide at its other end a second output from the stage, the second output having an impedance coupling to the first winding of the first transformer whereby the two transformers together form a broadband directional coupler.

5. An amplifying stage according to claim 4 including frequency dependent impedances connected in series and/or parallel with the second winding of the first transformer.

6. An amplifying stage according to claim 1 having a transistor as an amplifying device, the transistor being connected for operation in a common base mode, the one winding of the transformer being connected in series with the load and the second winding of the transformer being connected in parallel with the input.

7. A three terminal amplifying stage substantially as described with reference to Fig.

3, Fig. 4 or Fig. 5 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. age developed across the load, and by suitable choice of turns ratio subtracts part of it from the voltage fed back to the input. This effectively provides the stage with voltage gain, which greatly eases the design of the high frequency amplifier required to drive it. If the ratio of turns is n:l for the load and input windings respectively then the input winding substracts 1/n of the output voltage from the feedback voltage requirement at the input of the stage. A device that gives a high voltage gain in a common emitter mode can now provide a voltage gain approaching 11(1 - 1/n) in the common collector mode. In many cases a transformer or inductor will already exist at the output stage which can be modified to provide the action of T1. A typical application is depicted in Fig. 4. In this case improved performance is obtained by the use of two transistors. These may be formed on the same semiconductor substrate so that terminal 2 is shared. Transformers T1 and T2 form a broadband directional coupler derived from a design by C. G. Sontheimer et al (U.S. Patent 3,426,298) to split the output power into a transmission line at output 1 and a feedback network at output 2. Impedances Z1 and Z2 are frequency dependent and may be used to disconnect the effect of T1 from the input circuit at frequencies the amplifier is not required to carry. Where a common case connection is required to have a current gain of greater than unity a similar transformer coupled arrangement between the input and the output can be used, as shown in Fig. 5. Here the load winding of the transformer is in series with the load and the other winding shunts the input. A part of the input current requirement is supplied by current induced in the input circuit by the transformer when the load draws current through the other winding. WHAT WE CLAIM IS:-

1. A three terminal current (voltage) amplifying stage with a negative feedback having approximately unity voltage (current) gain including a transformer having one winding connected in the load circuit of the stage and a second winding connected in the input circuit of the stage, the respective polarities of and the ratio of turns between the windings being such that the negative feedback voltage (or current) is reduced to provide greater voltage (current) gain.

2. An amplifying stage according to claim 1 having a transistor as an amplifying device, the transistor being connected for operation in a common collector mode, the one winding of the transformer being connected in parallel with the load and the second winding being connected in series with the input.

3. An amplifying stage according to claim 2 in which a second transistor is connected with the first transistor to form a Darling pair, the second winding and the load being connected to the emitter of the second transistor the base of which is connected to the emitter of the first transistor, the collectors of the two transistors being conneced together.

4. An amplifying stage according to claim 3 in which the load circuit includes a first winding of a second transformer in series with the load, the second winding of the second transformer being connected at one end thereof to the common collector connection of the two transistors to provide at its other end a second output from the stage, the second output having an impedance coupling to the first winding of the first transformer whereby the two transformers together form a broadband directional coupler.

5. An amplifying stage according to claim 4 including frequency dependent impedances connected in series and/or parallel with the second winding of the first transformer.

6. An amplifying stage according to claim 1 having a transistor as an amplifying device, the transistor being connected for operation in a common base mode, the one winding of the transformer being connected in series with the load and the second winding of the transformer being connected in parallel with the input.

7. A three terminal amplifying stage substantially as described with reference to Fig.

3, Fig. 4 or Fig. 5 of the accompanying drawings.