A BALANCED AMPLIFIER OUTPUT STAGE

Abstract: A balanced amplifier output stage preferably intended to be used in a telephone instrument and which has its current supply from the telephone line includes two transistor amplifiers connected in parallel each with a top (T1, T2) and a bottom (T3, T4) transistor with series connected emitter-collector-circuits. Equally located transistors (e.g. T1, T2) in the series connections are driven in push-pull. To stabilize the current sum (1) running through the two transistor amplifiers (T1, T3; T2, T4) connected in parallel there is a regulation circuit (T5) which compares the current sum (1) with a reference quantity (Vref) and feeds a correction signal in equal phase inputs of the amplifiers connected in parallel.

Description: A BALANCED AMPLIFIER OUTPUT STAGE FIELD OF THE INVENTION The present invention relates to a balanced amplifier output stage pre¬ ferably intended to be used as a receiver amplifier in telephone instru¬ ments. DESCRIPTION OF PRIOR ART An output stage of a receiver amplifier in a telephone instrument shall be so constructed that it can work with high efficiency also when the line voltages are low. It is for example known through Texas Instruments Application Report B107, Audio Amplifier design Bedford 1971 p 36, 37 and 39 to use a bridge connection with two series connected transistors as an amplifier output stage where the load is connected between the mid-points of the two series connections. The transistors are operated in push-pull in such a way that during half a semi-cycle the current goes through the top transistor in one series connection and the bottom transistor in the other series connection, while it during the reverse semi-cycle passes through the top transistor in the second series con- nectioπ and the bottom transistor in the first series connection. The output obtained will therefore be considerably greater as the effective voltage deviation across the load will in principle be twice as great as in a single output stage of a so called "single ended push-pull" type. SUMMARY OF THE INVENTION The amplifiers described above present a relatively low impedance towards the current supply source. In normal audio amplifiers this does not imply any problem as the current supply source has a very low impedance, for example by being shunted with a high capacitance. If a known amplifier should be used as a receiver amplifier in a telephone instrument the current supply will take place through the subscriber's line which has a considerable impedance. There is then a great risk- that the line current varies according to the signal causing a non- desired feed-back which can lead to self-oscillation. OMPI A balanced receiver amplifier according to the invention includes two transistor amplifiers connected in parallel, each one including two transistors with series connected emitter collector circuits where th load is connected between the common points of said series connected emitter collector circuits and where equally located transistors in both amplifiers are fed with input signals in push-pull and the characteristics of the invention appear from the attached claims. By the circuit according to the invention an amplifier is obtained .- which utilizes the whole available voltage at the same time as it tak a constant current from the source so that the feed voltage is kept constant independent of the internal impedance of the current supply source. Moreover the dc-level of the amplifier will be well defined. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described more in detail in connection to the attached drawing, showing a driving and an output stage in a receiver amplifier. PREFERRED EMBODIMENT In the figure T1, T2, T3 and T4 denote four transistors which form two push-pull arranged amplifier stages T1, T3 and T2, T4 respectively . * which are connected in parallel and driven in push-pull. The load Rec is connected between the common points C and D for the two transistors in the stages T1, T3 and T2, T4 respectively. As the circuit is intend ed to be designed as a monolitic integrated circuit all the transistor T1 - T4 are of npn-type. However, it is naturally possible to use othe known combinations of transistors in the push-pull stages. The output stage is driven by a differential input stage T7, T8, the base circuit of which are connected to a signal input INa, INb through the resis¬ tances R7, R8. The emitters of the differential stage is as usual con¬ nected to a constant current source Is which is schemat cally drawn bu can consist of some transistor circuit known per se. The collector sid in the differential amplifier T7, T8 is connected partly to the bases OMPI of the transistors T1 and T2 partly to the terminal +V of a single voltage source through two of the collectors and the emitter of tran¬ sistor T6 connected as a current mirror. A third collector in the transistor T6 connected to the base is connected to the negative ter- minal pole -V through a series transistor T5 controlled by a reference voltage Vref and an emitter resistance R4 common to the transistors T3 and T4 in the output amplifier. The push-pull input signals to the transistors T3 and T4 are received from a voltage divider R1, R2, R3 connected between the points C and D. The base of the transistor T3 is connected to the common point between R2 and R3 whereas the base of the transistor T4 is connected to the common point between the resist tances R1 and R2. The points C and D are also connected to the base circuits in the transistors T7 and T8 through the feed back resist tances R5 and R6. The circuit operates in the following way. A balanced signal applied to the inputs INa and INb is amplified in the differential amplifier T7, T8 and fed in a balanced way to the base circuits in the transis¬ tors T1 and T2 in the output amplifier stage. The operating point for these transistors T1 and T2 is determined by the transistor T6 connect- ed as a current mirror where two collectors supply base current to the transistors T1 and T2. The load Rec connected between the emitters in the transistors T1 and T2 does not carry any current in case the emit¬ ter voltages are alike. The bottom transistors T3, T4 receive the base current feed from the emitter circuit in the opposite top transistor T2 and T1 respectively through the resistor network R1, R2, R3. An incoming signal which for example makes the transistor T1 more con¬ ducting than T2 means that also the transistor * T4 becomes more conduct¬ ing than T3 and a current will pass through the load Rec. When the signal polarity is the reverse on the bases of the transistors T1 and T2 the direction of current through the load is the reverse. The sum of the currents through the transistors T3 and T4 runs through the resistance R4. As long as the balance in the amplifier is perfect the voltage is not altered across the resistance R4 with the signal. An increasing current through one of the transistors for example T3 corresponds to an' identical current decrease through the transistor T If, however, the balance is not perfect, which is the case in integrat ed circuits which are not specially trimmed, a so called common mode signal will flow to the current source. If the impedance of the curren source is high, which is the case when a telephone instrument amplifi sets its current supply through the telephone line, the voltage of the supply point will- be modulated and there is a risk that the amplifier becomes unstable. The direct current sum through the resistance R4 can also vary for example with the temperature, as the whole amplifier is DC. connected. A small voltage alteration in an early stage is amplified and can caus that the current through R4 is altered. To stabilize towards these signal- and direct current alterations the voltage across the resistance R4 is compared with a reference voltage Vref in the transistor T5, the collector current of which is brought to vary with the current through R4. If for example the current throug R4 decreases the base current and accordingly the collector current in the transistor T5 increases. Also the emitter current in the transisto T6 increases aad accordingly the two collector currents increase to th same extent. The base currents in the two transistors T1 and T2 inr.- crease and causing the current sum through the resistance R4 to be restored to the value determined by the reference voltage Vref. A stabilization of the differential amplification and the operating point is obtained through the symmetrical feed-back from the points C and D in the output stage to the inputs of the amplifier stage T7, T8. _O PI

Claims: What we c laim i s: 1 A balanced amplifier output stage including two transistor ampli¬ fiers connected in parallel each one with a top and a bottom transistor (T1, T3, T2, T4) with series connected emitter collector circuits and where the load (Rec) is connected between the common points (C, D) of said series connected emitter collector circuits and where equally located transistors (T1, T3 and T2, T4 respectively) in the two series connections are operated in push-pull, characterized by a control cir¬ cuit (T5) controlled by the sum of the currents (I) through the two transistor amplifiers (T1, T3; T2, T4) connected in parallel and a re- ference quantity (Vref), the regulating circuit being arranged to keep the current sum (I) constant. 2 A balanced amplifier output stage according to claim 1, where the base of each of the top transistors (T1, T2) is directly connected to an output each of a balanced amplifier stage (T7, T8) the collector circuits of which are fed by a controllable constant current source (T6), characterized in that the output of the control circuit (T5) is connected to a control input of the constant current source (T6). 3 A balanced amplifier output stage according to claim 2, charac¬ terized in that the control circuit consists of a transistor (T5) the base of which is connected to a reference voltage (Vref) whereas the emitter is connected to a point in the output stage (R4) the voltage of which is proportional to the current sum (I) and the collector is connected to the emitter and a collector in multi collector transistor (T6) in a current mirror connection, another two collectors of which are connected to the collector circuits of the amplifier stage (T7, T8). 4 A balanced amplifier output stage according to any of the above claims, characterized in that the base circuits in each of said bot¬ tom transistors (T3, T4) are connected to the emitter circuit in the reverse top transistor (T1, T2) through a resistor network (R1, R2, R3). OMPI . / . WIPO ,

Inventors: BERG B
Applicants: BERG B, ERICSSON TELEFON AB L M
Application year: 1980
Main classification: H03F3
IPC codes: H04M1