Push–pull output

A push-pull amplifier is an electronic circuit and is used in a range of power amplifiers. It is a part of an amplifier circuit and has the task of amplifying an electrical signal so far that connected equipment can be operated with him. The name is derived from the fact that two components operate in the circuit in the opposite manner, depending on the design of only one is active by two each.

General

Applications of push-pull output, for example, audio amplifier, voltage converter or transmitter. Be push-pull amplifiers with transistors such as bipolar transistors or field effect transistors constructed. Historic designs use electron tubes.

A load resistance is not required for the push-pull circuit. Thereby, a much higher efficiency is achieved in comparison with other control principles. In power amplifiers, which is a significant advantage over " Eintaktschaltungen ".

A special form of push-pull stage is the bridge circuit. This offers the advantage that it is possible to output both positive and negative voltage despite a unipolar power supply.

Complementary output stage

The picture shows the basic circuit of a complementary output stage with Eintaktansteuerung and unbalanced operating voltage. The advantage is that the DC even series connection of transistors Q4 and Q5 the expensive transformers makes superfluous ( ironless output stage). However, a base bias for the output stage transistors necessary to minimize crossover distortion, which is done with the two diodes D1 and D2. These diodes are thermally conductively connected to the transistors, the forward voltage of the diode changes in the same way as the base-emitter junctions of the transistors, resulting in a bias change largely compensated (temperature compensation). Both Q4 and Q5 conduct just no quiescent current, it is B operation before. A higher base bias (eg with three diodes ) and by adding emitter resistors also operating with quiescent current (AB or A mode) can be set. In this case, the signal quality increases, but also the power loss increases.

Circuit description: R1 = R2 = 100 k and 20 k ensure that adjusts a voltage of 3.3 V at the connection point. Q1 and Q2 form a differential amplifier, the fraction of R7 / ( R7 R8) compares the output voltage with these 3.3V and any deviation immediately takes the occasion to counteract over Q3. The output voltage at the junction of Q4 and Q5 will be half the supply voltage, so that the dynamic range up and down symmetrically. In order to follow the values ​​of R7 = 20 k and R8 = 40 kOhm.

So that by the loudspeaker does not constantly DC current flows, the heat would himself and Q4, an electrolytic capacitor of approximately 1000 uF is placed in series.

Now, if the input voltage increases to 1 V, the output voltage has to increase to 3 V so that the difference amplifier ceases nachzusteuern Q3. Thus, the circuit amplifies the voltage by a factor of 3 when the input voltage increases to 1 V, the signal source must provide = 1 kOhm V/20 50 microamperes. Q4 and Q5 but can conduct secure 10,000 times more electricity from the power supply to the speaker, so the control power is amplified at the entrance to 30,000 -fold.

This internal circuit is to be found, with slight modifications in many ICs that contain everything except the two capacitors. At 20 V operating voltage, the output voltage can vary up to 10 V up or down from the mean, which corresponds to an effective voltage of 7.1 V. For a 4 -ohm speaker then the maximum power is 12.5 W. In practice, one must still close to 1 V per power transistor subtract and can then expect 10 W.

Quasi- complementary output stage

The quasi- complementary output stage consists of two identical transistors. Until the 1970s for two PNP transistors were used because there were no reliable germanium power transistors as NPN version. With the advent of power transistors based on silicon this connection method was also used for NPN transistors to appropriate complementary types were available.

Quasi complementary circuits are rarely used for audio amplifiers today.

A sample circuit shows the image on the right: T2 and T3 form the output stage, where T2 T3 works as a common collector and emitter circuit. T1 serves as a driver. The advantage of this circuit is that no NPN - PNP complementary type with identical electrical parameters is required.

Push-pull amplifier with transformer

The picture on the right shows a basic circuit of a push-pull amplifier with two B- npn transistors, push-pull control by input transformer and balanced output transformer, composed both half-waves again. This type of circuit is now particularly common in tube amps.

With the base voltage divider R1/R2, the operating point is set: the voltage across R1 must not exceed 0.55 V silicon transistors, otherwise the quiescent current increases sharply and the transistors can overheat. The operating characteristic of each half is an S-shape. The characteristic of the balanced amplifier results in a two-fold addition of graphical S-shaped curve, which results by fourier transform a wide range of harmonics with dominance of the odd-numbered harmonics. Since the overall circuit is linearized by any negative feedback, can be expected distortion.

Digital technology

In particular, TTL gates use a push-pull output stage, the totem-pole circuit at the output. The structure corresponds to a quasi- complementary output stage and has been optimized for fast switching times. The CMOS technology has now largely replaced TTL and works exclusively with complementary transistors.