What is amplified

Push-pull amplifier / push-pull output stage

Single ended amplifiers are transistor amplifier circuits with only one transistor. The single-ended amplifier works in A mode. When amplifying an alternating voltage, the transistor must amplify both half-waves. This limits the gain and the maximum output voltage.

Complementary series push-pull output stage

A push-pull amplifier has two gain elements. In this case two transistors that share the gain of the positive and negative half-wave. The push-pull amplifier then works in B mode. Unfortunately, small signals are then generated below the UBE-Voltage (approx. 0.7 V) not amplified. This is why a push-pull amplifier usually works in AB mode. This enables the amplification of large and small signals.
The push-pull amplifier here consists of an NPN and a PNP transistor.
The AC voltage input signal is alternately from both transistors T1 and T2 reinforced.
The NPN transistor T1 amplifies the positive half-wave, the PNP transistor T2 amplifies the negative half-wave of the alternating voltage.
The identical direct current properties of the transistors T are a prerequisite for proper operation of the push-pull amplifiers1 and T2.
Usually you need two operating voltages for the push-pull amplifier. A positive and a negative operating voltage, each for the amplifier parts of the positive and negative half-waves. If no negative voltage is available, then the load must be via the coupling capacitor C.K be coupled. The lower limit frequency can then only be kept low by using a large capacitance.

In B operation without UBEBias voltage causes takeover distortions in the output voltage U.a. The transfer distortion is particularly noticeable at low input voltages. Without UBEBias voltage, input signals below 0.7 V are not even amplified.
Transfer distortion creates other frequencies, so-called harmonics.
In order to get the output signal free of distortion, the UBE-Bias voltage by means of diodes D1 and D2 be generated.

Characteristics of the push-pull amplifier

  • A push-pull amplifier has a very low resistance because it hardly contains any resistors.
  • The two diodes D1 and D2 the push-pull amplifier is in AB mode.
  • The basic circuit is the collector circuit.

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