Push-Pull Amplifiers

A single-transistor common-emitter amplifier has a fundamental limitation: one transistor handles both the positive and negative halves of the output signal cycle. For Class A operation, it must be biased to conduct a large quiescent current so that it can sink or source enough current for the full output swing. This wastes DC power as heat whenever no signal is being amplified. For Class B or AB operation, a single transistor can only amplify one polarity of signal — you need a second transistor to handle the other half of the cycle.

The push-pull amplifier solves both problems elegantly. Two transistors are connected so that one handles the positive half-cycle of the signal (pushing current through the load) while the other handles the negative half-cycle (pulling current through the load). The two halves are combined at the output. This configuration delivers twice the output power of a single stage from the same supply voltage, cancels even-order harmonic distortion by symmetry, and permits efficient Class AB operation that eliminates crossover distortion while maintaining acceptable linearity.

Push-pull design is the dominant topology for virtually every power amplifier stage — from the output stage of a hi-fi amplifier or an op-amp to broadband RF power amplifiers, motor drivers and the output stages of switching amplifiers. Understanding push-pull operation is essential for anyone who wants to understand or troubleshoot power amplifiers of any kind.

The Push-Pull Principle

In a push-pull amplifier, the input signal is split into two equal and opposite signals — one drives transistor Q1, the other drives transistor Q2. Q1 amplifies the positive half of the cycle; Q2 amplifies the negative half. The outputs of Q1 and Q2 are combined in a way that reconstructs the full output waveform.

The key insight is symmetry. Because the two transistors handle equal and opposite signals:

• Odd-order distortion products (fundamental, 3rd harmonic, 5th harmonic…) add constructively in the output — they are preserved and amplified.
• Even-order distortion products (2nd harmonic, 4th harmonic…) cancel because they are in phase in both transistors and the output circuit subtracts one half from the other. This cancellation is automatic — it requires no tuning or adjustment.

The cancellation of even-order harmonics is a major advantage of push-pull design. A single-ended (one transistor) Class A stage produces significant 2nd harmonic distortion. The same transistors in push-pull produce no 2nd harmonic — their 2nd harmonic components cancel, leaving only 3rd, 5th, and higher odd-order distortion, which are typically much smaller in amplitude. This makes push-pull amplifiers substantially cleaner than their single-ended counterparts for the same transistors and bias conditions.

Transformer-Coupled Push-Pull

The classic implementation uses transformers at both input and output. The input transformer (driver transformer) has a centre-tapped secondary winding. When the signal goes positive, the top half of the secondary drives Q1's base positive and the bottom half drives Q2's base negative — giving Q1 and Q2 exactly opposite drive signals. The output transformer has a centre-tapped primary. Q1's collector current flows through one half of the primary winding; Q2's collector current flows through the other half. The two currents produce magnetic flux in the transformer core that adds to drive the secondary winding and the load.

Circuit components:

• Q1, Q2: matched NPN transistors (or matched MOSFETs in higher-power designs)
• T1: input/driver transformer with centre-tapped secondary
• T2: output transformer with centre-tapped primary, turns ratio chosen for impedance matching
• Bias resistors/diodes: set Class AB idle current for both transistors
• Emitter resistors (small value, 0.1–1 Ω): force current sharing between Q1 and Q2

Transformer-coupled push-pull is used in virtually all solid-state broadband RF power amplifiers producing more than a few watts, and in valve (tube) audio amplifiers. The transformers provide galvanic isolation, allow easy impedance transformation, and the center-tap arrangement ensures precise 180° phase splitting. Ferrite-core transformers (wound on toroidal or binocular ferrite cores) are the standard choice for RF work — they achieve low loss and wide bandwidth, for example from 2 MHz to 30 MHz, in a single winding.

Complementary-Symmetry Push-Pull

The complementary-symmetry topology replaces the output transformer with a matched NPN/PNP transistor pair. Q1 (NPN) handles positive half-cycles; Q2 (PNP) handles negative half-cycles. Because Q2 is a PNP transistor, it is connected so that a positive base signal turns it off and a negative base signal turns it on — the opposite of Q1. This provides the necessary push-pull action without a transformer.

Both transistors receive the same input signal — no phase-splitting transformer is required at the input. When the signal goes positive, Q1 (NPN) turns on and drives current through the load to the negative supply rail. When the signal goes negative, Q2 (PNP) turns on and drives current from the positive supply rail through the load. The load sees a full, bidirectional current waveform.

Complementary-symmetry is the standard topology for solid-state audio power amplifiers and for op-amp and IC output stages. Its advantages are fewer components, no transformer bandwidth limitation, and no core saturation risk. Its disadvantages are the difficulty of finding perfectly matched NPN/PNP pairs and the need for careful biasing to avoid crossover distortion.

Even-Order Harmonic Cancellation

The cancellation of even-order harmonics can be understood mathematically. Suppose each transistor has a non-linear transfer function and produces output with a second-harmonic component a₂:

This is why push-pull amplifiers have very little 2nd harmonic output (negligible in practice, if the circuit is well balanced). In an RF amplifier, the 2nd harmonic of a 7 MHz signal would fall at 14 MHz; a well-balanced push-pull stage can achieve 50–60 dB of 2nd harmonic suppression, dramatically simplifying the harmonic filter that follows. In an audio amplifier, cancelling the even harmonics removes a large part of the distortion before any feedback is applied.

Class AB Biasing for Push-Pull Stages

The most critical design element of a push-pull audio or linear RF amplifier is the idle (quiescent) current bias that places both transistors in Class AB operation. Without idle current, the stage operates in Class B and exhibits crossover distortion. With too much idle current, efficiency drops and the transistors overheat.

Diode Bias

The simplest approach for complementary-symmetry audio stages uses two silicon diodes (one for each transistor's VBE) in series between the bases. The diodes are thermally coupled to the transistors — mounted on the same heatsink. As the transistors heat up, VBE decreases; the diodes, also heating up, produce a lower voltage drop across the bias network, tracking the transistors' VBE reduction and keeping the idle current approximately constant. This is simple, cheap, and effective for audio power stages.

VBE Multiplier (Rubber Zener)

For more precise idle current control, a VBE multiplier circuit is used. This consists of a single transistor (Qbias) with a resistor divider from collector to base. It produces an output voltage that is a multiple of VBE, adjustable by the resistor ratio. The multiplied VBE voltage sets the idle current of the output pair. VBE multiplier circuits are thermally coupled to the output transistors for temperature tracking. They allow idle current to be set precisely by adjusting a single trimmer resistor.

Setting Idle Current

Idle current for push-pull RF power amplifiers is typically set at 50–200 mA total (25–100 mA per transistor). For audio amplifiers, 20–100 mA total is common. Too low gives crossover distortion and poor IMD. Too high wastes power and can cause thermal runaway in poorly heatsunk designs. The correct idle current is found experimentally by measuring distortion (THD, or IMD with a two-tone test) while adjusting bias, or by looking at the output waveform at low signal levels and adjusting until the crossover notch disappears. Many audio amplifier service manuals simply specify the voltage to set across the emitter resistors.

Output Power and Efficiency

For a Class AB push-pull stage with supply voltage VCC and a load resistance RL (reflected to the primary through the transformer):

Output Transformer Impedance Matching

The output transformer in a push-pull amplifier has two functions: it reconstructs the full output waveform from the two half-cycle contributions of Q1 and Q2, and it transforms the impedance from the transistors' optimum load to the load impedance — 50 Ω for an RF amplifier, or 4–8 Ω for a loudspeaker in a valve audio amplifier.

The transformer turns ratio required for impedance matching:

In practice, ferrite-core wideband transformers (wound on toroidal ferrite rings or binocular cores) are used instead of tuned transformers for broadband RF amplifiers. Wideband transformers can cover a range such as 2–30 MHz without retuning, so the amplifier works at any frequency in that range without adjustment. The transformer is wound with bifilar or trifilar wire so that each winding has equal coupling to the core.

Balun Transformers in Push-Pull RF Stages

A balun (balanced-to-unbalanced) transformer is used at the output of a push-pull amplifier when the load is unbalanced (e.g., a coaxial cable with one grounded conductor). The push-pull stage has a balanced output — equal and opposite voltages on the two collectors. The coaxial output is unbalanced — one conductor is at ground potential. The balun converts between these two forms without disturbing the symmetry of the push-pull stage or letting common-mode current flow on the outside of the cable.

Most broadband RF power amplifiers use a ferrite-core balun transformer at the output. A well-designed balun has insertion loss < 0.5 dB across its frequency range and provides 20–40 dB of common-mode rejection.

Push-Pull Output Power Calculator