Sziklai Pair / Compound or Complementary Pair

- tutorial and essential circuit design details for the Sziklai Pair, Connection or Compound / Complementary Pair, a complementary version of the Darlington.

The Sziklai pair is a very useful circuit configuration.

Using a very similar circuit configuration to the popular Darlington pair, the Sziklai pair offers a number of advantages in many occasions.

Being known by a variety of names the Sziklai pair may also be known as the compound pair or complementary pair.

The circuit may also sometimes known as the Sziklai connection on some occasions.

The circuit follows a similar beta boosting configuration to the Darlington pair, behaving like a transistor with a large beta gain.

The circuit consists of the two transistors with the collector of the first transistor being connected directly to the base of the second.

In the circuit, one transistor is NPN and the other PNP, the overall Sziklai pair taking on the appearance of the configuration of the type of the input transistor, e.g. if the input transistor is NPN and the second is PNP, then the overall configuration will appear like an NPN transistor to the external world.

The circuit Sziklai Pair / Compound Pair showong its resemblances to the Darlington.
Sziklai Pair Configuration (NPN)

It is also possible to have the Sziklai or compound pair in a PNP version:

Sziklai Pair Configuration (PNP)

Like the Darlington, it is wise to include a bypass resistor.

Circuit showing the Sziklai Pair with a bypass resistor between the base and emitter of the second transistor to improve performance
Sziklai Pair with Bypass Resistor

Sziklai compound pair gain

Although the gain of the Sziklai pair or compound pair is very nearly the same as that of the Darlington, it is not quite the same. The gain of the Darlington is given by the formula below:

Formula or equation enabling the gain of a Darlington pair

The gain of the Sziklai pair is slightly different as there is no individual contribution from Q2 as seen below.

Formula or equation allowing the gain of the Sziklai pair to be calculated

In view of the fact that the terms βQi and βQ2 on their own can be neglected, we obtain the more familiar equation which can be used for both the Darlington and Sziklai pairs..

Sziklai compound pair features

Although the Darlington is used in many applications, the Sziklai or compound pair has a number of advantages and can be used to good effect in a number of applications. Some of its features include:

  • Only a single base emitter drop between the overall base and emitter of the compound transistor.
  • Higher saturation voltage than a Darlington.
  • Very slightly lower gain than a Darlington
  • Can be used in a pseudo-complementary output with a Darlington - a true complementary pair would use both of the same circuit configurations. This configuration, which uses three NPN transistors and one PNP transistor. It offers a number of advantages including:

    • Previously silicon PNP transistors have been more costly than their NPN equivalents because of processing techniques and also the volume usage especially for the power transistor versions.
    • The performance of the lower "pull" pair, which uses a single NPN transistor, more closely matches the performance of the upper push pair, which consists of two NPN transistors (PNP transistors have lower carrier mobility). A true complementary pair would use all NPN for the lower pair and all PNP for the upper pair.
    As many PNP transistors with almost equivalent performance to their NPN counterparts are now available, the advantages of using the Sziklai / compound pair are less than they used to be.
  • The Sziklai pair is known to provide a better level of linearity than the Darlington pair which can be advantageous especially in audio applications.

In view of its characteristics the Sziklai pair or compound pair finds uses in circuits in a number of areas including audio amplifier outputs, general audio amplifiers and also for digital switching.

By Ian Poole

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