Although the Darlington configuration is widely used, the less known Sziklai pair provides a very useful circuit configuration that is also used, often in conjunction with the Darlington.
The Sziklai pair uses a very similar circuit configuration to the Darlington pair, and in many instances the Sziklai pair can offer a number of useful advantages in many occasions.
Although the name Sziklai pair is the most common terminology for the circuit, it may also be known as the Sziklai compound pair or complementary pair - occasionally it may also be referred to as the 'Sziklai connection.'
The Sziklai pair is named after the Hungarian, George Clifford Sziklai. He was born on 9th July 1909 in Budapest. He was educated at University of Budapest and then Technical University Munich.
In 1930 he emigrated to the USA where we worked for a number of companies including Radio Corporation of America and Westinghouse Electric Corporation and then in 1967 he settled at the Lockheed Palo Alto Research Laboratory.
Sziklai held around 160 patents including some for colour television and also credited with constructing the first Image Orthicon television camera. He also invented the transistor circuit configuration that bears his name: Sziklai pair.
Sziklai died on 9th September 1998 in Palo Alto, California, USA.
Sziklai pair basic circuit
The Sziklai pair circuit utilises a very similar topology to that of the Darlington and it provides a similar beta boosting configuration to provide very high levels of gain.
Like the Darlington, the Sziklai pair uses two transistors with the collector of the first transistor being connected directly to the base of the second.
The two transistors in the pair are different types, one PNP and the other NPN.
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.
It is also possible to have the Sziklai or compound pair in a PNP version:
Like the Darlington, it is wise to include a bypass resistor.
The bypass resistor is included to aid the switch-off process. Without the resistor in place, there is no discharge path for the base emitter capacitance. Including it enables the charge stored in this capacitor to dissipate and this aids a faster turn-off.
It is good design to include this resistor, but if speed is not an issue then the resistor can be omitted, but unless cost and component count are being reduced to the minimum then it is wise to include it.
Determining the resistor value is not an exact science. Smaller resistors will give a faster turn-off, but if they are made too small then a large proportion of the drive current for the second transistor passes through the resistor and gain is lost. If the value of the resistor is low and it takes current from the base of the second transistor, then current gain will be reduced and the equation for the overall gain of the Szlikai pair will need to incorporate this.
Typical values might be a few hundred Ohms for a power circuit and a few thousand Ohms for a small current transistor.
Sziklai 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:
The gain of the Sziklai pair is slightly different as there is no individual contribution from Q2 as seen below.
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 pair features
Although the Darlington is used in many applications, the Sziklai 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.
- Very slightly lower gain than a Darlington
- Higher saturation voltage 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:
- 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.
- 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 Sziklai pair is known to provide a better level of linearity than the Darlington pair which can be advantageous especially in audio applications.
The Sziklai compound pair finds applications in many areas of electronics. Possibly the most common areas are in audio output stages where it can be used as one half of a push pull amplifier, the other half being a Darlington. In this way the Sziklai pair enables the output to operate in a complementary symmetry fashion.
The Sziklai pair also finds uses in a number of digital circuits. Again it enables outputs to operate in a complementary symmetry fashion, and also the fact that it only requires a single diode voltage drop across the input instead of the two for a Darlington makes it attractive in many areas.
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