Sziklai Transistor Pair was first designed by George Sziklai to overcome some efficiency related issues with Darlington pair, which will be discussed later in this article. It is also known as compound or pseudo-Darlington pair. This transistor pair consists of two bipolar transistor pairs, where one is NPN and another is PNP. The Sziklai pair looks similar to the Darlington pair.
Sziklai Transistor Pair and its Configuration
Both Darlington and Sziklai have two types of configuration. But, Sziklai’s configurations have an equal base-emitter voltage drop to a normal transistor. While the base-emitter voltage drop of Darlington is twice of it. The sziklai pair are commonly used at the output stages of push-pull and class AB audio amplifier.
As you can see in the above image, Sziklai pair have two configuration. The first is NPN type sziklai pair, in which the transistor Q1 is NPN and Q2 is PNP. And, the second one is PNP type sziklai pair, in which the transistor Q1 is PNP and transistor Q2 is NPN.
The gain of the Sziklai and Darlington are approximately equal to each other.
Sziklai Pair Gain: β = βQ1x βQ2 + βQ1
Darlington Pair Gain: βQ1x βQ2 + βQ1 + βQ2
Practically, the total gain for both the pairs is approximately equal to:
β = βQ1x βQ2
Switching Voltage Test of Sziklai Pair with Darlington Pair
The main disadvantage with Darlington pair is that it requires twice the base-emitter voltage to start full conduction in comparison with normal transistor. A normal transistor requires 0.3-0.7v base emitter to fully saturate the transistor but Darlington pair need approx. 1.2v voltage drop between base-emitter for full conduction. This results in higher heat dissipation and slower response time. Learn more about Darlington Pair here.
These problems are resolved by Sziklai transistor pair as it has lesser turn-on voltage than the Darlington pair. It requires half or even less than half turn-on voltage as compared to Darlington pair. This can be easily understood by the proteus simulation of switching voltages of the Sziklai pair and Darlington pair.
Although the turn-off time of the Sziklai is greater than the Darlington pair but this turn-off time can be reduced by decreasing the value of the base drive resistor.
Circuit Diagram of Sziklai Transistor Pair
- 2N2222 – NPN Transistor
- 2N2905 – PNP Transistor
- Resistor – (100, 1k, 10k)
- Connecting Wires
Working of Sziklai Transistor Pair
Here we are demonstrating the switching test of the Sziklai pair by giving it a turn-on voltage of 0.7v. Sziklai pair starts conducting at this voltage and LED turns on, that means the turn-on base-emitter voltage for Sziklai pair is equal to normal transistor i.e. 0.7v. This is demonstrated properly in the video given at the end.
If we apply pulse input voltage at the base terminal of the NPN transistor Q1 to turn this ON, the PNP transistor Q2 is already in the forward biased state. Therefore, the current travel through the emitter of the transistor Q2 to the collector and emitter of the transistor Q1.
Which is better Sziklai Pair or Darlington Pair?
As Sziklai Pair addresses the issues with Darlington pair, so it is mostly advantageous to use Sziklai pair but it depends upon the application. Here are few advantages of Sziklai pair:
- Sziklai pair have lower quiescent current for better linear operations.
- Thermal stability of sziklai pair is superior to the Darlington pair.
- It has faster response time than Darlington pair.
- The turn-on voltage of sziklai pair is equal to a normal transistor, while Darlington takes twice the input voltage.
However there are few disadvantages like the gain of the sziklai pair is lesser than the Darlington pair.