Being known by a variety of names the Sziklai pair may also be known as the complementary feedback pair (CFP) or "compound transistor", and as a "pseudo-Darlington". The Sziklai pair is a configuration of two bipolar transistors, similar to a Darlington pair. In contrast to the Darlington arrangement, the Sziklai pair has one NPN and one PNP transistor, and so it is sometimes also called the "complementary Darlington". The configuration is named for its early popularizer, George C. Sziklai.
What is a Sziklai?
In essence, a high gain super transistor, somewhat similar to a Darlington. But there are major differences. Unlike Darlingtons, there is some voltage gain with Sziklais. Another unique feature is local feedback.
Why is it not as popular as EF?
Basically, more difficult to design then EF. When local feedback is nested in global feedback, an amplifier is more prone to instability. In other words, it is easier to break out into high-frequency oscillations. But a well designed Sziklai Output can often be outstanding.
Characteristics of Sziklai Pair
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| Sziklai Pair Configuration (NPN) |
The current gain of the pair is similar to that of a Darlington pair and is the product of the current gains of the two transistors. The figure above illustrates an NPN-PNP pair that acts like a single NPN transistor overall. By replacing Q1 with a PNP transistor and Q2 with an NPN transistor the pair will act like a PNP transistor overall. (Just exchange the two arrows in the figure to visualize the PNP-NPN pair.)
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| Sziklai Pair Configuration (PNP) |
Like the Darlington, it is wise to include a bypass resistor.
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| Sziklai Pair with Bypass Resistor |
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.
Applications
In a typical application, the Sziklai pair acts somewhat like a single transistor with the same type (e.g. NPN) as of Q1 and with a very high current gain (β). The emitter of Q2 acts the role of a collector. Hence the emitter of Q2 is labeled "C" in the figure to the right. Likewise, in a typical application, the collector of Q2 (also connected to the emitter of Q1) plays the role of an emitter and is thus labeled "E." As with a Darlington pair, a resistor (e.g., 100Ω–1kΩ) is usually connected between Q2's emitter and base to improve its turn-off time (i.e., its performance for high-frequency signals).
Advantages
One advantage over the Darlington pair is that the base turn-on voltage is only about 0.6V or half of the Darlington's 1.2V nominal turn-on voltage. Like the Darlington, it can saturate only to 0.6V, which is a drawback for high-power stages.
Sziklai-based output stages
Sziklai pairs are often used in the output stages of power amplifiers due to their advantages both in linearity and bandwidth when compared with more common Darlington emitter follower output stages. They are especially advantageous in amplifiers where the intended load does not require the use of parallel devices.
Sziklai pairs can also have the benefit of superior thermal stability under the right conditions. In contrast to the traditional Darlington configuration, the quiescent current is much more stable with respect to changes in the temperature of the higher power output transistors vs the lower power drivers. This means that a Sziklai output stage in a class AB amplifier requires only that the bias servo transistor or diodes be thermally matched to the lower power driver transistors; they need not (and should not) be placed on the main heatsink. This potentially simplifies the design and implementation of a stable class AB amplifier, reducing the need for emitter resistors, significantly reducing the number of components which must be in thermal contact with the heatsink and reducing the likelihood of thermal runaway.
The optimal quiescent current in an amplifier using Sziklai pairs also tends to be much lower than in Darlington-based output stages, on the order of 10mA vs. 100mA or more for some emitter follower output stages. This means that idle power consumption is on the order of a few watts versus tens of watts for the same performance in many cases. This is a very compelling reason to use the Sziklai pair in cases where output power is moderate (25-100W), fidelity is critical and relatively low idle power consumption is desired.
Quasi-complementary output stages
Historically, designers frequently used the "quasi-complimentary" configuration, which uses a Darlington push pair (i.e., two NPN transistors) and a Sziklai pull pair (i.e., one PNP and one NPN transistor). This configuration, which uses three NPN transistors and one PNP transistor, is advantageous because while the first transistors and the most common small signal transistors for decades were PNP Germanium devices, silicon PNP power transistors were slower to develop than and have historically been more expensive than their NPN counterparts. Alternately, if a germanium PNP device were used, it would have significantly different characteristics. In the Quasi-complementary topology, the performance of the lower pull pair, which used a single NPN transistor, more closely matched the performance of the upper push pair, which consists of two NPN transistors and an identical power device.
While for decades the Quasi-complimentary output stage made sense, recently PNP and NPN power transistors have become roughly equally available and have more closely matched performance characteristics, and so modern audio power amplifiers often use equivalent topologies for both pairs, either both Darlington emitter follower or both Sziklai pair.
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:
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.
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.
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