![]() 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. Optimal quiescent current in an amplifier using complementary feedback pairs also tends to be much lower than in Darlington-based output stages, on the order of 10 mA vs 100 mA or more for some emitter follower output stages. This significantly reduces the number of components which must be in thermal contact with the heatsink and reduces the likelihood of thermal runaway. This potentially simplifies the design and implementation of a stable class AB amplifier, reducing the need for emitter resistors. 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. In contrast to the traditional Darlington configuration, quiescent current is much more stable with respect to changes in the temperature of the higher power output transistors vs the lower power drivers. Ĭomplementary feedback pairs can also have the benefit of superior thermal stability under the right conditions. They are especially advantageous in amplifiers where the intended load does not require the use of parallel devices. Like the Darlington, it can saturate to only about 0.6 V, which is a drawback for high-power stages.Ĭomplementary feedback-based output stages Ĭomplementary feedback 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. One advantage over the Darlington pair is that the base turn-on voltage is only about 0.6 V, or about half of the Darlington's 1.2 V nominal turn-on voltage. As with a Darlington pair, a resistor (e.g., 100 Ω to 1 kΩ) can be connected between Q2's emitter and base to improve its turn-off time (i.e., improve its performance for high frequency signals). Likewise, in a typical application the collector of Q2 (also connected to the emitter of Q1) functions as an emitter and is thus labeled "E". Hence the emitter of Q2 is labeled "C" in Figure 1. The emitter of Q2 functions as a collector. In a typical application the Sziklai pair acts somewhat like a single transistor with the same type (e.g., NPN) as Q1 but with a very high current gain (β). By replacing Q1 with a PNP transistor and Q2 with an NPN transistor the pair will act like a PNP transistor overall. Figure 1 shows an NPN-PNP pair that acts like a single NPN transistor overall. The current gain of the Sziklai pair is similar to that of a Darlington pair and is the product of the current gains of the two transistors. 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". In electronics, the Sziklai pair, also known as a complementary feedback pair, is a configuration of two bipolar transistors, similar to a Darlington pair. Sziklai pair that acts like a single NPN transistor with collector C, emitter E, and base B. R1 is only for protection, so you can increase it's value depending on the collector-emitter-voltage of the optocoupler to limit the maximum current.Figure 1. If we assume a CTR of 50% then you need to drive the LED with about 2mA to get Q1 into saturation. The normal Current Transfer Ratio of most optocoupler lies in the range of 30% to 200%. So you need at least 1mA base-current to drive Q1 into saturation. From here we can determine the needed base-current to (96mA / 100) = 960uA. The 50Ohm load will in this case be driven with a current of (5V-0.2V)/50Ohm =96mA. The saturation voltage (collector-emitter-voltage) of a common NPN transistor like BC546 is about 0.2V. ![]() The resistor has to be small enough to drive Q1 into saturation.Ī transistor goes into saturation when both the base-emitter and base-collector junctions are forward biased e.g when the collector voltage drops below the threshold of the base-collector voltage (about 0.4V-0.6V). Depending on the CTR (Current Transfer Ratio) of the optocoupler and your driving current of the optocoupler LED, you need a resistor (R2) to limit the current through Q1. In this picture, you use the current driving capabilities of the optocoupler directly. In your circuit, when the optocoupler is not conducting, the second transistor always dissipates energy. While this configuration might work, there is a more efficient way of controlling the load. It simply boosts your current driving capabilities for the 50Ohm load and inverts the logic level from your optocoupler. Here is no name for this transistor configuration. ![]()
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