Add as many if you want to isolate the outputs with inverters. For example, pass-transistor logic ( transmission gates) makes it pretty simple :Įach pass element is a pair of complementary transistors, so this gate uses 2 NMOS and 2 PMOS. You can find circuits using 4, 6, 8, 9, 10 or 12 transistors, again with varied strengths for the inputs and the output. The XOR gate has a much wider range of implementations in MOS and CMOS. Does the gain in parts count affect the performance ? Apparently, it's pretty close to ideal because it's touted as a solution in Direct Coupled Transistor Transistor Logic:Īnother version has only one transistor but 4 diodes :Īnother question is : can this scheme (no amplification, just relying on the input's strength) be extended to other logic or sequential functions ? XOR is pretty important in CPUs because many mechanisms rely on it, for example ALUs. Thus, the question : is it the best method ? What about the switching speed or the capacitances ? These two interlocked transistors use a very unusual structure, which requires the least theoretical number of switching elements, but it depends on a trick : the input impedances matter a lot and the circuit depends on a "hard" 0 level, because the circuit behaves almost like a "pass" element. As you can see, when the collector current increases, h FE decreases.I sometimes find a small circuit with 3 resistors and 2 transistors that performs the eXclusive OR operation. The graph above shows h FE on the y-axis and collector current on the x-axis for a general-purpose transistor. Students often find it difficult to visualise the relationship between h FE and collector current. The h FE parameter is not a constant though, because a transistor may have many ratings for different collector currents Ic. Hence, the current flowing through the collector is proportional to the base current multiplied by gain, as shown by the formula below. How large this current flow is depends upon a gain factor known as "h FE", also sometimes called the DC current gain, and beta. Remember that a bipolar transistor is a current amplifier, because a small amount of current "Ib" through the base controls a larger amount of current "Ic" flowing through its collector. For hard saturation, engineers usually choose a value of 10. Value for transistor switching purposes we always choose the minimum rating as the worst case because we want the transistor to conduct in the saturation region. The parameter "h FE" represents the DC gain,Īnd this is the parameter to consider. Small case "h fe" represents the small-signal current gain or AC gain,Īnd we do not use this parameter when using the transistor as a switch. In transistor literature, there are two different types of gain parameters with the same three letters. There are two calculators in this multi-page section of the article, and the first one is for when the load resistance is known, whilst the second, is for when the load current is known.
A proper value of base resistance is therefore required for conduction in this region, and this value is different for different input switching voltages.
In these types of switching applications, we require the transistor to behave as a switch and conduct fully in For hard saturation, engineers usually use a DC current gain h FEĪn NPN transistor requires a positive voltage at the base junction to switch ONĪnd control a load (RL) such as a low-voltage relay with a known resistance value. This resistor determines the amount of saturation current I b(sat) flowing into the base junction, and that controls the amount of saturation current I c(sat) flowing through the collectorĪnd emitter junctions. Engineers often have to consider the required value of the base resistor that controls the amount of current entering the base junction of a bipolar junction transistor (BJT) to cause it to conduct in the saturation region.
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