![]() ![]() If the transistor had infinite gain, the input impedance would be zero !įor the time being, we'll ignore the AC performance, and just examine the biasing requirements. The DC feedback stabilises the operating conditions, and the AC feedback causes the input impedance to be reduced. This is due to the feedback provided by R2, which acts for both AC and DC. ![]() As shown (and perhaps surprisingly), the input impedance is around 660Ω - much lower than you'd expect. The circuit shown in Figure 1 will definitely work as an amplifier, but it needs input and output capacitors, and it has a very low input impedance. Note: This article is not intended to show the way to build a simple transistor amplifier, but to allow you to determine the parameters of a transistor. Emitter current is always equal to the sum of the base and collector currents. Transistors are essentially current-to-current converters, so a small base current controls a larger collector current. Ultimately, the terminology doesn't matter all that much, provided you understand the concept of current gain. Not all agree that the terms as shown represent the characteristics, and h fe and h FE are often used interchangeably. The terminology can be different, depending on what source material you're looking at. H FE: The h FE parameter describes the DC or large signal steady state forward current gain. The small letter 'h' indicates it is a small signal gain. The letter 'f' indicates that it is a forward transferĬharacteristic, and the 'e' indicates it is for a common emitter configuration. H fe: This is the current gain for a transistor expressed as an h (hybrid) parameter. Beta β: This is the basic notation for the forward current gain of a transistor. ![]() While it's common to see questions asked about substitutions, if you know these basic facts you can work out for yourself what will (or will not) work. That means that almost any transistor will work, provided it's rated for the current and voltage used in the circuit. In these cases, it's generally accepted that the base current should around 1/10 of the collector current, regardless of the transistor's β. The cutoff and saturated regions are only of importance in switching circuits. For amplification, we need to be in the active region. If nothing else, an AC test that includes distortion measurements is useful to determine the overall linearity - a truly linear circuit contributes no distortion.Ī transistor can be in one of three possible states, cut-off (little or no collector current flows), active (or 'linear') and saturated (collector voltage at the minimum possible). While it's not required, I expect that most readers will want to run AC tests, and they are informative (even if not actually very useful). This is not part of the process for determining the DC characteristics, and is only necessary to measure AC performance. In the circuits shown below, the input coupling capacitor has been selected to give a low frequency -3dB frequency of around 10Hz. While most circuits don't need matched devices, in some cases doing so improves performance. This is a useful tool to let you understand how the transistor functions, and is easily adapted to the task of matching devices if that's something you need to do. ![]() Only a few simple calculations are needed to let you determine the DC current gain (aka β / h FE), with the benefit that you can set the transistor's actual operating conditions when setting up the test. There are even applications where it's useful, particularly where there are no opamps in the circuit and you need a gain stage. While this topology is easily beaten by even the most pedestrian opamp for most things, it offers a fairly easy way to determine the transistor's parameters. A perfect example is a BJT amplifier circuit, using only a single transistor and a pair of resistors (as shown in Figure 1). Paradoxically, it's often the circuits that appear to be the simplest that cause the most problems. Some circuits are easy and don't require much more than Ohm's law, while others seem a great deal harder. There are many things about transistors that confuse the beginner and no-so-beginner alike. ![]()
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