![]() The TIP35/36 devices I suggest will have a 'typical' gain (h FE) of around 45, and around 100 for the BD139/140. Using a transistor emitter-follower means that we can use higher resistance and lower capacitance, with the transistor providing the current, rather than directly from the filter.Ī single transistor doesn't have enough gain to allow the use of comparatively high resistance. If the resistor is reduced to 1Ω, the capacitance would have to be 1F (that's 1 Farad !). For example, a filter using 1k and 1,000♟ has a -3dB frequency of 159mHz. This is impractical, because the capacitance needed to obtain a -3dB frequency of less than 1Hz becomes very large. Provided you understand that the original term is inaccurate (or just plain wrong) and understand how it works, it doesn't matter what it's called.Ī simple passive filter can't be used with significant current because the voltage drop across the resistors would be prohibitive unless they are very low values (less than 1Ω). Calling it a 'buffered passive filter' is more accurate, but doesn't convey the same idea, as the original term has been used for years and it's something that people are used to. This improves ripple rejection because the filter is converted from first-order (6dB/ octave) to second-order (12dB/ octave).ĭespite the name 'capacitance multiplier' being a misnomer because nothing of the sort happens, I'll still use the term in this article. ![]() In particular, there's a great deal to be gained by using two capacitors, separated by a second resistor. Capacitance is not multiplied by the gain of the transistor(s), only the current flowing through the base resistor. The behaviour is similar to a very much larger capacitance, but there are some significant differences.Ī 'capacitance multiplier' is really just a buffered filter, with the filter response set by the resistance and capacitance at the base circuit. Everyone who uses this type of circuit calls it a 'capacitance multiplier', and while you may think it's also a crude gyrator (simulated inductor), this isn't the case. While a capacitance multiplier is superficially simple, there's actually more to it than you might think. ![]() Parts 2 and 3 are also interesting, but don't cover high current supplies. The article Linear Power Supply Design should be considered essential reading before embarking on a capacitance multiplier, as many of the essential elements are discussed in detail. The original John Linsley-Hood version (see Simple Class A Amplifier, page 9) uses a single-pole filter, which is nowhere near as good as the version described here. There is also an article in the 'TCAAS' section of my site (see JLH Capacitance Multiplier), but this doesn't cover the design criteria in much detail either. While this is a useful resource, it doesn't delve into the design criteria, so this article is intended to provide you with enough information to design your own. In the Project 15 page, I have described a number of different approaches to a capacitance multiplier. 5 - Dual Capacitance Multiplier For Class-A Amps. ![]()
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