J162 | Electronic Project Circuits

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Mosfet Amplifier OTL 100W by K1058,J162

This includes a 220uF capacitor and two diodes to slow down the switch-on to reduce the output thump as the output capacitor charges through the speaker. This is not essential, depending on speaker sensitivity, with my own speakers there is just an unobtrusive low frequency sound at switch-on. With other applications such as a bi-amp or tri-amp system with active crossover filters, this addition may become more useful to avoid low frequencies reaching high frequency speaker drivers. It may also have some point if a high sensitivity speaker is used, so it makes sense to include it in this low noise version.

Be the first to comment - What do you think? Posted by admin - April 30, 2008 at 11:57 am

Categories: mosfet amp Tags: 100 watt amplifier, J162, K1058, Mosfet amps

Simple Audio Power Amplifier 60W by K1058, J162

mosfet amplifiers there is no clear distinction between class-B and class-AB. For a bipolar output stage it is common to call a design class-B if the quiescent current, Iq, is adjusted for minimum crossover distortion, and class-AB if the current is increased well above this optimum level so that operation remains in class-A for small signals. (Not everyone agrees on this convention but it seems a sensible choice, for audio amplifiers at least, because these two classifications have rather different design problems. For example ‘class-B’ usually requires far more accurate quiescent current setting and thermal compensation for best results.) Class-AB causes higher distortion than class-B at higher signal levels where device switching starts to occur. For mosfets the distortion is generally highest at low quiescent current and reduces as Iq is increased, so there is not a similar optimum value giving minimum distortion. My choice of 100mA for the circuit presented here is because the mosfets used have zero temperature coefficient around this value, but with adequate heatsinks the current could be increased. With reduced supply voltage the circuit can even be biased into class-A for the lowest distortion, and here mosfets have some advantage because of their nearly square-law characteristics. If they were perfectly square-law then the peak output current in class-A would be 4 x Iq instead of 2 x Iq for typical bipolar output stages, and up to this level the distortion would be zero for ideal accurately matched complementary devices. Unfortunately well matched pairs are unobtainable, and they are not very accurately square-law, and they have non-linear capacitances, so even class-A still has its problems.

Mosfet Power Amplifier 120W by K1058,J162

This amplifier evolved from the previous 6 transistor mosfet amplifier. That design was already so good there seemed no reason to look for improvements, but the high open-loop distortion at 20kHz and the resulting triangular ‘error voltage’ extracted suggested that the non-linear mosfet capacitance was responsible for almost all the distortion, and although the closed-loop distortion remains below 0.01% this could easily be reduced further by driving the mosfets from a lower impedance just by adding an emitter-follower.

Source: http://www.angelfire.com/ab3/mjramp/amp7.html