![]() This happens because the gate or base drive circuit inserts charge into the bases/gates of one side faster than it extracts from the gates/bases of the other side. Third: most amplifier circuits, both with MOS and bipolar output stages, suffer from current tails when they are asked to perform relatively quick zero-current crossing events. As expected, the multimeter shows a bias value that may be two or three times higher than expected (only 20% to 50% higher in clever designs with truly good compensation) and this value falls quickly to the expected value after a few seconds To circumvent that, I leave the multimeter turned on and connected to the test points and I disconnect the input signal and the load very quickly while looking at the display. After a few seconds, the dies are already at the same temperature as the heatsink and your bias measurement has been fooled. They may be, say, 20✬ or 30✬ hotter than the heatsink, but their temperature drops very quickly when you disconnect the load and the input signal in order to measure bias current. Second: when an amplifier is playing, the dies of the output devices are somewhat hotter than when it's idle. I have measured it and I'm fully aware of that issue.įirst: relevant changes in bias with small changes in supply voltage should be taken into account (this should only happen in poor amateur designs, despite any marketing). The optimum bias for these designs should just provide a smooth transition between each bank of output devices, compensating for the gain loss when Ic is only a few mA. That jump is a major source of intermodulation. ![]() When the bias is set too high in a class-AB bipolar output stage, the output impedance jumps abruptly between the usual value and half that value, depending on whether both sets of output devices are or not conducting at the same time. ![]() Thanks for the replies,I have looked into building a Class A amp and have not been able to make my mind up as to building one.A few years ago I did make one,sort of,I used a TIP 141 and biased it into Class A and added another TIP 141 as an Emitter Follower I ran it of a 2.5amp transformer and although it sounded better than my AB amp,a JVC AX-2,it just burnt itself out.It hummed big time,I used a 4700uf cap on the speaker output to remove a big DC offset that pushed the drivers cone right out,OUCH,I guess my calculations were off.The transformer heated up,the transistors heated up,which in turn must have heated the transformer up some more.I had to heatsinks bolted together and blasted them with a 9" office fan.I figured that when I calculated the emitter current I forgot that I had to take into account the current drawn by the rest of the circuit,like base bias current used by both used BJT's because I couldn't figure out how to use mosfets.Maybe I'll have ago again one day.Yes TNT should go Boom ha! Please,can someone clear this up for me.Maths is not my strong point. Hi all,Just been asked a question.An article on the TNT website about increasing an amplifier's bias current,gives you more Class A.I have posted this at the Audio Asylum,ages ago.What do you lot think about this?My amp is Class AB and if I increased my bias from 45ma to 120ma,say,do we get more Class A? Yes it runs hotter,but I've been told of increased distortion,somthing about harmonic distortion(I must buy a scope).The original article "Put a Tiger in your amp"gives a few numbers,but I havn't a clue about whats being calculated,and I've always assumed a Class A amp uses one power transistor.I would like to know how you would know if your listening in class A or AB.Am I to assume you would need sensitive speakers and the volume to be low,and wouldn't a transient signal send it into AB?Ĭan anyone explain this? I'd like an LED to show me when I'm listening in Class A.Do you think I'm barking up the wrong tree?At the end of the day,some sensitive speakers and an indicator and I've got me a Class A amp,right? ![]()
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