Simple class A amp improved with a constant current source

Comments · 82

• FR

  @fredfabris7187 · 5 years ago (edited)

  I’ve got a couple things. First I don’t think the video is too long I prefer a longer video than a short one. Thank you for re-explaining the constant current source because it’s helpful. I love your videos and the detailed conversation about how circuits work is hard to find online. Much appreciated

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• MA

  @markweikle5645 · 5 years ago

  "that's not going to keep greenpeace from kicking down your door." I launched a little of my eggroll on that. Lol!

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• NO

  @northox · 5 years ago

  Never get bored of John's video.

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• DA

  @darthbubba866 · 5 years ago

  Welcome to the world of Class-A audio amps; warm sound, warm listening room! 😄

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• TH

  @theschnilser7962 · 5 years ago

  Reminds me of the time when I had a very simple Class A amp with just a decoupling cap, two resistors, a TIP3055 and a 12V light bulb as a current source. It was not even soldered, but on a breadboard like this one. That thing was not very loud either, but enough to enjoy some music while doing some electronic stuff or gaming.

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• RE

  @RexxSchneider · 4 years ago (edited)

  The problem with biasing a MOSFET for class A is the relatively large range of Vt. its threshold voltage. The datasheet for IRFZ44 indicates a value between 2 and 4 volts. Coupled with a high forward transconductance of around 2.7 A/V at 1A, you can see why small variations of the bias voltage on the gate can produce large current swings at the drain.<br><br>In the original circuit the 100K pot was fed from a 22K resistor allowing the gate voltage to vary from 0V to 7V. The resulting change in drain current past the threshold voltage was ameliorated by the 0.47R source resistor and 8R drain resistor, which represents a dc gain of about 8R/(0.37R + 0.47R) = 9.5 (the 0.37R is the reciprocal of the estimated forward transconductance). That&apos;s not too bad for sensitivity.<br><br>When you replace the 8R drain resistor by a constant current source, you&apos;re providing a high dc impedance to the drain and dramatically increasing the dc gain, so that the bias point becomes very &quot;twitchy&quot;. In fact the mismatched temperature coefficients of two diodes against one PNP emitter junction will make that bias point very temperature dependant. However, the ac gain does double because originally the load is 8R in parallel with the 8R speaker, and now it is a high resistance in parallel with an 8R speaker.<br><br>So to stabilise the dc, you sensibly introduced a load of negative feedback from the drain to the gate. That has the effect of reducing the circuit gain, effectively becoming the ratio of the 10K feedback resistor to the value of the trim pot that&apos;s setting the gate bias voltage. Assuming the drain voltage is round about half the supply (4.5V), that suggests the trim pot will have a value between 8K (if Vt=2V) and 80K (if Vt=4V). The gain will therefore be between 1.25 and 0.125. Great for dc stability, but not so good for use as an amplifier. You could, of course, increase that 10K resistor and replace the trim pot by a trim pot in series with a resistor R, then bypass the trim pot with a capacitor that has a smaller reactance than the resistor R. That would allow you to set the dc gain small, but have a larger ac gain set by the ratio of the feedback resistor to the unbypassed resistor R. The ac gain is then independent of the setting of the trim pot.

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• PI

  @pilobond · 5 years ago

  I wonder if you can try using LM317 in current source mode as a constant current and do a comparison video. Thanks for wonderful videos.

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• AL

  @alfredo-g3 · 5 years ago

  Great video John, I recently started watching your DIY amp videos and they&apos;re very inspiring, amazing work! Thanks &amp; cheers!

• KU

  @kuadidzv8726 · 5 years ago

  very nice explanation sir😍😍😍

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• GK

  @gkdresden · 5 years ago (edited)

  11.6% COP ist a very good value for a constant current class A design. It corresponds to about 20% plus for a usual class A power amplifier design with an idle bias current of half of the current at its nominal maximum power. The limit of class A design is a COP of 25%.<br><br>BTW: I like your mixed type BJT / FET circuit design. :)

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• BR

  @briangoldberg4439 · 5 years ago

  Double the output power at the same current draw sounds like a win to me.

• RO

  @roberthorseman7432 · 5 years ago

  That&apos;s tidy up to my bench.