Electronics > Beginners
Why is this simple Class B amplifier not working?
colejohnson66:
I’ve been thinking about my requirements, and I don’t think I need a “rail-to-rail” op amp (yes, I know it’s just marketing). While my voltage rails will be -45 and 45V (my PSU only has one output, so I’m using buck converters to invert), I only need an output from -40~40V. As for the accuracy, it doesn’t need to be perfect. An output of 5V from the amplifier could have 250mV out and be fine.
I’ve been considering a discrete op amp based system, but I could never find a schematic simple enough. The one @magic posted seems promising, however, I have one question: I can’t find a 100V rated 100 pF capacitor on DigiKey that’s a good price (or any pF magnitude ones at that voltage). Does it need to be 100V rated, or could it be less? As for the transistors, I can easily find 100V surface mount transistors for pennies in the volume I’d need.
magic:
One end is at 0.7V above the negative rail, the other may swing all the way up to the positive rail. You know what rating you need >:D
I don't believe there are no 100V capacitors. Either MLCC or through hole ceramic disk should be found with ease at any distributor. Try other categories or just type "100pF 100V" into the search box.
And by the way, my circuit actually sucks :P I thought I'm being clever using that current mirror but I probably should have got a clue when SPICE told me the output current will be 1.5mA. That's because collector voltage on Q5 is much higher than on Q4. And another effect of that will be a serious temperature difference between them and even more current through Q5. I think it's risky business.
I recommend replacing Q4 with two series diodes or an LED and adding a resistor between Q5 emitter and the supply rail to set the current through Q5 and Q3 to 1~2mA or whatever you need. Mind power dissipation in Q5 and Q3, including when the output is at negative rail for prolonged time.
colejohnson66:
Ah. I was searching in the wrong category for the capacitor. I found one for 3.5¢ or less in volume. And now that I know the difference between a power and a voltage amplifier, what if I added a power stage to the output of the “op amp”? Something like a darlington push-pull buffer?
Also, how do you measure the maximum output current with LTSpice?
magic:
You need to add some realistic load to simulate the output stage properly. Then you will also know whether it's strong enough to drive your load.
To simulate short circuit protection, short the output to ground or the opposite supply rail with 1mΩ resistor and plot resistor current.
Audioguru again:
Your first schematic has a +/- 48V supply and a 10 ohms load. If its output swings 90Vp-p then it is 31.8V RMS. Then the maximum undistorted power in the 10 ohms load is (31.8V squared)/10 ohms= 101W.
The power in each output transistor will be almost 40W which is impossible with surface mount transistors.
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