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Why is this simple Class B amplifier not working?

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magic:
AN-1593 figure 1 is captioned "voltage follower" and I can believe that the whole appnote is about it. That is, unity gain.

Indeed your circuit will never work because the high voltage output is your opamp's output and it needs to swing ±48 while the opamp's inputs need to stay at ±5V. No way that both the output and the inputs will stay within the opamp's supply rails, whatever they are.

There are more complex bootstrapping circuits which work with gain, but I don't remember their names. That being said, their complexity probably approaches that of a fully discrete solution, so for completeness, here's one.

This is a fully discrete opamp which should work for this purpose. It's almost rail to rail. Output can source 1mA of current and sink enough to blow up Q3 :P
Modifications are possible to change this behavior if you need.

GerryR:

--- Quote from: hamster_nz on November 08, 2019, 09:49:33 pm ---Isn't that schematic set up to amplify current, not voltage? (i.e. It is a power output stage)

--- End quote ---

This!!

Psi:
I'm not sure of how important signal integrity/linearity is or what the actual data is,
but have you looked into opto isolators.
You can get them with high voltage transistors and they are definitely cheaper than $5

You might find they could work to get your  +/-5V signal up to +/- 50V with some tweaking and DC offsets.

rstofer:

--- Quote from: colejohnson66 on November 09, 2019, 02:46:15 am ---I only have one more problem now: the "flying rails" are not going all the way to the -45V and 45V rails. Instead, they only go to roughly -37~37V. This clips the output of the op amp to roughly -35~35V. Now, clipping at the rails is fine, if they were at the PSU rails, but they're not. How can I fix this?

--- End quote ---

Random Op Amps are unlikely to be 'rail-to-rail' on input and or output so the output can't get within a couple of volts of the rails.  Read the datasheet!

For the 741 op amp with +-15V supplies and a 2k load, it can't get within 5 volts of the rail (MIN) or 2 volts (TYP).  Look at "Output Voltage Swing".  A prudent design would use the 5V figure.  In fact, early analog computers used 741s and the anticipated voltage swing for input and output was +-10V.

http://www.ti.com/lit/ds/symlink/lm741.pdf  Page 5

Input Voltage Swing is similarly restricted, you can' put in more than +-12V (MIN) or +-13V (TYP).  Again, a prudent design would use the 12V figure.

When you buy rail-to-rail op amps, you need to realize that 'rail-to-rail' is a marketing term, not a reality.  There will still be limits, they just won't be as severe.

magic:
Those MOSFETs aren't exactly rail to rail either ;)

That circuit is hopless for the reason I already stated: the opamp can't be bootstrapped to the output voltage because its input needs to stay near ground potential.

A discrete voltage gain stage is needed, like Q3/Q5 on my schematic. There is no escape from that. An opamp, however, could replace Q1/Q2 although some additional trick would be required to translate its output down to the -49.3V potential of Q3 base. Which is why I said that it approaches complexity of a wholly discrete solution. Precision will be better with an opamp, though.

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