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Analog multiplier - wrong simulation or flaw in the circuit?

 

I run across the schematic shown in page 13 of the RC4136's datasheet (uploaded). After 3 days trying to have it "working" in LTSpice (uploaded) I realized I need help.?

I tend to think I know the basics of log / antilog amps and I understand this circuit would give (hopefully) a Vout = Vy * Vx / Vz. Instead, the prevailing result I get is Vout slightly above Vx, no matter what Vy or Vz are.?

With different op amps, or no matter how many diodes I pile up at the top (to get up to around 3.5V) to polarize the first stages, I always get a result not even close to the expected (and most of the times with >1 MHz noise riding the output).?

I am puzzled by what is wrong: my simulation or the circuit??


Sorry but I am not sure if I uploaded both files (RC4136 datasheet and Analog mutiplier 03.asc in the right place)
 

Hello Augustinto,

There are two problems in your circuit.

1. The offset adjust circuit only works with a very few hundred milli Volts. See my test circuit.



2. Your Opamps require a feedback capacitor of a few ten pico Farads.


The folder with the circuits:

?

It's also very important to use matched transistors.

1. Use matched quad transistors! MAT04, MAT14, THAT300, 2nd choice CA3046, CA3086, CA3127
2. Don't forget decoupling capacitors of > 0.1uF from +V to GND and -V to GND and a real circuit!

Best regards,
Helmut
 

You hit the nail again, Helmut.

Gracias!!

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